Question

Which of the following are “pure substances”?
ice, Milk, Iron, Hydrochloric acid, Calcium oxide, Mercury, Brick, Wood, Air.

Answer 1

Hint: In chemistry the substance which is composed of only one type of component is known as pure substance. Pure substances and mixtures are two contrary terms in chemistry. Elements and compounds are considered as a type of pure substance.

Complete Step By Step Answer:
As we know that solid form of liquid is ice so it is made up of only one type of constituent. Therefore, it is an example of pure substance.
Milk is composed of liquid water and oil droplets which together form a type of colloidal system therefore it does not contain only one type of constituent. Therefore, it is not an example of pure substance.
Iron is a pure element which is composed of only atoms of iron and nothing else. Therefore, it is an example of pure substance.
Hydrochloric acid contains only the liquid form of hydrochloric acid and no other constituent is present in it. Therefore, it is an example of pure substance.
Calcium oxide is made up of constituents which have a definite composition and no substance other than calcium hydroxide is present. Therefore, it is an example of pure substance.
Mercury is an element which contains only atoms of mercury and exits as liquid at room temperature. It does not contain any other component. Therefore, it is an example of pure substance.
Brick is made by mixing mud along with water and further heating it to a higher temperature. Since it is composed of more than one constituent it is not an example of pure substance.
Wood is natural product obtained from a plant which is made up of fibers, water, lignin and other organic components like hydrogen, carbon etc. since, it is made up of more than one constituent it is not an example of pure substance.
Air is composed of water droplets, dust particles, smoke, smog and many other particulates. Since it is made up of more than one constituent it is not an example of pure substance.
Form the above discussion we find that substance which are pure substances are-
$\Rightarrow$ ice, Iron, Hydrochloric acid, Calcium oxide, Mercury.

Note:
The process of breaking down the pure substance into its constituents is a tedious work and requires special methods like melting, boiling or chromatography etc. Any substance which is composed of more than one type of constituents is known as impure or mixed substance. Constituents particles of impure substances are combined physically instead of chemical combination.