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 Form 3 Physics Quantity of Heat Video Questions and Answers

(a) State two differences between boiling and evaporation. (2 marks)
(b) State three ways in which loss of heat by conduction is minimized in a vacuum flask. (3 marks)
c) In a certain experiment,50g of dry steam at #100^0C# was directed into some crushed ice at #0^0C#.(Latent heat of vaporization of water is #2.26 times 10^6# J/kg, latent heat of fusion for ice is #3.34 times 10^5#J/Kg and
specific heat capacity of water is #4.2 x 10^3# J/Kg) (i) Quantity of heat lost by steam to change to water at #100^oC#. (2 marks) (ii) Quantity of heat lost by water to cool to #0^oC#.(2 marks) (iii) Mass of ice melted at #0^oC#.(2 marks) (KCSE 2018 paper 1 question 14.)

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Answer Text:
1. Boiling
Takes place at fixed temperature.
Takes place throughout the liquid.
It is lowered by decreasing the atmospheric pressure.

2. Evaporation:
Takes place at all temperatures.
Takes place on the surface of the liquid.
It is increased by decreasing the atmospheric pressure.

b).-Having a double wall.
-Having a cork made of an insulator.
-Having a vacuum between the double walls.
c).i) #Q =ML_V = 50/1000 X 2.26 X 10^6 =113000J#
ii) #Q= MCϴ = (50)/(1000) X 4200 X 100 = 21000J.#
iii).Heat lost by steam = Heat gained by the melting ice.\
#ML_V + MCϴ = ML_f#
113000 + 21000 = M X 334000
134000 =334000M
M= 0.4012Kg.


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