Exercises: Effects of heat on matter

What is thermal expansion? Explain it in your own words and give an everyday example where it can be observed, other than a thermometer.

When it is hot, the red liquid in a thermometer rises up the tube; when it is cold, it goes down. Observe the figure.

1. a) What happens to the volume of the liquid when it is heated?
2. b) What is this effect of heat called?

Between the rails of a train track there are small gaps every few metres. Observe the figure.

1. a) What are these gaps for?
2. b) What would happen to the rails on a very hot day if the track did not have these gaps?

The metal lid on a jar of olives is impossible to open. Unai's grandmother holds it under the hot water tap for a few seconds and then opens it easily. Knowing that metal expands more than glass, explain his grandmother's trick.

When ice is heated, it turns into liquid water and, if heating continues, into steam.

1. a) What is the change from solid to liquid called?
2. b) What about the change from liquid to gas?
3. c) What are the two reverse changes called, those that occur when cooling?

After the rain, puddles in the park disappear as soon as the Sun comes out.

1. a) What change of state does the water in the puddles undergo?
2. b) Where does the water get the heat needed for this change?

The washing line cable in the patio is tauter and straighter in winter, whereas in summer it sags forming a curve. Observe the figure.

Explain this using thermal expansion.

When a candle is blown out, the melted wax next to the wick turns hard in a few minutes.

1. a) What is the change of state that the wax undergoes called?
2. b) In this change, does the wax absorb or release heat?

An ice cube comes out of the freezer at $-10\ ^\circ\text{C}$ and we leave it on a plate in the kitchen.

1. a) How high does the temperature of the ice rise before it starts to melt?
2. b) What is the name of the change of state that it undergoes next?
3. c) In this change of state, does the water absorb or release heat?

In the mountains, rainwater seeps into cracks in the rocks. At night it freezes and, little by little, the cracks get bigger until the rock eventually breaks. Look at the diagram.

Knowing that water increases in volume when it freezes, explain how it manages to break the rock.

Water freezes at $0\ ^\circ\text{C}$ and boils at $100\ ^\circ\text{C}$. State the state of matter of the water in each case and justify your answer. The diagram places the three cases on a temperature scale:

1. a) The water on the ice rink, which is at $-3\ ^\circ\text{C}$.
2. b) The water in a glass, which is at $25\ ^\circ\text{C}$.
3. c) The water coming out of a pot, which is at $110\ ^\circ\text{C}$.

If you suddenly pour boiling water into a thick glass, it may crack; in a thin glass it is less likely. Taking into account that the inner surface of the glass heats up and expands before the outer surface, explain why a thick glass breaks more easily.

When cooking pasta, the underside of the pot lid fills with small droplets of water. Look at the diagram.

1. a) What happens to the steam when it touches the lid, which is cooler?
2. b) What is this change of state called?
3. c) When turning into droplets, does the steam absorb or release heat?

In an alcohol thermometer, the column rises $10\ \text{cm}$ when the temperature goes from $0\ ^\circ\text{C}$ to $50\ ^\circ\text{C}$. Look at the diagram.

1. a) How many centimetres does the column rise per degree?
2. b) How much will the column rise if the temperature goes from $10\ ^\circ\text{C}$ to $35\ ^\circ\text{C}$?

At Noa's house, the wooden door to the terrace rubs against the frame and is difficult to close in August, but in January it closes easily.

1. a) Explain this using thermal expansion.
2. b) At what time of year will the door be slightly more "shrunk"?

When getting out of the shower, Nico feels cold even though the bathroom is warm.

1. a) What change of state does the water remaining on his skin undergo?
2. b) Where does this water get the heat it needs for the change?
3. c) Use the previous answers to explain why it is advisable to dry oneself with a towel straight away.

You tie a half-inflated balloon and leave it by the window in the sun: after a while, it is more inflated. Look at the diagram.

1. a) What happens to the air inside the balloon when it is heated?
2. b) Predict what will happen if you put that balloon in the fridge for a while.
3. c) Thinking about the balloon, the alcohol in the thermometer and a metal ruler, order from greatest to least how much a gas, a liquid and a solid expand with heat.

A metal bridge lengthens by $1\ \text{cm}$ for every degree its temperature rises. Look at the diagram of the bridge.

1. a) Between a winter night at $0\ ^\circ\text{C}$ and a summer afternoon at $35\ ^\circ\text{C}$, by how many degrees does the bridge's temperature rise?
2. b) How many centimetres does the bridge lengthen between those two days?
3. c) Will the expansion joints of the bridge be more open in winter or in summer? Give reasons for your answer.

A steel nut is seized on its bolt and the mechanic heats it with a blowtorch to loosen it.

1. a) What happens to the nut when it heats up?
2. b) Why does this help to loosen it?
3. c) What problem would arise if the bolt heated up as much as the nut?

We slowly heat ice that is at $-10\ ^\circ\text{C}$ until it turns into steam at $110\ ^\circ\text{C}$, measuring the temperature the whole time. The setup is shown in the diagram.

1. a) Make a list of the five stages of the process, indicating what is present in each stage (ice, water, steam or a mixture) and what the temperature is doing.
2. b) In which two stages does the temperature not rise even though we continue to supply heat? At what temperatures do they occur?
3. c) What is the heat being used for during those two stages?

Seawater evaporates with the heat of the Sun; the steam rises, cools in the upper layers of the atmosphere and forms clouds; finally, rain falls.

1. a) Name the change of state that seawater undergoes when it turns into steam.
2. b) Name the change of state that occurs when the cloud droplets form.
3. c) In each of these two changes, does the water absorb or release heat? Who supplies it or takes it away?

At a scientific base in Antarctica, the thermometer reads $-50\ ^\circ\text{C}$. Mercury freezes at $-39\ ^\circ\text{C}$ and alcohol at $-114\ ^\circ\text{C}$. The diagram summarizes the data on a temperature scale.

1. a) Which thermometer works there, the mercury one or the alcohol one?
2. b) What happens to the liquid in the other thermometer, making it unusable?
3. c) What is the name of the change of state that this liquid undergoes?

Water is an unusual substance: when it freezes its volume increases, so ice is less dense than liquid water.

1. a) Explain why ice cubes float in a glass.
2. b) In a lake in winter, does the surface freeze first or the bottom? Justify your answer.
3. c) What advantage does this have for the fish living in the lake?

We put an ice cube tray filled with water into the freezer and measure its temperature every five minutes: $20\ ^\circ\text{C}$, $12\ ^\circ\text{C}$, $5\ ^\circ\text{C}$, $0\ ^\circ\text{C}$… and for a long time it continues to read $0\ ^\circ\text{C}$ while the ice is forming. The graph shows the measurements.

1. a) Your brother says that the water should go from $0\ ^\circ\text{C}$ to $-5\ ^\circ\text{C}$ continuously. Who is right, him or you? Why?
2. b) While ice is forming, does the water continue to lose heat or not?
3. c) What is that heat lost by the water during the change of state used for?

Unit challenge. How can you prevent a hot drink from losing energy quickly? Become an energy auditor and design a homemade thermal insulator by analysing heat transfer.

1. Phase 1. Baseline measurement: Pour $150\text{ ml}$ of hot water (at about $50\text{ }^\circ\text{C}$) into a glass. Measure the temperature every $4\text{ min}$ for $16\text{ min}$ with a household thermometer (record its uncertainty, for example $\pm 0,5\text{ }^\circ\text{C}$). Check the AEMET website for the current temperature in your town as an ambient reference.

2. Phase 2. Prototyping and testing: Choose an everyday material (aluminium foil, wool, or cardboard) to wrap the glass. Repeat the experiment starting from the same initial temperature and recording measurements at the same intervals.

3. Phase 3. Report and conclusions: Present a data table and draw a temperature ($T$) versus time ($t$) graph comparing both curves. Justify which material best slows down cooling by reasoning based on thermal equilibrium and heat propagation.

Evaluation criteria:

• Rigour in data collection, indicating the uncertainty and integrating both the table and the comparative graph.

• Correct conceptual distinction between heat, temperature, and thermal equilibrium in the conclusions.

• Reasoned justification of the prototype's effectiveness against energy losses.