Question

When a substance is heated its density:
(a) increases
(b) decreases
(c) remains same
(d) None of the above

Answer 1

Hint: Think of what happens to the volume when the substance is heated. You will get to know the relationship between density and temperature and you can also use the fact that density describes how closely packed the particles are in a solid,liquid or gas.

Complete answer:
Let us understand density in simpler words, Density is the measure of the mass of an object per unit volume. It is denoted as \['\rho '\].
\[\rho =\dfrac{M}{V}\]
Where \[\rho \] is the density, \[M\] is the mass in kgs and \[V\] is the volume in \[{{m}^{3}}\]. Therefore, the SI unit of density is \[kg/{{m}^{3}}\].
As mentioned in the hint, it describes how closely particles are packed. So, if we increase the temperature of any substance, its particles will start vibrating with more kinetic energy and as a result intermolecular space increases, which means its volume will increase. Hence, the density will decrease as density is mass per unit volume.
This is generally the case that as temperature increases density decreases but there is one exception to this which is water. When the temperature of water is decreased, its density increases(the water contracts) but this behaviour is upto \[{{4}^{\circ }}\] celsius only. After \[{{4}^{\circ }}\] celsius, if we further decrease the temperature, its density decreases(the water expands) and this behaviour of water is called anomalous expansion of water.
So,the correct answer is option B.

Note:
The consequences of anomalous expansion of water are:
Ice floats on water.
Maximum density of water occurs at \[{{4}^{\circ }}\] celsius which is equal to \[1000kg/{{m}^{3}}\]
Survival of aquatic life in cold conditions is possible due to this behaviour only.