Reversible Changes and Dissolving

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1. Key words

2. Physical changes and reversible changes

In a physical change no new substance is made. Ice is still water; melted chocolate is still chocolate; salt in water is still salt and water. Because the substance is still there, many physical changes are reversible — we can get the original substance back.

In a chemical change (a chemical reaction) new substances are made, for example when wood burns to ash and smoke. Most chemical changes are irreversible. You will study these on the Chemical Reactions page.

Careful: not every physical change is easy to reverse. Tearing paper or crushing a can are physical changes (no new substance), but you cannot easily put them back as they were. In Checkpoint questions, "reversible" means you can get the same substance back again.

3. Dissolving

When you stir sugar into water, the sugar seems to disappear. It has not gone — it has dissolved. The sugar is the solute, the water is the solvent and the sweet clear liquid is a solution. You can still taste the sugar, and if you let the water evaporate, the sugar is left behind. This proves dissolving is a reversible change.

• Soluble substances dissolve in water: salt, sugar, instant coffee, washing powder.
• Insoluble substances do not dissolve in water: sand, chalk, flour (it makes a cloudy mixture), pepper.
• A solution is clear (you can see through it), even if it is coloured, like weak tea or blue copper sulfate solution. A cloudy mixture means some solid has not dissolved.

4. Mass does not change when a solid dissolves

The solute particles are still there, only spread out, so the mass of the solution = mass of solvent + mass of solute. If you dissolve 20 g of salt in 100 g of water, the solution has a mass of 120 g. Nothing is lost.

5. Getting the substances back (reversing)

1. Filtering separates an insoluble solid from a liquid. Pour the mixture through filter paper in a funnel. The solid (the residue) stays in the paper; the liquid (the filtrate) passes through.
2. Evaporating separates a dissolved solid from its solution. Heat the solution gently in an evaporating basin, or leave it in a warm place. The water becomes water vapour and the solid is left as crystals.
3. To collect the water as well, the water vapour must be cooled so it condenses back into a liquid (this is how solar stills make clean water from salty water).
4. Filtering cannot separate salt from water — the salt particles are so small that they pass through the filter paper with the water.

6. The particle model of dissolving

Scientists use a model to explain dissolving: everything is made of tiny particles that are always moving. In a liquid the particles touch but slide past each other.

1. Water particles are always moving and bump into the surface of the sugar crystal.
2. The bumps pull sugar particles away from the crystal one by one.
3. The sugar particles spread out and mix between the water particles, so the crystal gets smaller and smaller until it cannot be seen.
4. The sugar particles are still there (the mass is the same), just too spread out to see.

7. How temperature affects dissolving

For most solids, a higher temperature of the liquid has two effects:

• The solid dissolves faster (it takes less time to dissolve).
• More solid can dissolve in the same volume of liquid before the solution is saturated.

Particle explanation: when the water is heated, its particles get more energy and move faster. Fast particles hit the solid more often and with more force, so sugar particles are pulled away from the crystal and spread out more quickly. Because the particles are moving faster, there is also room for more sugar particles to spread out among them, so more sugar dissolves before the solution becomes saturated.

Other things also make a solid dissolve faster — stirring and using smaller pieces (powder instead of a cube). In a fair test about temperature, these must be kept the same (control variables).

Reading the graph: as the temperature goes up, the time to dissolve goes down. The curve gets less steep at higher temperatures: going from 20 °C to 30 °C saves 45 s, but going from 50 °C to 60 °C saves only 15 s.

8. Worked examples

Worked example 1 — Here are five changes: melting wax, burning wax, dissolving coffee in hot water, freezing orange juice, cooking an egg. Write down the three changes that are reversible. [2 marks]

1. Ask for each one: is a new substance made? Burning wax makes new gases (carbon dioxide and water vapour) and cooking an egg changes it into a new substance — both are chemical and irreversible.
2. The others make no new substance: melting wax (cool it → solid wax again), dissolving coffee (evaporate the water → coffee left), freezing juice (warm it → liquid juice).
3. Answer: melting wax, dissolving coffee, freezing orange juice. (2 marks for all three correct, 1 mark for two correct.)

Worked example 2 — Omar puts 200 g of water in a beaker. He dissolves 25 g of salt in it. (a) What is the mass of the salt solution? [1] (b) Explain your answer. [1]

1. (a) mass of solution = mass of water + mass of salt = 200 + 25 = 225 g.
2. (b) The salt particles are still there, spread out between the water particles; nothing is lost, so the mass does not change.
3. Common mistake: writing 200 g because "the salt disappeared". Dissolved substances still have mass.

Worked example 3 — Laila has a mixture of sand and salt. Describe how she can get dry sand and salt crystals from the mixture. [3 marks]

1. Add water and stir — the salt dissolves, the sand does not. (1 mark)
2. Filter the mixture — the sand stays in the filter paper (dry it in a warm place). (1 mark)
3. Evaporate the water from the salt solution that passed through — salt crystals are left. (1 mark)
4. Exam tip: always say what each step separates and where each substance ends up.

Worked example 4 — Use the graph in section 7. (a) How long did the sugar take to dissolve at 40 °C? [1] (b) Describe the pattern. [1] (c) Explain the pattern using the particle model. [2]

1. (a) Go up from 40 on the temperature axis to the line, then across: 80 s.
2. (b) The higher the temperature, the shorter the time to dissolve (the sugar dissolves faster).
3. (c) At a higher temperature the water particles move faster (1); they hit the sugar more often/harder so sugar particles are pulled away and spread out more quickly (1).
4. Common mistake: "the sugar particles melt". Particles never melt, and the sugar is dissolving, not melting.

Worked example 5 — Youssef predicts: "More sugar will dissolve in 100 ml of water at 70 °C than at 30 °C." Is his prediction correct? Give a reason. [2 marks]

1. Yes, his prediction is correct (1).
2. Reason: for most solids, more solute can dissolve in hotter water before the solution is saturated, because the faster-moving particles make room for more sugar particles to spread out among them (1).
3. To test it: add sugar one level spoonful at a time, stirring, until no more dissolves; count the spoonfuls at each temperature, using the same volume of water each time.

Practice

All pages in this course

Start here

• Checkpoint 6 Science: The Test and How to Prepare
• Thinking and Working Scientifically: Investigations, Variables, Tables and Graphs

Biology

• The Heart and Circulatory System
• The Lungs and Respiratory System
• Human Reproductive System and Puberty
• Diseases, Hygiene and the Body's Defences
• Food Chains, Food Webs, Energy and Toxins

Chemistry

• Properties of Substances: Melting and Boiling Points, Gases, Conductors
• Reversible Changes and Dissolving
• Boiling and Evaporation
• Chemical Reactions: Reactants, Products and Evidence

Physics

• Mass, Weight and Gravity
• Force Diagrams and Forces on Moving Objects
• Floating and Sinking
• Light: Reflection and Refraction
• Electric Circuits, Symbols, Series and Parallel

Earth and space

• Rocks and Fossils
• Types of Soil
• The Rock Cycle
• The Solar System and the Phases of the Moon

Practice tests

• Checkpoint 6 Science — Practice Test 1
• Checkpoint 6 Science — Practice Test 2
• Checkpoint 6 Science — Practice Test 3