Question

what is a chemical reaction? what changes occure in a chemical reaction? give 5-6 examples of a chemical reaction.​

Answer 1

Answer:

Chemical reaction

A chemical reaction a is change in which one or more substance(s) or reactant(s) react(s) to form new substance(s) with entirely different properties.

The reacting species (molecule, atom, ion) are known as reactants (the substance that undergo chemical change reaction) and the new species formed as a result of the reaction are called products (the new substance formed during reaction).

Example

2 Na (s)  +  2 H₂O (l) → 2 NaOH (aq) + H₂ (s) ↑

Sodium      Water           Sodium        Hydrogen

                                      hydroxide

More examples -

3 Fe (s) + 4 H₂O (l )    →   Fe₃O₄ (s) + 4 H₂ (g)

2 Pb (NO₃) (s) → 2 PbO (s) + 4 NO₂ (g) + O₂ (g)

Na₂SO₄ (s) + BaCl₂ (s) → BaSO₄ (s) + 2 NaCl (s)

Answer 2

(iv) Absorption or evolution of heat

For example :

• When water is added to quicklime, heat energy is evolved.

A change which alters the specific properties of a material by bringing about a change, is called a chemical change. On the other hand we can say that when two or more substances react in such a way that there is formation of one or more new substances, the change is called a chemical change or chemical reaction.

CHARACTERISTICS OF A CHEMICAL CHANGE :-

(i) Evolution of gas

For example :

• When Zinc granules are added to dilute sulphuric acid, hydrogen gas is evolved. If a burning matchstick is brought near the mouth of a test tube, it burns with a pop sound.

Sulphuric acid + Zinc --> Zinc Sulphate + Hydrogen

(ii) Change of colour

For example :

• When solid lead nitrate is heated, reddish-brown nitrogen oxide is evolved. Also, a yellow coloured lead monoxide is formed.

Lead nitrate --> (heat supply) Nitrogen dioxide (reddish-brown) + lead oxide (yellow) + oxygen

(iii) Formation of Precipitate

For example :

• When hydrogen sulphide gas is passed through blue coloured copper Sulphate solution, black Precipitate of copper sulphide is formed.

Copper Sulphate + hydrogen sulphide --> Copper sulphide + sulphuric acidQuicklime + Water --> Calcium hydroxide + heat

(v) Sound may be produced

For example :

When baking soda is added to vinegar, carbon dioxide gas is produced with a hissing sound.

Vinegar + Baking Soda --> Carbon dioxide + Hissing sound