Exercises: Physical changes and chemical changes

You leave a glass with ice cubes on the worktop and, after a while, they have turned into liquid water, as shown in the diagram.

Has any new substance appeared? Is it a physical change or a chemical change? Justify your answer.

You burn some old papers in the fireplace and they are transformed into ash, smoke and heat. Is it a physical or chemical change? Could you recover the original paper in any way?

You add a spoonful of salt to the water where you are cooking pasta, and the salt disappears from view, as shown in the diagram.

Has it transformed into another substance? What do we call this type of change? How could you recover the salt?

A nail left outdoors on the balcony ends up covered in a reddish layer of rust. Is it a physical or chemical change? Explain what has happened to the iron in the nail.

After disinfecting a cut with alcohol, the skin dries by itself in a few seconds because the alcohol evaporates. Has it stopped being alcohol? Classify the change and give reasons for your answer.

When frying an egg, the egg white, which was transparent and liquid, becomes white and solid. Is it a physical or chemical change? Could you get the raw egg back by letting the pan cool down?

Complete the sentence and give an everyday example for each case: "In a physical change, no $\dots$ substances are formed, whereas in a chemical change, the initial substances are transformed into $\dots$ substances with different properties."

Think about these two kitchen situations: you freeze water to make ice cubes and then let them melt, and you toast a slice of bread until it turns golden brown. State which of the two changes is reversible and which is not, and explain how this relates to whether they are physical or chemical.

To make a sponge cake, Nora melts chocolate in a bain-marie and, after mixing it with the other ingredients, bakes the batter. The diagram summarises the two processes:

1. a) Which of the two processes is a physical change and which is chemical?
2. b) Give reasons for your answer by thinking about which of them could be "undone" by cooling.

In a lighter, the liquid fuel in the reservoir turns into gas when exiting through the valve, and then this gas burns, producing the flame. The diagram summarises the sequence of processes:

Identify the physical change and the chemical change that occur one after the other and explain how they differ.

Grandma's silver cutlery tarnishes over the years even though it is kept in a drawer. Is it a physical or chemical change? Explain what clue is given by the fact that this black layer cannot be removed by blowing or shaking.

In a pottery workshop, Iker models a figure using wet clay (process A) and then bakes it in a kiln, so that it no longer softens even when wet (process B).

1. a) Classify each process as physical or chemical.
2. b) Justify your classification using the reversibility of each process.

At concerts, dry ice is used, which is solid carbon dioxide at around $-78\ \celsius$: it leaves no puddle because it passes directly from solid to gas (invisible); this very cold gas makes the moisture in the air condense, forming that eye-catching fog. The diagram shows the possible changes of state; the orange arrow indicates the one dry ice undergoes:

1. a) What change of state does dry ice undergo?
2. b) Is it a physical or chemical change? Explain why there is nothing left to "collect" at the end.

You take a cold bottle out of the fridge and, after a while, it is wet on the outside, as shown in the diagram.

Your cousin claims that the bottle is leaking and that the water comes from inside. Explain what physical change actually occurs and where that water comes from.

Aisha mixes plaster of Paris powder with water to make a figurine and, after a while, the mixture becomes hard as a rock. Compare this process with wetting sand at the beach, which turns back into loose sand when it dries: which is physical and which is chemical? Justify your answer by considering whether the original material can be recovered.

You grate cheese to sprinkle over pasta (situation A) and, another day, you find a forgotten piece of cheese in the fridge covered in green mould (situation B). Classify both situations as physical or chemical, explaining what happens to the substances in each case.

You heat water and, when it boils, huge bubbles rise. In another glass, you drop an effervescent tablet into cold water and bubbles rise everywhere as well, as shown in the diagram.

1. a) In which of the two cases is there a chemical reaction?
2. b) Steam from the saucepan, upon touching a cold lid, turns back into water droplets: what does this prove about the first situation?
3. c) Explain why "bubbles appear" is not sufficient evidence to claim that a chemical change is taking place.

You heat sugar over low heat to make caramel: first it melts into a golden liquid; but if you get distracted, it darkens, gives off a burnt smell and ends up tasting bitter.

1. a) Which part of the process is a physical change?
2. b) Which part is a chemical change and what signs give it away?
3. c) Explain why liquid caramel, when cooled, becomes a sweet solid again, whereas there is no going back from burnt sugar.

In the laboratory, the teacher strongly heats a shiny metal ribbon inside a covered crucible and it turns into a whitish powder that no longer bends or conducts electricity like the metal.

1. a) Is it a physical or chemical change?
2. b) Describe two tests you would perform to demonstrate that a new substance has been formed.
3. c) Why is it not enough to say "its appearance has changed" to conclude that it is chemical?

Your friend claims: "Water from a puddle evaporates under the sun and then falls back as rain; therefore, all changes are reversible and chemical changes do not exist."

1. a) Give an everyday example, different from hers, that supports the first part of her reasoning.
2. b) Give an example that proves her conclusion is false.
3. c) Write a correct conclusion relating reversibility and the type of change.

When you eat a sandwich, your teeth chop it up as you chew and then saliva and digestive juices in the stomach transform the food into new substances that the body can absorb.

1. a) Identify the physical change and the chemical changes in the process.
2. b) Explain why chopping alone would not be enough to nourish your cells.
3. c) Is any of these changes reversible? Relate this to their classification.

The batteries in the TV remote "run down" and have to be thrown away, whereas the mobile phone battery recharges every night and works again.

1. a) What type of change occurs inside a battery when it runs down?
2. b) If the mobile phone battery "recovers" when recharged, does that mean a physical change occurs inside it? Reason your answer applying the criterion of new substances.

The tumble dryer heats the drum, the water in the damp clothes turns into steam and then a cold area of the appliance converts it back into liquid, which is collected in a reservoir. The diagram summarises the path of the water:

1. a) Name the two changes of state that occur sequentially inside the tumble dryer.
2. b) Are they physical or chemical? Justify your answer.
3. c) Explain why it can be said that the water "has made a round trip" and what would happen if any of those changes were chemical.

In summer, the swimming pool needs to be topped up because the water slowly disappears, and new chlorine must also be added periodically because the previous chlorine has been consumed while disinfecting.

1. a) Classify the two processes (losing water and chlorine being consumed) as physical or chemical.
2. b) Why is the water topped up with a hose, but chlorine has to be bought again?
3. c) Explain what would happen to the microorganisms in the water if chlorine did not transform into new substances when acting.