Explain the process of melting of ice with explanation of freezing, melting and melting point.
The change of state from solid to liquid is called melting and from liquid to solid is called fusion.
It is observed that the temperature remains constant until the entire amount of the solid substance melts.
Both the solid and liquid states of the substance coexist in thermal equilibrium during the change of states from solid to liquid.
The temperature at which the solid and the liquid states of the substance is in thermal equilibrium with each other is called its melting point.
It is characteristic of the substance. It also depends on pressure.
The melting point of a substance at standard atmospheric pressure is called its normal melting point.
An activity to explain the process of melting of ice :
Take a slab of ice.
Take a metallic wire and fix two blocks, say $5 \mathrm{~kg}$ each, at its ends. Put the wire over the slab as shown in Fig. You will observe that the wire passes through the ice slab. This happens due to the fact that just below the wire, ice melts at lower temperature due to increase in pressure.
When the wire has passed, water above the wire freezes again. Thus the wire passes through the slab and the slab does not split. This phenomenon of refreezing is called regelation.
Skating is possible on snow due to the formation of water below the skates. Water is formed due to the increase of pressure and it acts as a lubricant.
The graph shows the variation of temperature $(T)$ of one kilogram of a material with the heat $(H)$ supplied to it. At $O,$ the substance is in the solid state. From the graph, we can conclude that
A liquid boils when its vapour pressure equals
A child running a temperature of $101\,^{\circ} F$ is given an antipyrin (i.e. a medicine that lowers fever) which causes an increase in the rate of evaporation of sweat from his body. If the fever is brought down to $98\,^{\circ} F$ in $20$ minutes, what is the average rate of extra evaporation caused, by the drug (in $g/min$). Assume the evaporation mechanism to be the only way by which heat is lost. The mass of the child is $30\; kg$. The spectfic heat of human body is approximately the same as that of water, and latent heat of evaporation of water at that temperature is about $580\; cal \;g^{-1}$.
A given mass $m$ of a hypothetical solid is supplied with heat continuously at a constant rate and the graph shown in the adjacent figure is plotted. If $L_f$ and $L_v$ are latent heats of fusion and latent heats of vaporization and $S_l$ and $S_s$ are specific heats of liquid and solid respectively. It can be concluded that
On a hot day, placing a cup full of ice water on the table heats up as cup of hot tea on the same table cools down. write the reason.