Which of the following materials fall in the category of a 'pure substance'?
$(a)$ Ice $(b)$ Milk $(c)$ Iron $(d)$ Hydrochloric acid $(e)$ Calcium oxide
$(f)$ Mercury $(g)$ Brick $(h)$ Wood $(i)$ Air.

When is a compound said to be pure?

$(i)$ Name the compound formed on heating a mixture of iron filings and sulphur.
$(ii)$ If dilute $HCl$ is added to the above compound,name the gas evolved and write down its two properties.

Classify each of the following as a physical or a chemical change. Give reasons.
$(a)$ Drying of a shirt in the sun.
$(b)$ Rising of hot air over a radiator.
$(c)$ Burning of kerosene in a lantern.
$(d)$ Change in the colour of black tea on adding lemon juice to it.
$(e)$ Churning of milk cream to get butter.

When a bar magnet is brought closer to a mixture of iron filings and sulphur powder,iron particles stick to the magnet,whereas in the case of iron sulphide,no change is observed. When a mixture of iron and sulphur is added to carbon disulphide,sulphur dissolves,whereas in the case of iron sulphide,no change is observed. Justify the statement that the properties of a compound are different from its constituents,while a mixture shows the properties of its constituent elements using the example of iron and sulphur.

Which of the following is an element?