Pure Substance in Chemistry – Definition and Examples

The definition of a pure substance in chemistry is not the same as that of what we use commonly. Most of the substances, in our daily life, which we assume as pure are not pure substances as per the chemistry. Generally, we consider that substance as a pure substance, which is free of impurities or unwanted materials. For example – milk, honey, tap water, fruit juice, medicine, etc. Actually, these are mixtures made up of elements and compounds. To learn more about mixtures, click here.

DEFINITION OF PURE SUBSTANCE IN CHEMISTRY

The basic definition of pure substance in chemistry is as follows:

Pure substances are either elements or compounds. They contain the same kind of atoms or molecules and have a definite set of physical and chemical properties.

Another definition of a pure substance is:

A pure substance is a homogenous material which contains atoms of one kind and has a definite set of physical and chemical properties. However, if a substance consists of two or more differenct kinds of atoms, combined together, then the proportion of these combining atoms by weight must remain constant.

PROPERTIES OF PURE SUBSTANCES

A pure substance exhibits the following properties:

• A pure substance has a fixed melting and boiling point.
• It has its characteristic taste, color, and odor.
• It is always homogeneous.
• We cannot break a pure substance into simpler substances by any physical means.

PURE SUBSTANCE EXAMPLES

As per the above definitions, all elements and compounds are pure substances. Some examples of pure substances are –

• Pure gold, pure silver, pure copper, pure iron, etc. are elements with the same kind of atoms.
• Pure sugar, pure water, etc. are compounds with the same kind of molecules. They contain different kinds of atoms, but the proportion of these combining atoms by weight is always constant.

Pure Substance in chemistry Definition and examples

IMPURE SUBSTANCES WITH EXAMPLES

Impure substances are those in which some other substances are also present in smaller amounts. These are commonly called mixtures. Most of the substances which we commonly call pure are not considered pure substances in chemistry. For examples –

• A clear-looking tap water is not a pure substance, because it contains a small number of dissolved minerals and gas in it. It is these dissolved minerals provide taste to water, otherwise, water is tasteless.
• Fruit juice is an impure substance, as it contains sugar, minerals, salts, and a number of other organic compounds.
• Milk is not a pure substance, as it contains a number of different compounds such as – fats, carbohydrates, protein, salts, vitamins, and water. Further, the proportions of these compounds vary from one cow to that of another.
• Honey is an impure substance, as it contains a number of molecules of other substances in addition to sugar.
• Brass an alloy made by combining copper and zinc is not a pure substance in chemistry. It contains atoms of two different elements not chemically combined with each other.

FREQUENTLY ASKED QUESTIONS

Question 1. Is air a pure substance?

Ans: Air is not a pure substance. It is a mixture of 21% oxygen, 78% nitrogen, and 0.3% carbon dioxide. The remaining 0.7% comprises an inert gas like argon and traces of other gases. This composition of the air changes from place to place due to climatic conditions and levels of pollutants present in that region. It also varies with height above the earth’s surface. At higher altitudes percentage of oxygen decreases, therefore mountaineers carry an oxygen cylinder with them. Air also contains dust particles whose composition in air changes from place to place.

Question 2. Is water a pure substance?

Ans: Water in its pure form or distilled water is a pure substance. Pure water is a compound made of hydrogen and oxygen with both combined in the ratio of 2:1, respectively. However, clear-looking tap water is not a pure substance, because it contains a small number of dissolved minerals and gas in it. It is these dissolved minerals provide taste to water, otherwise, water is tasteless.

Question 3. Is sugar a pure substance?

Ans: Yes. Sugar is a pure substance. Sugar in its pure form is a compound with the same kind of molecules. It contains carbon, hydrogen, and oxygen combined together in a fixed proportion. Therefore, although it contains different kinds of atoms, the proportion of these combining atoms by weight is always constant.

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