How much heat energy is gained when 5 kg of water at \$20 ^ { \circ } \mathrm { C }\$ is brought to its boiling point?

\$Q = mc\Delta \theta \$\$ = 5 \times \left( {1000 \times 4.2} \right) \times \left( {100 - 20} \right)\$\$ = 1680 \times {10^3}J\$\$ = 1680\,KJ\$Hence,option \$(A)\$ is correct answer.

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Melting and Boiling

Q: In a pressure cooker the cooking is fast because.

Cooking is fast in pressure cooker just because the trapped steam increases the atmospheric pressure inside the cooker. At that pressure, boiling point of water is increased and this higher temperature cooks food faster.

Q: You are provided with a mixture of naphthalene and ammonium chloride by your teacher. Suggest an activity to separate them with well labelled diagram.

Napthalene is insoluble in water but soluble in benzene (organic solvent). Ammonium chloride is soluble in water but insoluble in benzene. Napthalene changes into vapours at room temperature whereas ammonium chloride changes into vapours on heating. Method :(a) Add water to the mixture and shake it vigorously to dissolve ammonium chloride.(b) Filter the mixture. Napthalene is obtained as residue whereas, filtrate contains ammonium chloride. Crystallize the filtrate by heating till saturated solution of ammonium chloride is obtained.(c) Cool the hot saturated solution to get crystals of ammonium chloride.

Q: Latent heat of fusion of ice is:

Latent heat of fusion of ice\$=80 cal g^{-1}\$\$=80\times 4.2 J\times 10^3 kg^{-1}\$\$=336\times 10^3 J kg^{-1}\$

Q: Give a reason for the following observations:\$(a)\$ Naphthalene balls disappear with time without leaving any solid.\$(b)\$ We can get the smell of perfumes sitting several metres away.

(a) Naphthalene balls undergo sublimation due to which it converts directly into vapours and disappear into air without leaving any solid.(b) The particles of perfume diffuse rapidly into the air and its smell can be felt while sitting several metres away.

Q: Define latent heat of vaporization.

Latent heat of vapourisation is a physical property of a substance. When a material in liquid state is given energy, it changes its phase from liquid to vapour without change in temperature, the energy absorbed in the process is called latent heat of vaporization. The latent heat of vaporization of water is about \$2260kJ/Kg\$ which is equal to \$40.8kJ/mol\$.

Q: Look at the given figure and suggest in which of the vessels \$A,B, C\$ or \$D\$ the rate of evaporation will be the highest? Explain.

The rate of evaporation increases with the increase in surface area and wind speed. The surface areas of vessel \$C, A\$ and \$D\$ are similar but the wind speed is greater above the vessel \$C\$ because of fan, hence the rate of evaporation will be highest in the vessel \$C\$.

Q: What is the effect of increase in pressure on the boiling point of a liquid ?

As the pressure applied to the liquid surface is increased, the energy needed for the liquid molecules to expand to gas phase also increases. Hence, a higher temperature is required to change liquid to gas phase. So, boiling point of liquid rises on increasing pressure.