Thermal expansion of liquids:

Thus, there are two types of thermal volume expansion for liquid

• Apparent volume expansion
• Real volume expansion

 

Activity:

Take a long-necked flask. Fill it with some colored liquid up to mark A on its neck as shown in the figure. Now start heating the flask from the bottom. The liquid level first falls to B and then rises to C.

The heat first reaches the flask which expands and its volume increases. As a result, liquid descends into the flask and its level falls to B. After some time, the liquid begins to rise above B on gets hot. At a certain temperature, it reaches C. the rise in level from A to C is due to the apparent expansion in the volume of the liquid. The actual expansion of the liquid is greater than that due to the expansion because of the expansion of the glass flask. Thus, the real expansion of the liquid is equal to the volume difference between A and C in addition to the volume expansion of the flask. Hence

 

Real expansion of the liquid or BC

=

Apparent of the flask AC + AB

=

Expansion of the liquid

 

 

The expansion of the volume of a liquid taking into consideration the expansion of the container also is called the real volume expansion of the liquid. The real rate of volume expansion $\beta_{r}$ of a liquid is defined as the actual change in the unit volume of a liquid for a 1K (or 1°C) rise in its temperature. The real rate of volume expansion $\beta_{r}$ is always greater than the apparent rate of volume expansion $ \beta_a$ by an amount equal to the rate of volume expansion of the container \beta_{g}.

Thus, $\beta_{r}$  = $ \beta_{a}$+ $ \beta_{g}$   ………………  (ii)

It should be noted that different liquids have different coefficients of volume expansion.