The order of increasing densities of the given substances is:

Air < Exhaust from chimneys < Cotton < Water < Honey < Chalk < Iron

The forces of attraction between the particles of gases are the weakest. It is very strong in case of solids and intermediate in case of liquids. Gases have a lot of empty spaces between them which results in decrease in density of gases as compared to liquids and solids. Solids have the higher density as compared to liquids and gases. Due to strong forces of attraction particles are packed together with least number of empty spaces which results in increase in the ratio of mass per unit volume.

(i) Air is mainly a mixture of comparatively light gases like nitrogen and oxygen etc. so it has the lowest density.

(ii) Exhaust gases from chimneys, in addition to air, consists of a mixture of heavier gases like carbon dioxide, sulphur dioxide and nitrogen dioxide, etc. They have slightly higher density than air.

(iii) Cotton is a fluffy solid with a lot of pores. These pores have air trapped in them. Due to a large amount of trapped air, solid cotton has higher density than exhaust gases from chimneys but lower density as compared to water.

(iv) Water is a liquid in which the particles are closely packed as compared to air. This results an increase in mass per unit volume. So the density of water is higher than that of cotton and air.

(v) Honey is a thick liquid. Particles of honey are heavier than water and are closely packed. Hence, the density of honey is higher than that of water.

(vi) Chalk is a porous solid in which the particles are comparatively less closely packed as compared to iron. But it has higher density than honey which is a liquid but less density as compared to iron.

(vii) Iron is a highly compact solid. The particles are very closely packed due to strong forces of attraction. As a result, it has higher density than chalk.