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 materials fall under the category of a "pure substance," we need to understand what a pure substance is. A pure substance consists of only one type of particle, while a non-pure substance contains two or more types of particles.

### Step-by-Step Solution:

1. **Identify the Definition of a Pure Substance**:
- A pure substance is defined as a material that has a consistent composition and properties throughout. It contains only one type of particle.

2. **Evaluate Each Material**:
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