Question

17.1. State the differences between chemical and physical changes.
When we keep a piece of iron in the open area for few days, a brownish, flakey substance, called
rust, is deposited on it.
a. Is rust different from iron?
b. Can you change rust back into iron by some simple method?
Give two other similar type of changes.​

Answer 1

a) Rust is a brown dust of 'iron oxide'. So, rust is a totally different form of iron.

b) No, rust is an irreversible chemical reaction.

c) While rusting (iron) it mixes with oxygen in the existence of moisture. This produces new compound named 'iron oxide'. The iron oxide is also known as rust. Hence rusting is a chemical change.

d) The two examples are:

i) Burning of 'magnesium ribbon' to produce 'magnesium oxide'.

ii) Formation of curd from the milk.