Heating Curve MCQ - Practice Questions with Answers

Heating Curve

If a solid of mass (m) is heated at a constant rate such that it is undergoing change of phase from solid to liquid and liquid to gas on a graph of Temperature and time is called Heating curve.                                           

Now we will discuss each phase one by one - 

(1) In the region OA , the temperature of a solid changes with time so, $Q=m c_S \Delta T \Rightarrow P \Delta t=m c_S \Delta T \quad[$ as $Q=P \Delta t]$

Here, P is the power and $\Delta t$ is the time interval.

So, from here we can deduce that -

$$
c_s \propto \frac{1}{\text { Slope of line } O A}
$$
Because the $\frac{\Delta T}{\Delta t}$ is the slope of OA

(2) Now come to the region $A B$, here temperature is constant, so it represents a change of phase, i.e., a change of phase from solid state to liquid state. Now you can see that between $A$ and $B$ substance is partly solid and partly liquid.

If $L_f$ is the latent heat of fusion $Q=m L_f$

$$
L_F=\frac{P\left(t_2-t_1\right)}{m} \quad\left[\text { as } Q=P\left(t_2-t_1\right)\right]
$$

i.e., the Latent heat of fusion is proportional to the length of the line of zero slope. So, this line is parallel to the time axis.í

(3) In the region BC temperature of liquid increases so, again $c_L \propto \frac{1}{\text { Slope of line } B C}$. Since it is sensible heat, the temperature will change in this zone BC. 

(4) In the region CD temperature is again constant, so it represents the change of phase, i.e., boiling with boiling point $\mathrm{T}_2$. In this, the liquid is changing its phase from liquid to gas. The length of line CD is proportional to latent heat of vaporization ( $\mathrm{L}_{\mathrm{w}}$ )

Here, $Q=m L_v$

$$
L_v=\frac{P\left(t_4-t_3\right)}{m} \quad\left[\text { as } Q=P\left(t_4-t_3\right)\right]
$$

So, $L_v \propto$ Length of line $C D$. It means that the line is parallel to the time axis.
(5) The line DE represents the gaseous state of substance. Here, its temperature increases linearly with time. Just like solid and liquid, the reciprocal of slope of line will be proportional to the specific heat or thermal capacity of substance in vapour state.