When an iron blade of a knife is dipped in a copper sulphate solution, then iron blade is coated with reddish brown deposits of copper.
And the blue colour of copper sulphate solution changes to light green due to the formation of iron sulphate. So, it is a chemical change.

Harmful Changes are
(i) Rusting of iron.
(ii) Decaying of fruits.
Rusting of iron is harmful because it slowly destroys iron articles and makes them useless. Since, iron is used in making large number of objects or articles such as bridges, grills, railings, gates and bodies of cars, buses, trucks and ships, etc. Rusting of iron causes a great loss over a period of time.
Prevention Rusting can be prevented by oiling, greasing or painting. It can also be prevented by galvanisation.
Decaying of fruits causes health hazards. Due to decaying of fruits, there is a lot of monetary loss in food industry.
Prevention Fruits can be preserved by keeping them at low temperature and by using some specific preservatives.

In a chemical change, the products are quite different from the reactants. Therefore, a chemical change cannot be normally reversed.

When we burn a cracker, it explode. Explosion produces heat, light, sound and unpleasant gases that pollute the atmosphere. Many new products are formed. So, it is a chemical change.

(i) Burning of a candle Chemical change as well as physical change and energy evolved.
(ii) Lightning of a bulb Physical change, energy evolved.
(iii) Preparation of food by green plants Chemical change, energy absorbed.
(iv) Volcanic eruption Chemical change, energy evolved.
(v) Evaporation of petrol Physical change as no new chemical substance is formed, energy absorbed.
(vi) Burning of LPG Chemical change because LPG on burning form CO₂ and H₂O, energy absorbed.

In the process of digestion, the various food materials break down to form new substances which can be absorbed by the body. So, the process of digestion is a chemical change.

We know that for rusting, the presence of both oxygen and water (water vapours) is essential.
Thus, in coastal areas, the air contains high moisture which means more humid environment and rusting becomes faster.
Whereas in deserts, moisture in air is less, hence rusting of iron is very slow there.

Crystals of copper sulphate are prepared in the following manner:
(i) Take a cup full of water in a beaker.
(ii) Add a few drops of dilute sulphuric acid.
[Caution Never add water to the acid as the acid may splash on you].
(iii) Heat the water.
(iv) When it starts boiling, add copper sulphate powder slowly with constant stirring.
(v) Continue adding copper sulphate powder till no more powder can be dissolved.
(vi) Filter the solution.
(vii) Allow it to cool.
(viii) Do not disturb the solution, when it is cooling.
(ix) Look at the solution after some time and wait till it changes into crystals.

The setting of curd is a chemical change because we cannot get the original substance (milk) back. The new substance, i.e. curd is different from the milk in taste, smell and chemical properties.

When baking soda (sodium bicarbonate) is mixed with lemon juice (citric acid), a chemical change occurs. In this reaction, new substances like carbon dioxide is formed and heat is evolved. This change is irreversible. It is a chemical change.
Sodium bicarbonate (NaHCO₃)+ Citric acid \(\longrightarrow\) Sodium citrate + Carbon dioxide\(\uparrow\) + Water + Heat

(a) Seawater is collected in shallow pits. It is then allowed to evaporate in the sun. As the water evaporates, the salt solution becomes supersaturated (concentrated). This supersaturated solution cannot hold the excess salt. Thus, it separates out in the form of salt crystals. These salt crystals are collected and are redissolved in water and filtered to remove insoluble impurities. The clear solution is again evaporated to obtain the crystals of pure salt.
(b) Salt is obtained from sea water by the process of evaporation.
(c) The values shown by Karan are curious, aware and intelligence

(a) Chemical reaction between copper sulphate solution and iron metal. In this reaction, blue colour of copper sulphate solution changes to light green colour due to the formation of iron sulphate.
Copper sulphate + Iron \(\longrightarrow\) Iron sulphate + Copper
(Blue)                    (Grey)       (Light green)     (Brown)
(b) When baking soda and vinegar are mixed together then a chemical change takes place and bubbles of carbon dioxide gas are formed along with some other substances.
Baking soda              +                Vinegar          \(\longrightarrow\)     Sodium acetate + Carbon dioxide + Water
(Sodium hydrogen carbonate)   (Acetic acid)
(c) Explosion of a firework produces heat, light, sound and unpleasant gases. Explosion of a firework is a chemical change.