Question

When a liquid is heated its density
A. Decreases
B. Increases
C. Does not change
D. Decrease or increases depending upon the pressure to which it is subjected

Answer 1

When a liquid or gas gets heated the molecules inside the liquid will receive more energy and will move faster. Due to this the molecules will collide with each other more which results in increasing the interatomic distances. So the liquid will take up more space. The more separation between the molecules of a liquid the liquid will be less dense.

Formula used: Density is defined as mass per unit volume. i.e.
$\text{density=}\dfrac{\text{mass}}{\text{volume}}$

Complete step by step answer:
Qualitatively speaking density means the amount of given substance in a given space. If the space is less the density is more.
Quantitatively speaking density is defined as mass per unit volume. i.e.
$\text{density=}\dfrac{\text{mass}}{\text{volume}}$
If the mass remains constants and volume increases then the density will decrease.
The molecules inside a liquid are always in random motion called Brownian motion. As the motion is random the effective motion considering the whole liquid is zero.
When we apply some heat to a liquid the molecules constituting the liquid will receive the heat and gain kinetic energy and move more rapidly. So the molecules will; collide with each other more often and they will collide with more energy. So the intermolecular separation will increase so by increasing the temperature the liquid will occupy more space that means by increasing the temperature the liquid will occupy more volume but the amount constituting the liquid remains constant. i.e. with increase in temperature the mass of the liquid remains the same but the volume increases. So the density will decrease by increasing the temperature.

So, the correct answer is “Option A”.

Note: Note that this is also applicable to decrease in temperature. If we decrease the temperature the kinetic energy of the molecules constituting the liquid will decrease so they will collide less often and the intermolecular separation will decrease that means the volume will decrease if we decrease the temperature the molecules will take up less space . So the density will increase with decrease in temperature.
Also every substance has its own density which depends upon the amount of mass present per volume. Conventionally it is chosen that the Water has a density of one. So the Liquids and solids which have density greater than one will sink in water. And then Liquids and solids which have density less than one will float in water.