physics!! help!?

Question

1) Explain in molecular terms, why considerable heat energy is needed to convert 1g of water into 1g of steam at 100°C, without any temperature change taking place.

2) In an experiment, 120g of ice at 0°C is to be melted. the specific latent heat is of ice is 340J/g. Assume all the energy from the heater is going to be used to melt the ice.

Calculate the expected time for which the 60W heater is switched on.

(please explain answers!!)

Answer 1

first question:
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in order to boil water, you must compeletely separate the water molecules from each other. however, water molecules exert attractive forces on each other. these attractive forces are called hydrogen bonds. since hydrogen bonds are relatively strong, it's hard to break them, which means a large amount of energy (in the form of heat) is required to separate the water molecules from each other.

the reason why the temperature doesn't change during the liquid to vapor phase change is becasue the energy that is being added is being used to break those hydrogen bonds between the water molecules. therefore the energy is *not* being used to increase the kinetic energy of the molecules (which would cause the temperature to increase)

second question:
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first determine the amound of energy needed to melt the water:

120 g x [340 J / 1 g] = 40800 J

since the heater is 60 W, it's supplying heat at a rate of 60 J per second:

60 J = 1 s

we can now determine how much time is required for the heater to melt the ice:

40800 J x [1 s / 60 J] = 680 s

Answer 2

1) That considerable amount of energy will be used to convert the state of the water from Liquid to Gas and as you know it is called the hidden heat of evapuration.
the molecules of the gas are more active(have more motion and moving faster) than liquid state. that considerable energy will be used for this movement.


2)
assume that the 0°C Ice will be melted into 0°C water , so :
340*120 = 40800 j ( the amount of energy that is needed)

(the heater is 60w)
so the heater will transfer 60 j in each second , so

40800/60 = 680 sec = 11 min and 20 sec.

Answer 3

Try this ,water when it evaporates has a refrigeration effect . Each cc of water that is changed to steak absorbs 60 calories from the water . That is why it never gets hotter.