A hot liquid X was allowed to cooled in air. The temp. was measured every 5 sec. Y axis shows temp. in deg. C of 10 deg. intervals & X axis shows
time in sec. of 5 sec. intervals. Starting from 70 deg. it falls to 50 deg. in 5 sec., stayed same up to 15 sec., than starts falling again to 33 deg. in 25 sec., stayed same onwards. What is the melting point of X, room temp. & in which part of the graph is energy being given out?
What is happening here is that you have a liquid which has been heated above its melting point. As you allow the liquid to cool the temperature drops until it reaches its melting point. The entire time the temperature is falling, the liquid is radiating heat energy to the surrounding air. At the melting point the temperature remains constant as the liquid starts solidifying and changing to a solid. During this phase change from liquid to solid the temperature remains constant but heat energy is still continually being released to the surrounding air. Once the liquid has changed to a solid, you now have a solid at a temperature of 50 C. Because this temperature is higher than the surrounding temperature the solid substance will continue to cool by again radiating heat energy to the surrounding air until it reaches room temperature at which point equilibrium conditions have been reached.
Thus, the melting point is 50 C and the room temperature is 33 C and energy is being released by the substance throughout the cooling process.
The pattern of the process as described above is that the temperature of a substance originally in liquid form and above the room temperature when allowed placed in open air, drops to a lower temperature and stays there for some time. Again the temperature falls and after some time stabilizes at that temperature for a very long period.
This pattern of temperature indicates to the possibility that when the temperature stopped falling for some time, the liquid was loosing latent heat of fusion. Due to this though heat was being removed from the substance its temperature did not change. Instead the loss of heat resulted in solidification of the liquid. Thus this intermediate stable temperature is the melting point of the liquid.
The final stable temperature is because the solidified substance has reached room temperature. After this, in absence of temperature difference between the substance and the surrounding air, there is no further energy loss.
Based on the above logic, answer to the question are:
Melting point of substance X = 50 degrees C.
Room temperature = 33 degrees C.
Energy is being lost continuously from start of the graph till temperature reaches 33 degrees C. Once this temperature is reached there is no further energy loss.
The heat energy is no doubt being given out by the liquid till the liquid attains the room temperature of 33 degree. But by Newton's law of cooling the rate of cooling of an object of higher temperature is proportional to the difference its own temperature and the surrounding. Here the hot looses its energy from 70 degree to 50 degree in 5seconds.
But the at 50 degree temperature it remains loosing heat energy for 15 secods (3 times the duration it took to fall down from 70 degree to 50 degree). So without liquid's temperature coming down, keeping a substaintial temperature difference of (50-33) degree with the room temperature the loss of heat energy must be very high. This constant temperature of 50 degree for 15 seconds is the time the liquid to took to completely turn into solid.