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.

Explanation

To determine which of the given materials fall into the category of pure substances, we need to understand what a pure substance is. A pure substance is defined as a material that has a uniform and definite composition. This means that if you take samples from different parts of the substance, they will have the same composition without any impurities.

Now, let's analyze each option:

1. **Ice (H2O)**: Ice is made entirely of water molecules (H2O) and does not contain any impurities. Therefore, ice is a pure substance.

2. **Milk**: Milk is an emulsion that contains fats, proteins, and water, which means it has varying compositions and impurities. Thus, milk is not a pure substance.

...