Heating Curve - Practice Questions & MCQ

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 , temperature of solid is changing with time so, $Q=m c_S \Delta T \Rightarrow P \Delta t=m c_S \Delta T \quad$ [as $\left.Q=P \Delta t\right]$

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 change of phase, i.e., change of phase from 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., Latent heat of fusion is proportional to the length of line of zero slope. So, this line is parallel to the time axis.
(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, so 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 its phase from liquid to gas. The length of line $C D$ is proportional to latent heat of vaporisation $\quad\left(L_v\right)$

Here, $Q=m L_v$
${ }_{\text {So, }} L_v=\frac{P\left(t_4-t_3\right)}{m} \quad\left[\right.$ as $\left.Q=P\left(t_4-t_3\right)\right]$
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 gaseous state of substance. Here, its temperature increasing linearly with time. Just like solid and         liquid, the reciprocal of slope of line will be proportional to specific heat or thermal capacity of substance in vapour state.