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Chapter: 7th Science : Term 1 Unit 3 : Matter Around Us

Compounds

A compound is a pure substance that is formed when the atoms of two or more elements combine chemically in definite proportions.

Compounds

A compound is a pure substance that is formed when the atoms of two or more elements combine chemically in definite proportions.


Compounds exhibit properties entirely different from the properties of their constituent elements. For example, the atoms of the elements hydrogen and oxygen combine chemically in a fixed ratio to form the compound water. However, water does not have the exact same properties as hydrogen and oxygen. For example, at room temperature water exist as liquid while hydrogen and oxygen exist as gases. Also, oxygen supports fire whereas water is used as a fire extinguisher.

Similarly, common salt (sodium chloride) is a compound made up of elements sodium and chlorine. It is used in our food, whereas sodium and chlorine are poison, are both unsafe for consumption.


ACTIVITY

Complete the following table by writing compounds of its constituents.

Compound : Constituent Elements

Water : Hydrogen and Oxygen

Salt (Sodium chloride) : Sodium and Chlorine

Sodium soda (sodium bicarbonate) : Sodium, Carbon and Oxygen

Baking soda (sodium bicarbonate) : Carbon, Hydrogen and Oxygen

Sugar : Carbon, Hydrogen and Oxygen

Calcium oxide : Calcium and Oxygen

Calcium hydroxide : Calcium, hydrogen and Oxygen

Sodium hydroxide : Sodium, hydrogen and Oxygen

Potassium hydroxide : Potassium, hydrogen and Oxygen


 

Properties of Compounds

* A compound is formed only when the constituent elements combine in a fixed proportion.

* The properties of a compound are different from those of its constituent elements

* A compound can not be broken down by physical methods. This is because acompound is made up of different elements that are chemically combined. Sodium chloride cannot be separated by physical methods such as filtration.

* A compound can be separated into its constituent elements by chemical methods only.


Difference between an element and a compound


Elements

• An element is the simplest substance

• Elements combine to form compounds

• Atoms are the fundamental particle of an element

Compounds

• A compound is a chemical substance formed by the combination of two or more elements

• Compounds can be split into elements

• Molecules are the fundamental particles of a compound

 

ACTIVITY

Complete the following table by counting the number of different elements in a compounds and give appropriate name.



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