Question

Explain the change of phase from liquid to solid in naphthalene by plotting a cooling curve graph.

Accepted Answer

Take some powdered naphthalene in a test tube fitted with a thermometer.
Place the test tube in a beaker of water (or a water bath). Note the temperature.
Heat the beaker from below over a flame and note the temperature after every half minute till all the solid has melted and then $10 ° C$ after melting.
Now remove the burner and allow it to cool.
Note the temperature after every half minute as before, till the naphthalene has again solidified and the temperature of the solid has decreased further by more than $10 ° C$ .
Plot graph for temperature against time while heating and cooling.

As the naphthalene cools, the temperature of the liquid naphthalene falls from $(100 ° C - 80 ° C)$ as shown by the portion AB.
The temperature remain steady as the naphthalene solidifies as shown by the portion BC which is known as freezing point.
When all the naphthalene has solidified, the naphthalene falls again as shown in the portion CD.
The naphthalene's temperature falls until it starts to solidify.
During the time when it is solidifying heat is given out resulting in no change in temperature.
Once the solidification process is complete the temperature of the naphthalene begins to fall again.

Substances	Specific heat capacity ( $J {kg}^{- 1} ° C^{- 1}$ )
Water	4200
Oil	2100
Iron	490
Lead	130

Explain the change of phase from liquid to solid in naphthalene by plotting a cooling curve graph.

Important Questions on Solved Physics Paper-2

Learn and Prepare for any exam you want !