Explain Classification of Elements

Explain Classification of Elements

Elements
An element is a substance which cannot be split up into two or more simpler substances by the usual chemical methods of applying heat, light or electric energy.
An element cannot be split up into two (or more) simpler substances because it is made of only one kind of atoms.
Ex.       Hydrogen is an element because it cannot be split up into two or more simpler substances by the usual methods of carrying out chemical reactions by applying heat, light or electricity.

ElementSymbol
AluminiumAl
ArsenicAs
BariumBa
BromineBr
CadmiumCd
CalciumCa
ChlorineCl
ChromiumCr
CobaltCo
FluorineF
HydrogenH
IodineI
MagnesiumMg
ManganeseMn
NitrogenN
OxygenO
PhosphorusP
SulphurS
UraniumU
ZincZn
(symbols from latin names)
Antimony (stibium)Sb
Copper (Cuprum)Cu
Gold (Aurum)Au
Iron (Ferrum)Fe
Lead (Plumbum)Pb
Mercury (Hydrogyrum)Hg
Potassium (Kalium)K
Silver (Argentum)Ag
Sodium (Natrium)Na
Tin (Stannum)Sn

All the Elements can be divided into three groups.

  1. Metal         
  2. Non-metal  
  3. Metalloid

1. Metals :
A metal is an element that is malleable and ductile, and conducts electricity. All the metals are solids except one metal mercury, which is a liquid.
Ex.      Iron, Copper, Aluminium, Zinc.

Properties of metals

  1. Metals are malleable : This means that metals can be beaten into thin sheets with a hammer (without breaking).
    Ex.       Aluminium metal is quite malleable and can be converted into thin sheets called aluminium foils. Aluminium foils are used for packing food items like biscuits, chocolates, medicines, cigarettes, etc.
  2. Metals are ductile :       This means that metals can be drawn (or stretched) into thin wires. All the metals are not equally ductile. Some are more ductile than the other.
    Ex.       Copper and aluminium metals are also very ductile and can be drawn into thin wires which are used in electrical wiring.
  3. Metals are good conductors of heat and electricity : This means that metals allow heat and electricity to pass through them easily. Silver metal is the best conductor of heat. It has the highest thermal conductivity.
    Ex.       The cooking utensils and water boilers, etc., are usually made of copper or aluminium metals because they are very good conductors of heat.
    Ex.       The electric wires are made of copper and aluminium metals because they are very good conductors of electricity.
  4. Metals are lustrous (or shiny), and can be polished : The property of a metal of having a shining surface is called metallic lustre (chamak). The shiny appearance of metals makes them useful in making jewellery and decoration pieces
    Ex.       Gold and silver are used for making jewellery because they are bright and shiny. The shiny surface of metals makes them good reflectors of light. Silver metal is an excellent reflector of light.
  5. Metals are generally hard :
    Most of the metals are hard. But all the metals are not equally hard. The hardness varies from metal to metal they can not cut with a knife. (except sodium and potassium which are soft metals).
    Ex.       Iron, copper, aluminium.
  6. Metals are usually strong. They have high tensile strength : This means that metals can hold large weights without breaking.
    Ex.       Iron metal (in the form of steel) is very strong having a high tensile strength. Due to this iron metal is used in the construction of bridges, buildings, railway lines, girders, machines, vehicles and chains etc.
  7. Metals are solids at the room temperature :
    All the metals like iron, copper, aluminium, silver and gold, etc., are solids at the room temperature. Only one metal, mercury, is in liquid state at the room temperature.
  8. Metals generally have high melting points and boiling points :  This means that most of the metals melt and vaporise at high temperatures.
    Ex.       Iron is a metal having a high melting point of 1535ºC. This means that solid iron melts and turns into liquid iron on heating to a high temperature of 1535ºC.
  9. Metals have high densities : This means that metals are heavy substances.
    Ex.       The density of iron metal is 7.8 g/cm3 which is quite high.
  10. Metals are sonorous : This means that metals make a ringing sound when we strike them.
    Ex.       Plate type musical instruments like cymbals (manjira), and wires (or strings) for stringed musical instruments such as violin, guitar, sitar and tanpoora, etc.
  11. Metals usually have a silver or grey colour : (except copper and gold). Copper has a reddish-brown colour whereas gold has a yellow colour.
  12. Metallic Bonding
    The bonding which holds the metal atoms firmly together on account of force of attraction between metal ions and the mobile electron is called metallic bonding.
    Explain Classification of Elements 1 X-rays analysis of metal crystal has revealed that each atom in metal crystal is surrounded by 8 or 12 other metal atoms. In metal atoms, the valency electrons are few (1, 2, and 3) and thus, it is not possible for a metal atom to form 8 to 12 covalent bonds with neighbouring atoms.  Thus, it was assumed that the atoms in metal crystal are bonded with each other with a special type of bonding known as metallic bonding. Drude in 1900 proposed the theory of metallic bonding which was later on modified by Lorentz. According to these authors, metals having 1, 2 or 3 electrons in outermost shells, being electropositive lose their electron readily because of low IE values to form free electrons and remainder portion of atom with a Kernel (core of stable nature) carrying positive charge. The free electrons are mobile in nature and move from the one Kernel to another which are closely packed in regular fashion throughout the crystal lattice. thus, the metal crystal is represented by an arrangement of positively charged Kernels in a sea of mobile electrons (Figure) shared by each Kernal to give metallic bonds. Ad the shared electrons are delocalized, the metallic bonds have neither direction nor saturation. There are two essential conditions for metallic bonding :

        1. The metal atoms should have low ionization energy.
        2. There should be sufficient number of vacant orbitals.

    The strength of metallic bonds increases with increase in :
    (i) Number of valence electrons
    (ii) Charge on the nucleus.
    It is therefore, explained that alkali metals are soft and have low melting point, boiling point in comparison to transition metals which are hard and have high m.p., b.p. since, transition metals possess higher number of valence electrons as well as the higher charge on nucleus.
    It is metallic bonding which explains the electrical and thermal conductance, metallic luster, malleability, ductility m.pt., b.pt., hardness in metals.

2. Non–Metals :  
A non-metal is an element that is neither malleable nor ductile, and does not conduct electricity. All the non-metals are solids or gases, except bromine which is a liquid non-metal at room temperature.
Ex.       Some of the examples of non-metals are : Carbon, Sulphur, Phosphorus, Hydrogen, Oxygen, Nitrogen, Chlorine, Bromine, Iodine, Helium, Neon, Argon, Krypton, and Xenon. Diamond and graphite are also non-metals.

Properties of non-metals          
The physical properties of non-metals are just the opposite of the physical properties of metals.

  1. Non-metals are not malleable.
  2. Non-metals are brittle.
  3. Non-metals are not ductile. This means that non-metals cannot be drawn into wires. They are easily snapped on stretching.
  4. Non-metals are bad conductors of heat and electricity.
  5. Non-metals are not lustrous (not shiny). They are dull in appearance.
  6. Non-metals are generally soft
  7. Non-metals are not strong. They have low tensile strength.
  8. Non-metals may be solid, liquid or gases at the room temperature.
  9. Non-metals have comparatively low melting points and boiling points
  10. Non-metals have low densities.
  11. Non-metals are not sonorous.
  12. Non-metals have many different colours.

Comparison Among the Properties of metals and non-metals.

 Metals

Non-Metals

1. Metals are malleable and ductile. That is, metals can be hammered into thin sheets and drawn into thin wires.

1. Non-metals are brittle. They are neither malleable nor ductile.

2. Metals are good conductors of heat and electricity.

2. Non-metals are bad conductors of heat and electricity (except diamond which is a good conductor of heat, and graphite which is a good conductor of electricity).

3. Metals are lustrous (shiny) and can be polished.

3. Non-metals are nonlustrous (dull) and cannot be polished (except iodine which is a lustrous non-metals).

4. Metals are solids at room temperature  (except mercury which is a liquid metal).

4. Non-metals may be solid liquid or gases at the room temperature.

5. Metals are strong and tough. They have high tensile strength.

5. Non-metals are not strong. They have low tensile strength.

6. Metals are sonorous. They make a ringing sound when struck.

6.Non-metals are not sonorous.

3. Metalloids : 
The elements which show some properties of metals and some other properties of non-metals are called metalloids. Their properties are intermediate between the properties of metals and non-metals. Metalloids are also sometimes called semi-metals.
Ex.       Boron (B), Silicon (Si), and Germanium (Ge).

What are the types of Pure substances and Mixtures

What are the types of Pure substances and Mixtures

All the matter around us is not pure. The matter around us is of two types.
In the previous chapter, we have learnt about the three states of matter. Before, understanding the chemical nature of matter, let us first understand the scientific meaning of the term chemical substance.
The scientific meaning of the term chemical substance is different from its everyday meaning. In terms of science, substance is a kind of matter that cannot be separated into other kinds of matter by any physical process. In other words, substance is a pure form of matter and not a mixture of several different kinds of matter. Most of the things that we use in our day-to-day life are in the form of mixtures. Pure substances are rare. For example, the dissolved sugar can be separated from its solution by some physical process (evaporation or distillation). However, sugar is itself a substance and cannot be separated by physical processes into its constituents. Similarly, common salt (sodium chloride), iron, mercury, calcium oxide, hydrochloric acid are substance.
What are the types of Pure substances and Mixtures 1
Pure substances

  • A pure substance consists of a single type of particles.
  • Pure substances are always homogeneous.
  • All the elements and compounds are pure substances because they contain only one kind of particles.
  • A pure substance cannot be separated into other kinds of matter by any physical process.
  • A pure substance has a fixed composition as well as a fixed boiling point and melting point.

Ex.      Hydrogen, Oxygen, Copper, Gold, Silver.

Pure substances can be divided into two types.

  1. Elements  
  2. Compounds

Mixtures
A mixture is a substance which consists of two or more elements or compounds not chemically combined together. All the solutions are mixtures. The various substances present in a mixture are known as “constituents of the mixture” or “components of the mixture”.
Ex.       Lemonade (nimbu pani) is a mixture of water, lemon juice, sugar and salt.
A mixture consists of two or more different type of particles having different chemical nature. Mixture may be homogeneous or heterogeneous. All the mixtures are impure substances. A mixture does not have a fixed composition or a fixed melting point and boiling point.

Types of mixtures
Mixtures are of two types :

  1. Homogeneous mixtures. 
  2. Heterogeneous mixtures.

What is the Need for Classification of Elements?

What is the Need for Classification of Elements?

Need of Classification : It is difficult to study each and every element individually and to know its properties and uses. Therefore, they have been classified into groups on the basis of their similarities in properties.
Basis of Classification : Classification is done on the basis of similarities in properties so that the systematic study could be made about them.

Advantages of classifying elements in the Periodic Table

  • About 116 elements are known today.
  • Most of the elements known today were discovered during the 18th and 19th centuries.
  • Many scientists had contributed their priceless efforts in arranging the elements systematically in a table. This had led to the development of the Periodic Table that we use today.
  • The systematic classification of the elements in the Periodic Table has the following advantages.
    • It enables chemists to analyse and understand the properties of the elements and their compounds more systematically and orderly.
    • It enables chemists to predict the properties of the elements and their compounds based on their positions in the Periodic Table, and vice versa.
    • It becomes easier to study, understand, compare and contrast the related properties among the elements and their compounds from different groups.

Early attempts of Classification

Lavoisier‘s Classification :
Lavoisier classified elements into metals and nonmetals. This classification was based on certain distinctive physical properties such as hardness, malleability and luster. On the basis of these properties, sodium and lead were classed together as belonging to the group of metals.
Limitations
(1) Hardness, malleability and luster were found to be the only common properties of sodium and lead, otherwise the two elements were entirely different.
(2)  In such a classification there was no place for elements with properties resembling those of metals as well as nonmetals.
There fore, Lavoisier’s classification was found to be inadequate.

People also ask

Dobereiner’s Classification :
Law of triads In 1817, German chemist Johann Dobereginer classified elements having similar chemical properties into groups of three. These groups were called triads. He proposed a law known as Dobereiner’s law of triads. According to this law, when elements are arranged in the order of increasing atomic mass in a triad, the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the atomic masses of the other two elements.

Set ISet IISet-III
ElementAtomic massElementAtomic massElementAtomic mass
Calcium40Lithium7Chlorine35.5
Strontium87.5Sodium23Bromine80
Barium137Potassium39Iodine127
Average of the atomic masses of calcium and bariumAverage of the atomic masses of lithium and potassiumAverage of the atomic masses of chlorine and iodine
Atomic mass of strontium = 87.5Atomic mass of sodium = 23Atomic mass of bromine = 80

The classification of elements into triads was very successful in predicting the atomic mass and properties of the middle element. Further, this classification showed that there exists some relationship between the properties of elements and their atomic masses. This paved the way for future attempts at classification of elements.
Limitation : All the elements could not be grouped into triads.

Newlands’ Classification :
Law of octaves In 1864, John Newlands, and English chemist, showed that when elements are arranged in the order of their increasing atomic masses, the eighth element, starting from a given element, was a kind of repetition of the first one, like the eighth note in an octave of music, i.e.,
sa   re   ga   ma pa   dha         ni   sa,
where the first and the eighth note are same.
A part of Newlands’ classification is given below where the figures under the symbols show the atomic masses

Octaves of music and Newlands’ arrangement of elements

Indian :

saregamapadha

ni

Octaves

sa

Western:

doremifasola

ti

do

Newlands’s arrangement of elements with atomic masses

 

 

 

 

H

1.0

Li

7.0

Be

9.0

B

11.0

C

12.0

N

14.0

O

16.0

F

19.0

Na

23.0

Mg

24.0

Al

27.0

Si

28.0

P

31.0

S

32.0

Cl

35.5

K

39.0

Ca

40.0

Cr

52.0

Ti

48.0

Mn

55.0

Fe

56.0

Co and Ni

58.93 and 58.71

Cu

63.54

Zn

65.37

Y

88.90

In

114.82

As

74.92

Se

78.96

Br

79.90

Rb

85.47

Sr

87.62

Ce and La

140.12 and 138.91

Zr

91.22

Starting from lithium (Li) the eight element is sodium (Na). The eight element starting from sodium is potassium. The properties of lithium, sodium and potassium are similar. The properties of beryllium, magnesium and calcium are similar too.
Limitation :
(i)   This  law worked well for lighter elements (up to calcium), but it could not be applied to heavier ones (elements of higher atomic masses) because starting from calcium every eight element was found to have properties different from those of the first element.
(ii)  Newlands emphatically said that only 56 elements do exist in nature and no more element is likely to be discovered in future. But this concept was later on found to be untrue with the discovery of many new elements which defined the law of octaves.
(iii) In arranging elements in the form of a table, Newlands clubbed two elements together at the same place and in the same column. Not only this, he also placed some dissimilar elements in the same column. For example, cobalt (Co) and nickel (Ni) were clubbed together in the column of fluorine (F), chlorine (Cl) and bromine (Br) (under sa/do). We know that cobalt and nickel have properties entirely different from those of fluorine, chlorine and bromine. It is also known that cobalt and nickel have properties similar to those of iron. But iron (Fe) was placed in a column (under ni/ti) different from the column of cobalt and nickel.
However, this law support to the idea that the properties of elements depend upon the atomic masses. It also showed that the properties of elements are repeated after a certain interval, i.e., the properties of elements are periodic in nature.

 

Modern Periodic Table and Its Significance

Modern Periodic Table and Its Significance

Modern Periodic Table :
Henry Moseley, an English physicist found that the atomic number (Z) was the fundamental property of an elements and not the atomic mass for classification of elements.
Modern Periodic Law :
‘‘Properties of elements are periodic functions of their atomic numbers, i.e., the number of protons or electrons present in the neutral atom of an element.’’
Long form of Periodic Table :
Arranged in increasing order of their atomic numbers.
The prediction of properties elements and their compounds can be made with precision. All drawbacks of Mendeleev’s Periodic Table vanish when the elements are arranged on the basis of increasing atomic numbers.
Elements in a Group :
(1)  They show similar chemical properties due to similar outer electronic configuration, i.e., same number of valence electrons.
(2)  They have gradation in properties due to gradually varying attraction of the nucleus and the outer valence electrons as we go down the group.
Main Features of the Long Form of the Periodic Table :
(1)  It shows arrangement of elements based on modern periodic law.
(2)  There are 18 vertical columns known as groups.
(3)  There are 7 horizontal rows known as periods.
(4)  Elements having similar outer electronic configurations, i.e., having same valence electrons have been placed in same groups, e.g.,

Group-1
KLMNOPQ
H(1)1
Li(3)2,1
Na(11)2,8,1
K(19)2,8,8,1
Rb(37)2,8,18,8,1
Cs(55)2,8,18,18,8,1
Fr(87)2,8,18,32,18,8,1

 

Group-2
KLMNOPQ
Be(4)22
Mg(12)2,8,2
Ca(20)2,8,8,2
Sr(38)2,8,18,8,2
Ba(56)2,8,18,18,8,2
Ra(88)2,8,18,32,18,8,2

 

Group-13
KLMNOP
B(5)23
Al(13)2,8,3
Ga(31)2,8,18,3
In(49)2,8,18,18,3
Tl(81)2,8,18,32,18,3
Group-14
KLMNOP
C(6)24
Si(14)2,8,4
Ge(32)2,8,18,4
Sn(50)2,8,18,18,4
Pb(82)2,8,18,32,18,4

 

Group-15
KLMNOP
N(7)25
P(15)2,8,5
As(33)2,8,18,5
Sb(51)2,8,18,18,5
Bi(83)2,8,18,32,18,5
Group-16
KLMNOP
O(8)26
S(16)2,8,6
Se(34)2,8,18,6
Te(52)2,8,18,18,6
Po(84)2,8,18,32,18,6
Group-17
KLMNOP
F(9)27
Cl(17)2,8,7
Br(35)2,8,18,7
I(53)2,8,18,18,7
At(85)2,8,18,32,18,7
Group-18
KLMNOP
He(2)2
Ne(10)2,8
Ar(18)2,8,8
Kr(36)2,8,18,8
Xe(54)2,8,18,18,8
Rn(86)2,8,18,32,18,8

People also ask

(5) In periods, elements in which the number of electrons in the outermost shell increases gradually in step one are placed, e.g.,

Period 1

(K-shell)

H

1

He

2

Second Period

(K, L, shells)

Li(3)

2,  1

Be(4)

2, 2

B(5)

2, 3

C(6)

2, 4

N(7)

2, 5

O(8)

2, 6

F(9)

2, 7

Ne(10)

2, 8

(6)  Each group in the table signifies identical outer shell electronic configuration i.e., same valence electrons, e.g., group 1 has 1 valence electron, group 2 has 2 valence electrons, group 13 has 3, group 14 has 4 valence electrons.
(7)  Each period starts with filling of new shell, e.g.,

1st Period    –    K shell (1st shell) starts filling with Hydrogen and ends at Helium.

2nd Period    –    L shell (2nd shell) starts filling from Li (3) upto Ne (10)

3rd Period    –    M shell (3rd shell) start filling from Na (11) upto Ar (18)

4th Period    –    N shell (4th shell) starts filling from K (19) upto Kr (36) and so on.

(8)  The periodic table is divided in four blocks :
(a)  s-block elements :     Group 1 and 2 elements are called s-block elements.
(b)  p-block elements : Group 13 to 18 elements are called p-block elements
(c)  d-block elements : Group 3 to group 12 are called d-block elements or transition elements (in between s- block and p-block elements)
(d)  f-block elements : The elements placed at the bottom of the periodic table are known as f-block elements. The fourteen elements after La(57) (Lanthanum) are called Lanthanoides and 14 elements after Actinium Ac (89) are called Actinoides.

Modern Periodic Table and Its Significance 1

New Simplified Chemistry Class 7 ICSE Solutions – Elements, Compounds and Mixtures

New Simplified Chemistry Class 7 ICSE Solutions – Elements, Compounds and Mixtures

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

Points to Remember:

  1. Every substance is made up of very tiny particles, called molecules. Molecules are formed from even smaller particles called atoms.
  2. Element—
    (a) Element is the simplest pure substance. It cannot be divided further into simpler substances by any chemical method, e.g. oxygen, hydrogen, sulphur, etc.
    (b) At present 116 elements are known, of which 92 are natural elements.
  3. Based on their properties, elements are classified into : metals, non-metals, metalloids, noble gases.
  4. Metals are ductile, malleable, good conductors of heat and electricity, high melting and boiling points. Metals are sonorous, e.g., Iron, Gold, Silver, etc.
  5. Non-metals are solids and brittle in nature, bad conductor of heat and electricity (exception Graphite) low melting and boiling points, e.g. sulphur, carbon, hydrogen, etc,
  6. Metalloids— These elements show properties of both metals and non-metals. They are hard solids, e.g. Boron, Silicon, Arsenic.
  7. Inert or noble gases— These elements do not react chemically with other elements or compounds are called noble (Inert) gases, e.g., helium, neon, argon, etc.
  8. Symbols of Elements— Each element is denoted by a symbol usually to first letter.
    Examples: Oxygen by O Hydrogen by H.
  9. Atom— “An Atom is the smallest particle of an element that can take part in a chemical reaction but may or may not have independent existence.”The atom of an element exhibits all the properties of that element.
  10. Molecule— A molecule is the smallest particle of a pure  substance of element or compound which has independent existence. It exhibits all the properties of pure substance.
  11. Atomicity— The number of atoms of an element that join together to form a molecule of that element is known as the atomicity.
  12. Molecular Formula— Molecular formula of an element is the symbolic representation of its molecule. It indicates the number of atoms present in it. e.g. Magnesium oxide- MgO.

Exercise

Question 1.
Differentiate between the terms elements, compounds and mixtures with suitable examples.
Answer:
Element — is a pure substance
e.g. Potassium

  1. It is the basic unit of matter  and cannot be broken down into two or more simpler substances by any means.
  2. It is mainly classified into metals and non-metals.

Compound — Is a pure substance
e.g. Potassium chloride

  1. It is formed by combination of two or more elements.
  2. The elements are combined together in a fixed ratio.

Mixture — is an impure substance
e.g. Potassium chloride solution

  1. It is formed by combination of two or more pure substances.
  2. The pure substances are mixed together in any ratio.

Question 2.
State which element exists in the highest percentage in – (a) earth’s crust (b) atmosphere (c) human body.
Answer:
(a) Oxygen (O) — 46.1%
(b) Nitrogen (N) — 78%
(c) Oxygen (O) — 65%

Question 3.
Elements are mainly classified into metals and non-metals. State six properties of metals which differ from non-metals.
Answer:
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 1

Question 4.
Give the symbols of the following elements – and state in each case whether they are metals, non-metals, metalloids or noble gases.
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 17
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 2
Answer:
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 3

New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 4

Question 5.
State what is meant by the term ‘activity series of metals’. State the most reactive and the least reactive metal from the following – Zn, Ag, Na, Fe, Cu, Pb.
Answer:
Activity series of metals:
The positive of metals in the decreasing order of their reactivity is called ‘activity series of metals.’
It is a series of metals arranged according to their reactivity in which the most reactive metal i.e. potassium is at the top of the series and the least reactive metal i.e. gold is at the bottom.

The most reactive metal is Na.
The less reactive metals are Zn, Fe, Pb.
The least reactive metals are Ag, Cu.

Question 6.
Give three reasons why – carbon dioxide is considered a compound, while carbon – an element.
Answer:
Carbon dioxide (C02) has a formula and properties of C02 to extinguish fire etc. are quite different from the properties it constituents carbon and oxygen. Oxygen (O) and carbon (C) are elements as these are made up of one kind of atoms. Also carbon is combustible and oxygen is supporter of combustion

Question 7.
State what information is provided by the formula of calcium hydroxide – Ca(OH)2
Answer:
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 5

Question 8.
Four atoms of hydrogen combine with one atom of carbon to give methane [CH4]. State the valency of carbon.
Answer:
Methane is formed by the combination of four hydrogen atoms and one carbon atom where valency of hydrogen is 1 and valency of carbon is 4.

Question 9.
Write the symbols of the following elements and radicals along with their valencies.
(a) Sodium
(b) magnesium
(c) chlorine Ichioridel
(d) nitrate
(e) suiphite
(f) alurniniuni
(i) zinc
(j) sulphur [sulphide ]
(h) broniine [broiiiide]
(k) sulphate
(l) hydroxide
(m) calcium
(n) caibon.te
(o) potassium
(p) phosphate
(q) iodine[iodide]
Valency list: (i) 1+ (ii) 2, (iii) 3, (iv) 1, (y) 2-(vi) 3
Answer:
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 6

New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 7

Question 10.
Write the chemical formula
(a) magnesium oxide
(b) Sodium bromide
(c) calcium sulphide
(d) magnesium sulphate
(e) aluminium chloride
(f) zinc oxide
(g) calcium carbonate.
Answer:

(a) MgO
(b) NaBr
(c) CaS
(d) Mg SO4
(e) AIC3
(f) ZnO
(g) CaCO3

Question 11.
Give the formulas of the following acids:
(a) hydrochloric
(b) nitric
(c) sulphuric
(d) carbonic — acid.
Answer:

(a) Hydrochloric — HCI
(b) Nitric —HNO3
(c) Sulphuric —H2SO4
(d) Carbonic acid —H2CO3

Question 12.
Give the formulas of the following gases:
(a) hydrogen chloride
(b) ammonia
(c) carbon monoxide
(d) nitric oxide
(e) nitrous oxide
(f) nitrogen dioxide
(g) nitrogen
(h) sulphur dioxide
Answer:

(a) hydrogen chloride — HCI
(b) ammonia — NH3
(c) carbon monoxide— CO
(d) nitric oxide— NO
(e) nitrous oxide — N2O
(f) nitrogen dioxide — NO2
(g) nitrogen – N2
(h) sulphur dioxide— SO2

Question 13.
Explain the term mixture. Differentiate between a homogenous and a heterogeneous mixture with one example of each in the (a) solid-solid (b) solid-liquid If (c) liquid-liquid state of the mixture.
Answer:
Mixtures:
“Are made up of two or more elements or compounds or both mechanically mixed together in any proportion.”
Examples:

  1. Iron and sand mixture.
  2. Sand, iron and common salt mixture.
  3. Ice-cream is mixture of milk, sugar absence, dry-fruits etc.
    Differences between  homogenous and heterogeneous  mixtures.

Homogenous mixture

  1. Constituents – uniformly mixed.
  2. Properties and composition same throughout the mixture.
    Example: Alloys, (salt + water),  air etc.

Heterogeneous mixture

  1. Constituents – not uniformly mixed.
  2. Properties and composition not same but varies throughout the mixture.
    Examples: (oil + water), (iron + sulphur).
    Solid – solid homogenous mixture is brass (Cu + Zn)
  3. Solid – solid heterogenous mixture is (iron + sulphur)

Question 14.
State four differences between – elements, compounds and mixtures with suitable examples.
Answer:

  1. Term —     
    Elements – Pure substance made up of one kind of atoms only. e.g. Iron [Fe], Sulphur [S]
    Compounds – Pure substance made up of two or more elements, e.g. Iron sulphide [FeS]
    Mixture – Impure substance made up of two or more elements or compounds, e.g. Iron sulphur mixture.
  2. Existence—
    Elements –
    Elements i.e. atoms of one kind are present on their own.
    e.g. Iron and sulphur exist on their own as elements iron and sulphur.
    Compounds – Components in a compound present in a definite proportion.
    e.g. Iron and sulphur are chemically combined in a fixed ratio in iron sulphide.
    Mixture – Components in a mixture present in any proportion.
    e.g. Iron and sulphur are mixed in any ratio in the mixture of iron and sulphur.
  3. Properties —
    Elements –
    Elements have a definite set of properties. Elements classified into metal and non-metals each with its own properties.
    Compounds – Compounds have a definite set of properties.
    Elements of a compound do not retain their original properties.
    Mixture – Mixture not have a definite set of properties. Components of a mixture do retain their original properties.
  4. Separation —
    Elements –
    Elements occur on their own or as compounds and can be separated by chemical and physical methods.
    Compounds – Elements in a compound are chemically combined and can be separated by chemical methods only.
    Mixture – Components in a mixture are chemically mixed and can be separated by physical methods only.
  5. Examples —
    Elements –
    Metals – Iron, copper, silver, gold.
    Non-metals – Sulphur, chlorine.
    Compounds – Iron sulphide, copper oxide, water.
    Sodium chloride, copper, sulphate.
    Mixture – Iron + sulphur, copper + silver, air, brass. Sodium chloride + water, copper sulphate + water.

Question 15.
State the correct technique for separation of the following mixtures.
(a)  a sublimable solid and a non-sublimable solid.
(b) a liquid component from soluble impurities in the liquid component.
(c) a lighter liquid from a heavier liquid.
(d) a low boiling point liquid from a high boiling point liquid.
(e) solid constituents in a liquid constituent by adsorption.
Answer:
(a) By sublimation
Common salt + ammonium chloride on heating ammonium chloride sublimes common salt does not sublime and remains in the evaporating dish.
(b) By evaporation: Liquid is made to evaporate leaving behind the solid.
(c) By separating funnel: The heavier lower layer passes into the beaker on opening the stop-cock. Close the stop-cock as lighter layer reaches the tip. Lighter layer will remain in the funnel.
(d) By fractional distillation: Miscible low boiling point liquid will evaporate and is collected in the receiver, leaving behind the miscible high boiling point liquid in the flask.
(e) The method used is chromatography

Question 16.
Give –
(a) The principle involved in separation of the mixture
(b) The technique of separation for each of the following mixtures.
(1) Naphthalene and sodium chloride
(2) Common salt from a solution of common salt in water
(3) Pure water from impure water
(4) Kerosene and water
(5) Methyl alcohol and water
(6)Dyes of an ink
Answer:
(1) By Sublimation

Principle — Based on the differenc in sublimable and non- sublimable nature of solids.
Sublimation: It is the process of conversion of a solid directly into vapour and back to solid without passing through the liquid state.
Sublimable solids thus turn directly into vapour on heating and back to pure solid on cooling the vapours. The non- sublimable solid in the mixture thus remains behind.

Technique of Separation — The mixture of the sublimableand non-sublimable solids are heated in an evaporating dish covered with a funnel plugged at one end with cotton. Sublimable solid on heating sublimes and the vapours condense and collect in the pure form on the inner side of the funnel, from where the sublimable solid is scrapped off. Non-sublimable solid remains behind in the evaporating dish.
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 8

 

(2) By Evaporation

Principle — Based on evaporation of the liquid component in a soluble solid-liquid mixture.
Evaporation – It is a method used for recovery of the soluble solid from a solution by heating the mixture slowly, in a porcelain crucible on a flame.
For smoother steady heating the porcelain crucible maybe kept in a sand bath [a shallow metallic dish filled with sand].

Technique of Separation — The soluble solid can be separated from its liquid component by allowing the liquid component to evaporate either on its own or by heating. The liquid component is lost to the atmosphere The soluble solid component remains behind in the evaporating dish
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 9

(3) By Distillation

Principle — Based on the distillation of the liquid component in a soluble solid-liquid mixture.
Distillation — It is the process of converting a liquid into vapour by heating in a distillation flask & subsequent condensation of the vapour back into the liquid.
The method is used to separate a liquid from a soluble solid or pure liquid [water] from impure liquid [impure water].
Technique of Separation — The soluble solid can be separated from its liquid component or pure water from impure water by placing the mixture i.e. impure water in the distillation flask.
On heating the distillation flask
The solid or solid impurities in water remains behind in the distillation flask
The liquid or water vaporizes, condenses in the Liebig’s condenser and is collected in the receiver.
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 10

(4) By Separating funne

Principle — Based on the separation of two in-miscible liquids, one heavy the other light.
Separating funnel – It is a pair-shaped or cylindrical bulb with a stock-cock fitted at its end. The immiscible liquids are allowed to stand in the separating funnel, till the heavier liquid settles below the lighter liquid.
Technique of Separation — The liquid-liquid mixture is added to the separating funnel and the funnel kept aside for sometime.
The heavier immiscible liquid settles down in the separating funnel and is then removed from below on opening the tap and collected out separately.
The lighter immiscible liquid remains above the heavier liquid, (immiscible liquids do not mix)
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 11

(5) By Fractional Distillation

Principle — Based on the separation of mixture of two miscible liquids, with different boiling points.

Fractional distillation: It is the mixture of two miscible liquids with different boiling points is heated in a distillation flask when the two liquids separate out due to their difference in boiling points.

Technique of Separation – The mixture of two miscible liquids e.g. alcohol and water having different boiling points is taken in the distillation flask and heated slowly.
The mixture boils are the respective boiling points of the components and the component with the higher boiling point remains behind in the distillation flask condensation.The component with the lower boiling point collects in the receiver, after condensation in the Leibigs condenser.

The fractionating column contains several traps to allow the vapours of the higher boiling point components to condense, there by only the lower boiling point component enters the condenser and is collected in the receiver.
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 12

(6) By Chromatography

Principle — Chromatography is-a method by which the various components of a mixture e.g. dyes in an ink, are separated due to their difference in rate of flow over an absorbent medium i.e. stationary phase e.g. filter paper under the influence of a solvent i.e. the mobile phase.
Chromatography — There are two phases and they are:
Stationary phase in chromatography, the adsorbent medium is known as the stationary phase e.g. filter paper made of cellulose.
Mobile phase the solvent or the solution that moves due to capillary action is known as the mobile phase.
Technique of Separation — Separation of dyes in ink by paper chromatography Ink prepared by dissolving different dyes e.g. A, B, C, D in a suitable solvent, is taken.A line is drawn with a pencil over a long strip of a filter paper.
Place the ink spot containing the different dyes on the filter paper as shown below.
The filter paper is then hung with its lower end dipped in a suitable solvent.
The solvent maybe a mixture of ethyl alcohol and water poured into a tall jar, such that only a small lower part of the jar is filled.
The paper strip with the spot of ink at its, is dipped into the solvent in a way that the spot itself remains above the liquid surface.
The solvent flows over the ink spot and the various components of the mixture, of dyes in the ink are separated as shown below.
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 13

Objective Type Questions
Elements, Compounds & Mixtures

1. Select the correct answer from A, B, C, D and E for each statement given below:
A: Chalk    B: Oxygen   C: Silicon      D: Bromine   E: Nitrous oxide

An element present in the earth’s crust, atmosphere and human body.
Ans. B: Oxygen

The chemical name for dinitrogen oxide [N20].
Ans. E: Nitrous oxide

A compound containing carbon, oxygen and calcium.
Ans. A: Chalk

A metalloid.
Ans. C: Silicon

A non-metal which exists in the liquid state.
Ans. D: Bromine

2. Select the correct answer from the choice in bracket.

The least reactive metal, [magnesium/silver/copper]
Ans. silver.

The positively charged particle of an atom, [electron/ proton/neturon]
Ans. proton

The formula of caustic soda. [KOH/Ca(OH)./NaOH]
Ans. NaOH

The ideal method to separate iodine and KCI. [sublimation/ evaporation/distillation]
Ans. sublimation

A homogenous mixture, [brass/dust in air/chalk and water]
Ans. brass

3. Match the ideal method of separation of components in a mixture in List I with the – appropriate process in List II.
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 14
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 15
Answer:
New Simplified Chemistry Class 7 ICSE Solutions - Elements, Compounds and Mixtures 16

4. Give reasons for the following statements :

Question 1.
If fractional distillation is carried out using a liquid-liquid mixture, one liquid will remain in the flask and the other  will be collected in the receiver.
Answer:
The vapours of the higher boiling point components to condense, thereby only the lower boiling point component enters the condenser and is collected in the receiver.

Question 2.
Evaporation of a common salt solution or sea water, leaves behind common salt inside the evaporating dish after heating.
Answer:
The liquid component is lost to the atmosphere on heating the mixture in an evaporating dish.

Question 3.
Components in a mixture are present in varying proportions and not in a fixed proportion.
Answer:
A mixture is an impure substance containing more than one element or compound, mechanically mixed together in any proportion and the mixture retains the properties of its constituents i.e. elements or compounds.

Question 4.
Gunpowder is an example of a heterogenous mixture.
Answer:
Because Gun powder is produced by mixing two or more substances in any proportion by weight.

Question 5.
In chromatography the absorbent medium e.g. What man filter paper is known as the – stationary phase.
Answer:
Chromatography is a method by which the various components of a mixture e.g. dyes in an ink. What man filter paper is known as the stationary phase because it act as a substance that stays fixed inside the column.

5. Name the following: 

The non-sublimable solid from a mixture of iodine and potassium nitrate.
Ans. Potassium nitrate.

The heavier liquid component from – mercury and water.
Ans. Mercury.

The lower boiling point component from methyl alcohol and water.
Ans. Methyl alcohol.

The compound containing one atom of sulphur and two atoms of oxygen.
Ans. Sulphur dioxide.

An acid whose formula is ‘H2CO3’.
Ans. Carbonic acid