Edexcel iGCSE Physics (4PH1) 5.4 Ideal Gas Molecules Exam Style Question Paper 2B - New Syllabus
Question
The photograph shows a water bath that a technician uses to heat some water.
(a) The water bath is filled with water at an initial temperature of \(15^\circ\mathrm{C}\). Calculate the initial temperature of the water in kelvin.
(b) The technician heats the water to a final temperature of \(60^\circ\mathrm{C}\).
(i) Describe how the energy of the water molecules changes as the temperature of the water increases.
(ii) The table shows some information about the heating element in the water bath and the heating process.
Calculate the energy transferred by the heating element in the water bath during the heating process.
(iii) Calculate the mass of water being heated. Assume that all the energy is transferred to the thermal store of the water.
[for water, specific heat capacity = \(4200\,\mathrm{J\,kg^{-1}\,^\circ C^{-1}}\)]
(c) Some water evaporates as a gas from the water bath.
(i) Describe the arrangement of particles in a gas.
(ii) Describe two differences between evaporation and boiling.
Syllabus Topic Codes (Edexcel International GCSE Physics 4PH1):
• 5.18–5.19: Temperature, Molecular Speed, and Kinetic Energy — part (b)(i)
• 2.4–2.5: Power, Current, Voltage, and Electrical Energy Transfer — part (b)(ii)
• 5.12–5.13P: Specific Heat Capacity and Thermal Energy Change Equation — part (b)(iii)
• 5.10P: Particle Model of Solids, Liquids, and Gases — part (c)(i)
• 5.9P: Changes of State — part (c)(ii)
▶️ Answer/Explanation
(a) Temperature in kelvin [1 mark]
Use the conversion:
\(T_{\mathrm{K}}=T_{^\circ\mathrm{C}}+273\)
\(T_{\mathrm{K}}=15+273=288\,\mathrm{K}\)
\(\boxed{288\,\mathrm{K}}\)
(b)(i) Energy of the water molecules [2 marks]
As the temperature increases, the energy of the water molecules increases.
The average kinetic energy of the molecules increases, meaning that the molecules move faster on average.
(b)(ii) Energy transferred by the heating element [3 marks]
Use:
\(E=VIt\)
The heating time is \(45\,\mathrm{min}\), so convert it to seconds:
\(t=45\times60=2700\,\mathrm{s}\)
Substitute \(V=230\,\mathrm{V}\), \(I=1.5\,\mathrm{A}\), and \(t=2700\,\mathrm{s}\):
\(E=230\times1.5\times2700\)
\(E=931\,500\,\mathrm{J}\)
Therefore:
\(\boxed{E\approx9.3\times10^5\,\mathrm{J}}\)
(b)(iii) Mass of water [3 marks]
Use the thermal energy equation:
\(Q=mc\Delta T\)
The temperature change is:
\(\Delta T=60-15=45^\circ\mathrm{C}\)
Since all the energy is transferred to the thermal store of the water:
\(9.3\times10^5=m\times4200\times45\)
Rearranging:
\(m=\dfrac{9.3\times10^5}{4200\times45}\)
\(m\approx4.92\,\mathrm{kg}\)
\(\boxed{m\approx4.9\,\mathrm{kg}}\)
(c)(i) Arrangement of particles in a gas [2 marks]
- The particles are arranged randomly.
- The particles are widely spaced with large gaps between them.
(c)(ii) Evaporation and boiling [2 marks]
- Boiling occurs at a specific or fixed temperature, whereas evaporation can occur at any temperature.
- Boiling occurs throughout the liquid, whereas evaporation occurs only at the surface.
Question
A sample of steam (water in a gas state) is cooled using a very cold freezer.
(a) The steam is cooled from an initial temperature of \(150^\circ\mathrm{C}\). Calculate the initial temperature of the steam in kelvin (K).
(b) The temperature-time graph shows how the temperature of the steam changes during the cooling process. The steam eventually becomes ice (water in a solid state).
(i) The graph shows five stages, A, B, C, D and E, of the cooling process. State which stages of the cooling process show a change of state.
(ii) Describe the differences in the arrangement of particles when the sample is in a gas state (steam) and a solid state (ice). You may draw a diagram to help your answer.
(iii) Explain whether or not the energy in the kinetic store of the particles changes when the sample is changing state.
Syllabus Topic Codes (Edexcel International GCSE Physics 4PH1):
• 5.8P–5.9P: Energy Changes During Heating and Changes of State — part (b)(i)
• 5.10P: Particle Model of Solids, Liquids, and Gases — part (b)(ii)
• 5.18–5.19P: Temperature, Molecular Speed, and Kinetic Energy — part (b)(iii)
▶️ Answer/Explanation
(a) Converting Celsius to kelvin [1 mark]
Use the relationship:
\(T_\mathrm{K}=T_{^\circ\mathrm{C}}+273\)
\(T_\mathrm{K}=150+273\)
\(\boxed{T=423\,\mathrm{K}}\)
(b)(i) Changes of state [1 mark]
A change of state occurs during the horizontal sections of the temperature-time graph, where the temperature remains constant while the substance changes state.
Correct answer: B and D only.
(b)(ii) Arrangement of particles [2 marks]
- In a solid, the particles are much closer together than in a gas.
- In a solid, the particles have a fixed, regular arrangement, whereas particles in a gas are arranged randomly and are much further apart.
The particles in a gas can move freely throughout the available space, while particles in a solid vibrate about fixed positions.
(b)(iii) Kinetic energy during a change of state [3 marks]
The energy in the kinetic store of the particles stays constant while the sample is changing state.
- The temperature remains constant during the change of state.
- The average speed of the particles does not change.
- Kelvin temperature is directly related to the average kinetic energy of the particles.
During a change of state, energy is transferred to or from the particles without changing their average kinetic energy. Instead, the energy is associated with changes in the arrangement and separation of the particles.