2.2.3. Melting, boiling and evaporation
A subsection of Physics, 5054, through 2. Thermal physics
Listing 10 of 166 questions
A student makes a diagram of the molecules in a liquid, as shown in . motion molecule Suggest two ways in which the student’s diagram does not show the positions and motion of the molecules accurately. 1. 2. Some of the molecules of a liquid evaporate. Describe what happens during evaporation. Explain why evaporation has a cooling effect on the liquid. The beaker shown in contains pure water at a temperature of 80 °C. A thermometer, initially at room temperature, is placed in the air just above the water. The reading on the thermometer starts to increase slowly. water thermometer Explain how convection in the air causes the reading on the thermometer to increase. A heater is placed underneath the beaker and the water boils vigorously to produce steam. The thermometer reading increases quickly. Explain how the water in the gas state gives thermal energy to the thermometer. The heater placed underneath the beaker has a power of 200 W. The mass of water in the beaker is initially 100 g. The water in the beaker is heated for a time of 120 s. During this time, the temperature of the water in the beaker rises from 80 °C to 100 °C and then the water boils. At the end of this time, the mass of water in the beaker is 95 g. The specific heat capacity of water is 4.2 J / (g °C). The specific latent heat of vaporisation of water is 2250 J/ g. Calculate the thermal energy 1. produced by the heater in 120 s, thermal energy = 2. used to warm the water from 80°C to 100°C, thermal energy = 3. used to reduce the mass of water in the beaker, thermal energy = 4. produced by the heater that is not transferred to the water. thermal energy =
5054_s15_qp_21
THEORY
2015
Paper 2, Variant 1
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Two different kettles are used to heat water, as shown in . electric kettle gas heated kettle gas burner Data for the two kettles is shown in . energy supplied to the kettle in one minute / J thermal energy supplied by the kettle to the water in one minute / J electric kettle 120 000 95 000 gas heated kettle 130 000 90 000 Calculate the efficiency of the electric kettle. efficiency = Calculate the useful power of the gas heated kettle. power = Both kettles contain the same mass of water, at the same initial temperature. State and explain which kettle brings the water to boiling point first. The boiling water produces steam at 100 °C. State one difference between molecules in the steam and molecules in the boiling water.
5054_s15_qp_22
THEORY
2015
Paper 2, Variant 2
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A metal can and a plastic bottle, both containing liquid, are cooled by placing them in a jug of melting ice, as shown in . melting ice jug The can and bottle each contain 330 g of the same liquid at 15 °C. The specific heat capacity of the liquid is 4.2 J / (g °C). Calculate the thermal energy released when 330 g of the liquid at 15 °C cools to 2 °C. energy = When water at 0 °C is used in the jug, instead of the melting ice, the cooling is slower. Explain why cooling is faster when using melting ice in the jug, rather than water at 0 °C. The liquid in the metal can cools down faster than the liquid in the plastic bottle. Suggest why this happens.
5054_s15_qp_22
THEORY
2015
Paper 2, Variant 2
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State what is meant by melting point. The temperature of a liquid is between its melting point and its boiling point. Using ideas about molecules, describe the changes that occur when the liquid evaporates. The boiling point of nitrogen is –196 °C. Liquid nitrogen, below its boiling point, is stored in a vacuum flask, as shown in . glass walls vacuum liquid nitrogen The flask has two glass walls with a vacuum between them. Suggest where, in the apparatus shown in , evaporation occurs. State and explain the advantage of having a vacuum between the two glass walls. The liquid nitrogen reaches –196 °C, its boiling point. A small piece of metal at 20 °C is lowered slowly into the liquid nitrogen. Bubbles form within the liquid nitrogen as it boils. State what is inside the bubbles. The small piece of metal has a mass of 50 g. When it is lowered into the liquid nitrogen, the metal cools to –196 °C. The specific heat capacity of the metal is 0.39 J / (g °C). The specific latent heat of vaporisation of nitrogen is 200 J / g. Calculate 1. the thermal energy lost from the metal as it cools, thermal energy = 2. the mass of nitrogen that boils away. mass =
5054_s16_qp_21
THEORY
2016
Paper 2, Variant 1
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