7. 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.
Answer: Iron, Hydrochloric acid, calcium oxide, mercury are pure substances.
Ice: Ice made of pure water freezes to form pure ice. while if we use tap water or mineral water, ice also contains traces of minerals and salts.
Milk: Milk is a suspension of various particles in water.
Hydrochloric acid: Hydrochloric acid in its concentrated form is pure. But if the question mentions the hydrochloric acid solution or dilute HCl then it is a mixture of water and HCl.
Calcium oxide: Calcium oxide is also called quick lime. We mix it water to form slaked lime. We use Slaked lime in homes to whitewash the buildings.
Mercury: Mercury is the only liquid metal on earth. (at 25°C). We use it in pure form in a mercury thermometer.
Brick: The soil on baking changes into a brick. So it consists of various minerals, salts and other organic compounds which are present in the soil.
Wood: Wood is a plant product. It consists of various minerals, salt, and organic compounds.
Air: Air is a mixture of oxygen(21%), Nitrogen (78%), Carbon dioxide (0.003%), water vapour and other gases.