3.3 Changes of state and internal energy
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Which of these is a physical change?
Choose one answer.
Correct Not quite: the answer is Ice melting
Mark scheme
C: ice melting
Model answer
Ice melting (C), because no new substance is made and the change can be reversed.
0.50 kg of ice melts completely. State the mass of the water formed and explain your answer.
Mark scheme
0.50 kg (1); mass is conserved / the number of particles stays the same (1)
Model answer
0.50 kg. Mass is conserved because the number of particles stays the same during a change of state.
Describe what is meant by the internal energy of a substance.
Mark scheme
the total kinetic energy (of the particles) (1); and potential energy of all the particles in the substance (1)
Model answer
The internal energy is the total of the kinetic energy and the potential energy of all the particles in the substance.
State the two effects that heating a substance can have on it.
Mark scheme
the temperature rises (1); the substance changes state (1)
Model answer
The temperature of the substance can rise, or it can change state.
Ice at 0 °C is heated and melts, but its temperature stays at 0 °C until it has all melted. Explain why, in terms of energy.
Mark scheme
the energy supplied increases the potential energy of the particles (1); as the particles move further apart / the forces between them are overcome (1); the kinetic energy of the particles does not change, so the temperature stays constant (1)
Model answer
The energy supplied increases the potential energy of the particles as they are pulled apart. The kinetic energy of the particles does not change, so the temperature stays constant until all the ice has melted.
Explain why condensing steam into water is a physical change and not a chemical change.
Mark scheme
no new substance is made / it is still water (1); it can be reversed / by heating the water again (1)
Model answer
No new substance is made, because the water is still water. The change can be reversed by heating the water to make steam again.
Describe the arrangement and the movement of the particles in a liquid and in a gas.
Mark scheme
liquid: particles are close together, in a random arrangement (1); they move around each other (1); gas: particles are far apart and random (1); they move quickly in all directions (1)
Model answer
In a liquid the particles are close together in a random arrangement and move around one another. In a gas, the particles are far apart, and move quickly in all directions.
Explain why sweating helps to cool the body.
Mark scheme
the fastest particles in the sweat escape from the surface (evaporation) (1); so the average kinetic energy of the remaining particles falls (1); so the temperature of the sweat and the skin falls / energy is transferred from the skin (1)
Model answer
The fastest particles escape from the sweat as it evaporates, so the particles left behind have less kinetic energy on average. The sweat and skin cool down.
A bath of water and a cup of water are both at 30 °C. State which has the greater internal energy and explain why.
Mark scheme
the bath (1); it has many more particles, so the total kinetic and potential energy is greater (1)
Model answer
The bath has the greater internal energy, because it contains many more particles, and internal energy is the total energy of all of them.
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