Page 70 - Science Class 06

substance are different from the Expansion and Contrac on of Materials
proper es of the reac ng substances. Hea ng water changes to steam. Thus,

v The substances that react with each hea ng a material can change it. Water
other in a chemical reac on are called changes to ice on cooling. Thus, cooling a
reactants. material can also change it. But the effect of
hea ng is opposite to that of cooling.
v Each chemical reac on leads to chemical
changes. These changes results in Let us see how:
energy changes as well. Hea ng a substance can make it expand,

v The new substance produced in a but the amount of expansion differs in
chemical reac on cannot be reversed in solids, liquids and gases. Gases expand the

order to obtain the original reactants. most while solids expand the least.

Expansion in Solids Expansion in Liquids Expansion in Gases

Railway tracks consist of Water expands on hea ng. If you keep an inflated
two parallel metal rails Try this with the help of an balloon in the sun for some
joined together. Small gaps, adult! Take a glass filled with me, what will happen? It
called expansion gaps are water to its brim. Pour the will grow in size as the air
deliberately le between the water into a container and inside it expands on taking
rails as there is an expansion heat it (do not boil). Now try heat from the surroundings.

of the rails in hot weather. to pour the water back into
the same glass. The water Heat
overflows.

Expansion gaps in
railway tracks

The fact that materials expand on hea ng is level rises in the glass tube, indica ng the
used in thermometers. In many thermo- temperature.

meters, mercury is used. When the bulb of Cooling just does the opposite of hea ng!
the thermometer comes in contact with a Cooling causes a material to contract. Solids
hot object, the mercury expands and its contract the least while gases contract the
most.

Mercury expands due to heat.

70 Science-6