Chemical Change and Chemical Bond Science Notes

Chemical Change and Chemical Bond Science Notes

Important Points:

Chemical change and word equation

→ In a chemical change the chemical composition of the original matter changes and new substances having different properties and different chemical composition are formed.
→ A chemical equation can be written for a chemical change, if the exact change in chemical composition is known.

→ The word equation can be written for a chemical reaction, baking soda with lemon juice as follows:

Citric acid + Sodium bicarbonate → Carbon dioxide + Sodium citrate
Acid + Alkali → CO2 + Salt

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→ Always remember

→ First step of writing a chemical equation is to write a word equation by using the names of the concerned substances. When the chemical formula is written in place of each of the names, it becomes a chemical equation.
→ While writing a chemical equation, original substances are written on the left side and newly formed substances are written on right side and an arrow is drawn in between Arrow head points towards the substances formed.
→ Arrow indicates the direction of the reaction. Substances written on the left side of the arrow are original substances that take part in the reaction. They are called reactants. New substances formed as a result of the reaction are called products. Place for the products of a reaction is on the right side of the arrow.

Natural chemical changes:

→ Respiration:

→ Respiration is a continuously occurring biological process. In this process air is inhaled, oxygen present in this inhaled air reacts with glucose present in the cells of the body forming carbon dioxide and water.

→ The word equation and the chemical equation of this chemical reaction are as follows: (Here, the chemical equation is not balanced.)

→ Word equation

Chemical Change and Chemical Bond Science Notes 1

→ Chemical equation

Chemical Change and Chemical Bond Science Notes 2

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Green plants perform photosynthesis in sunlight. A word equation and a chemical equation (unbalanced) is written for this natural chemical change as follows:

→ Word equation

Chemical Change and Chemical Bond Science Notes 3

→ Chemical equation

Chemical Change and Chemical Bond Science Notes 4

Man-made chemical changes

→ Combustion of fuels

Wood, coal, petrol or cooking gas are burnt for producing energy. Carbon is the common substance that burns in all these fuels. The product carbon dioxide is formed when carbon combines with oxygen in the air during the combustion process. A common equation for all these combustion processes is as follows:

→  Word equation

Carbon + Oxygen → Carbon dioxide

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→  Chemical equation

C + 02 → CO2

→  Combustion of fuel is a fast and irreversible chemical change.

→ Cleaning Shahabad tile with dilute hydrochloric acid: The main chemical constituent of Shahabad tile is calcium carbonate. During its cleaning with hydrochloric acid the upper layer of the tile reacts with hydrochloric acid and three products are formed.

→ One of them is calcium chloride, which being soluble in water, gets washed away with water. The second product is carbon dioxide; its bubbles mix up in air. The third product, water mixes with water. The following equation is a chemical change.

→ Word equation:

Calcium carbonate + Hydrochloric acid → Calcium chloride + Carbon dioxide + Water

Softening of hard water

Water in some wells or tube wells is hard water. It is brackish to taste and does not form lather with soap. This is because hard water contains the chloride and sulphate salts of calcium and magnesium in dissolved state. Hard water is softened by adding solution of washing soda to it.

This results in the formation of a precipitate of insoluble salts of calcium carbonate and magnesium carbonate. As the dissolved salts of calcium and magnesium go out in the form of precipitate of the carbonate salts, the water is softened. The following equation is written for this chemical change.

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→ Word equation:

Calcium chloride + Sodium carbonate → Calcium carbonate + Sodium chloride

→ Chemical equation (unbalanced):

CaCl2 + Na2CO3 → CaCO3 + NaCl

Chemical bond

Noble gases do not form any chemical bond as their electron octet/duplet is complete while the atoms with incomplete electron octet/duplet form chemical bonds. During the formation of a chemical bond an atom uses its valence electrons. Moreover on forming chemical bonds equal to its valency the atom attains the electronic configuration of complete octet/duplet.

Ionic bond

→ The chemical bond formed due to an electrostatic force of attraction between the oppositely charged cation and anion is called an ionic bond or an electrovalent bond. The compound formed by means of one or more ionic bonds is called ionic compound.

→ One ionic bond is formed due to the electrical charge +1 or -1 on an ion. The valency of an ion is equal to the magnitude of positive or negative charge on it. An ion forms the same number of ionic bonds as its valency. Examples: Sodium chloride (NaCl), Magnesium chloride are ionic compounds.

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Covalent bond

→ The chemical bond formed by sharing of valence electrons of two atoms with each other is called a covalent bond. One covalent bond is formed by sharing of two valence electrons. Examples: Hydrogen, oxygen, water are covalent compounds.

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