A pure substance is a material composed of only one type of particle, so the pure substances in this list are: a) ice, c) iron, d) hydrochloric acid, e) calcium oxide, and f) mercury.
b) milk, g) brick, h) wood, and i) air are not pure substances because they are mixtures of multiple components.
Explanation:
Ice: While water can exist in different states (solid, liquid, gas), it is still composed of H2O molecules, making it a pure substance.
Iron: An element consisting only of iron atoms.
Hydrochloric acid: A compound consisting of hydrogen and chlorine atoms.
Calcium oxide: A compound made up of calcium and oxygen atoms.
Mercury: A liquid metal composed of mercury atoms.
Why the others are not pure substances:
Milk: A mixture of various fats, proteins, carbohydrates, water, and other substances.
Brick: Made up of clay, sand, and other materials.
Wood: Composed of cellulose, lignin, and other organic compounds.
Air: A mixture of different gases like nitrogen, oxygen, carbon dioxide, and more.