Page 41 - ICSE Chemistry 6

• The energy is absorbed or released during the • There is no change in energy during the formation of
formation of a compound. a mixture.
• The components of a compound cannot be separated • The components of a mixture can be separated by
by simple physical methods. They can be separated by physical methods.
chemical processes.
• The properties of a compound are entirely different • The constituents of a mixture retain their properties.
from its constituents.
• A compound has ϐixed melting and boiling points. • A mixture does not have a ϐixed melting or boiling
point.
Why is Water a Compound and Air a Mixture?

Table 2.9 will help us to understand why water is a compound and air a mixture.
Table 2.9: Diff erences between Water and Air

Water Air
• Water has a homogeneous composition, i.e., the • Air contains a number of gases like nitrogen, oxygen,
components of water–hydrogen and oxygen are carbon dioxide, water vapour, etc. which retain their
combined in a ϐixed ratio of 1:8 by mass. individual properties.

• The chemical composition of water remains the same, • The composition of air varies from place to place.
irrespective of the source from where it is obtained. At high altitudes, the amount of oxygen is less. Near
industrial complexes, the concentration of carbon
monoxide is high.
• The properties of water are totally different from the • The constituents of air retain their individual
properties of its constituent elements, i.e., oxygen and properties.
hydrogen.
• Energy change takes place during the formation of • There is no change in energy when constituents of air
water. are mixed together.

SEPARATION OF MIXTURES

The process of separa ng the components of a mixture is called separa on of mixtures. We separate
mixtures in our daily lives to obtain the desired components.
Need for Separation of Mixtures

Separa ng various components of a mixture some mes becomes essen al because it may contain
undesirable and harmful substances. The following points will help us to understand the need for
separa on of mixtures.

• To remove harmful or undesirable substances: Food items such as cereals, pulses and spices may
contain stones, husk, insects and their eggs, etc. as impuri es which are harmful if consumed.
Therefore, they need to be separated before cooking to get rid of the unwanted substances present
in them.
• To obtain a useful substance: Separa on of mixture is also important when we need to obtain
pure substances from the mixture. For example, when crude petroleum oil is separated, we get
products such as petrol, diesel, kerosene oil and wax.

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