Q1 Explain the changes occurring in …

1. When a candle burns, both physical & chemical changes occur. On heating, candle wax melts and form liquid wax. It is a physical change. Since it again turns into solid wax on cooling. This is a reversible change. When a candle is burnt in air, the mass of the product is increased due to atmospheric oxygen which show a that it is a chemical change.

3. 

Rusting of Iron

When an iron object is left in damp air (or water) for a considerable time, it gets covered with a red-brown flaky substance called rust. This is called rusting of iron. During the rusting of iron, iron metal combines with the oxygen (of air) in the presence of water (moisture) to form a compound iron oxide. This iron oxide is rust.

Iron + Oxygen +Water ——> Iron Oxide

Fe + O₂ + H₂O ——> Fe₂O₃

Rust is iron oxide (Fe₂O₃). Thus, rust and iron are not the same substance. Rusting of iron is a chemical change.

Methods of preventing iron from rusting

(1) Rusting of iron can be prevented by galvanisation

The process of depositing a thin layer (or coating) of zinc metal on iron objects is called galvanisation.

Galvanisation is done by dipping an iron object in molten zinc metal. A thin layer of zinc metal formed on the surface of an iron object protects it from rusting (because zinc metal remains unaffected by air and moisture). The iron sheets used for making buckets, drums, dust-bins and sheds (roofs) are galvanised to prevent their rusting. The iron pipes used in our homes to carry water are also galvanised to prevent rusting.

(2) Iron is coated with chromium to prevent rusting

This is called chrome-plating. Chromium metal is resistant to the action of air and moisture. So, when a layer of chromium is deposited on an iron object, then the iron object is protected from rusting.

Chromium-plating is done on steel furniture, taps, bicycle handle bars and car bumpers, etc, made of iron and steel to prevent them from rusting.