class -IX sample paper of FA 1
Chemically pure substance is a kind of matter that cannot be separated into other kinds of matter by any physical process. It has the same colour, taste, texture and composition at a given temperature and pressure.
Meaning of pure substance different for a common man than to a chemist
For a common man, pure substance means that it is not adulterated.
For example, for a common man, pure milk means the liquid material that is given by cow or buffalo and it is not mixed with water or other materials to thicken it.
But for a chemist, any milk is not a pure substance. Milk is a mixture of water, fats and proteins.
Q. Which of the following materials could be a pure substance? Butter, ghee, water, ink, glass, paper, sugar, banana, orange, marble, blood, wood.
Purity of a given substance
A pure substance always has the same colour, taste or texture at a given temperature and pressure. Also it has a fixed melting point or boiling point at constant pressure. For example, pure water boils at 373K at 1 atmospheric pressure. But water containing some invisible dissolved substance boils at a temperature above 373K.
If two or more substances (elements, compounds or both) mixed together in any proportion, do not undergo any chemical change, but retain their characteristics, the resulting mass is called mixture.
Kinds of Mixtures
Heterogeneous mixture: A mixture in which various constituents are not mixed uniformly is called
heterogeneous mixture. Examples: A mixture of sand, salt and sulphur is a heterogeneous mixture.
Similarly, a handful of soil is a heterogeneous mixture.
Homogeneous mixture: A mixture in which different constituents are mixed uniformly is called
homogeneous mixture. Examples: Brass is an alloy of copper and zinc and is a homogeneous
mixture. Similarly, all solutions are homogeneous mixtures.
Characteristics of a Mixture
- Variable composition: The constituents of a mixture are present in any ratio. Example: A mixture of sand and salt can be in a ratio of 1:2 or 5:6, etc, by weight.
- Physical change: The mixture is a result of physical change. The constituents of a
mixture do not bind each other by chemical bonds. Example: In air the main constituents, i.e., oxygen, nitrogen and carbon dioxide, do not bind each other with chemical bonds.
- No specific properties: The properties of a mixture are the average of the properties of its constituents.
Example: The properties of air are midway between properties of nitrogen and oxygen.
- Homogeneity: Most of the mixtures are heterogeneous, i.e., their constituents are not
spread evenly throughout. However, some mixtures are homogeneous. Example: In the mixture of iron and sulphur, at some places iron is more and at some places sulphur is more.
- Separation: Generally, the constituents of mixture can be separated by employing
suitable physical means. Example: Iron can be separated from the mixture of iron and sulphur with the help of a magnet.
- Energy changes: No energy is released or absorbed during the formation of a mixture. Example: On mixing iron and sulphur, heat energy is neither absorbed nor evolved.