Rusting of iron is due to the exposure of iron to the atmosphere for some time. To prevent rusting of iron gate, we paint it. Painting prevents the iron gate to come in direct contact of atmosphere, water or both. Thus, a coat of paint cuts the contact between iron and atmosphere and it prevents rusting of iron.
Also we know that
Iron (Fe)+ Oxygen (O₂)+ Water (H₂O) \(\longrightarrow\) Rust (iron oxide, Fe₂O₃)
Thus, it is necessary to cut the oxygen contact with iron to prevent rusting.

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.

Burning of wood is a chemical change while cutting of wood is a physical change because, during burning, new substances are formed. After burning, we cannot get original substance (i.e. wood) back. Cutting of wood into small pieces is a physical change because no new substance is formed.

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.

Magnesium oxide is basic in nature because it turns red litmus solution to blue.

Ca (OH₂)           +          CO₂              \(\longrightarrow\)        CaCO₃         +       H₂O
Calcium hydroxide   Carbon dioxide             Calcium carbonate    Water

Souring of milk is a chemical change because original substances present in milk lose their nature and identity and form new chemical substances.

Ripening of a fruit is a chemical change because after ripening, a new product with different properties is formed.

(a) Yes, rust is iron oxide (Fe₂O₃). Thus, rust and iron are not the same substance.
(b) No, rusting of iron is a chemical change because in this reaction, a new substance, rust (iron oxide) is formed. It cannot be reversed by any method.
(c) Yes, rusting of iron is a chemical change. During the rusting of iron, it combines with the oxygen in the presence of water (moisture) to form a new compound 'iron oxide'. This iron oxide is a rust.
Iron + Oxygen + Water \(\longrightarrow\) Iron oxide
(From damp air)                    (Rust)
It is a permanent change which cannot be reversed back. So, rusting of iron is a chemical change.
(d) Two other examples are
(i) Setting of curd from milk.
(ii) Burning of magnesium ribbon to form magnesium oxide.

(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