SUBSTANCES
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Matter is made up of one or more components known as substances. In terms of science, a substance is a kind of matter which cannot be separated into any other types of matter by physical means. Such a substance which has only one component and nothing else in it is called pure substance. Substances mostly mix with one another and their combination is known as mixture.
Pure Substances:
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A pure substance means a single substance (or matter) which cannot be separated into other kinds of matter by any physical process.
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A pure substance is made up of only one kind of atoms.
For example:
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Metals like gold, silver, copper, Iron, tin in completely pure form are pure substances.
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Ghee or Oil is not a pure substance because it is a mixture of many substances. Air is also not a pure substance as number of gases like oxygen, carbon dioxide, water vapours etc. are present in it.
Characteristics of Pure Substances:
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A pure substance is homogeneous throughout its mass.
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A pure substance cannot be separated into other kinds of matter by any physical process.
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A pure substance has a fixed composition as well as a fixed melting point and boiling point.
Impure Substances: Mixture:
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A mixture is the combination of two or more substances (elements or compounds) which do not combine chemically with each other and may be present in any proportion.
For example:
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Salt solution is a mixture of two pure substances: salt and water, sugar solution is also a mixture, air is a mixture of different gases; milk is also a mixture etc.
Characteristics of Impure Substances:
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A mixture may be homogeneous or heterogeneous.
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A mixture can be separated into other kinds of matter by physical processes.
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A mixture does not have a fixed composition or a fixed melting point and boiling point.
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Most of the matter around us exists as mixture of two or more pure substances.
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Pure substances are classified into two types –
Elements
Compounds