Dissolving: A Physical Change

Introduction

Stir a spoonful of sugar into a cup of warm water and the crystals seem to vanish. Yet the water now tastes sweet, and the cup is heavier by the mass of the sugar. The sugar has not been destroyed or turned into something else — it has dissolved . Dissolving is one of the most important physical changes in chemistry, and understanding it prepares you for work on solutions, separation and reactions in water.

Core explanation

Key words. The substance that dissolves is the solute . The liquid it dissolves in is the solvent . Together they form a solution . In sweet tea, sugar is the solute, water is the solvent and the tea is a solution. A substance that dissolves is soluble ; one that does not is insoluble .

What happens to the particles. A solid solute is made of particles held together in a regular arrangement. When it is placed in a solvent, solvent particles surround the solute particles at the surface and pull them away. The solute particles then spread out and mix evenly between the solvent particles. They become far too small and too spread out to see, so the solution is transparent , although it may be coloured.

Why it is physical. The solute particles are the same particles as before. A sugar molecule in solution is still a sugar molecule; it has simply moved away from its neighbours. No bonds within the sugar molecules are broken and no new substance is formed.

Evidence that the solute is unchanged.

- Taste or colour. Sugar solution tastes sweet; copper sulfate solution is blue, like the crystals. - Recovery. If the water is evaporated, the solute is left behind unchanged. Salt water left to evaporate leaves salt crystals. - Mass. The mass of the solution equals the mass of the solvent plus the mass of the solute. Nothing is lost.

Dissolving versus melting. Dissolving is not the same as melting. Melting needs only heat and one substance; dissolving needs a solvent. Sugar melts at about 186 °C, but it dissolves in water at room temperature, well below its melting point.

Dissolving versus reacting. Some substances seem to "dissolve" in acids but actually react. When magnesium ribbon is placed in dilute hydrochloric acid, it fizzes and disappears, but the magnesium becomes magnesium chloride and hydrogen gas is given off. Evaporating the liquid leaves a white salt, not magnesium metal. That is a chemical change, not dissolving.

Saturated solutions. There is a limit to how much solute a given amount of solvent can hold at a particular temperature. When no more will dissolve, the solution is saturated . For most solids, more dissolves in hot water than in cold.

Step-by-step reasoning

To show that dissolving salt is physical:

1. Weigh some salt and some water, then dissolve the salt. 2. Note that the solution tastes salty and weighs the same as the two parts together. 3. Evaporate the water gently. 4. White crystals remain that look, taste and behave like the original salt. 5. The salt was unchanged, so the change is physical.

Visual explanation

Imagine a tight cluster of blue circles (solute) placed among many small white circles (solvent). In the next picture, white circles surround the edge of the cluster and tug blue circles away. In the final picture, the blue circles are scattered evenly among the white ones. Every blue circle is the same shape as at the start.

Real-world analogy

Dissolving is like a group of friends arriving at a busy party. At the door they stand together, but soon each one drifts off into the crowd and mixes with other guests. They are harder to spot, but every friend is still there and could gather together again at the end.

Real-world example

Sea water is a solution of salts, mainly sodium chloride, in water. In hot coastal regions, sea water is let into shallow salt pans and the Sun evaporates the water. The salt crystals left behind are collected and sold as sea salt, showing that the dissolved salt was unchanged all along.

Why?

Why does a coloured solute give a coloured solution but a clear one? The solute particles keep their own properties, including colour, but they are spread out as individual particles far too small to scatter light, so you can see straight through the liquid.

Common misconception

"When sugar dissolves, it disappears and the mass goes down." The sugar is still present as particles spread through the water. Weighing shows that the total mass of the solution equals the mass of the water plus the sugar.

Worked example

Question: 5.0 g of salt is dissolved in 100.0 g of water. What is the mass of the solution, and what mass of salt could be recovered by evaporation?

Reasoning: Mass is conserved when dissolving, and the salt is unchanged, so all of it remains in the solution and is left when the water evaporates.

Answer: Mass of solution = 105.0 g; about 5.0 g of salt could be recovered.

Quick check

1. In salt water, which substance is the solvent? Answer: Water.

Exam focus

Use the words solute, solvent and solution correctly, and do not say "melt" when you mean "dissolve". To explain why dissolving is physical, state that the solute particles spread out between solvent particles and can be recovered by evaporation, so no new substance forms.

Advanced insight

When ionic solids such as sodium chloride dissolve, their ions separate and become surrounded by water molecules. Chemists sometimes debate whether breaking up the crystal lattice makes this more than a purely physical change. At this level, the key evidence is decisive: the same salt is recovered on evaporation, so dissolving is treated as a physical change.

Summary

Dissolving is a physical change in which solute particles spread out between solvent particles to form a solution. The solute keeps its identity, as shown by its unchanged taste or colour, by conservation of mass and by recovering it through evaporation. Dissolving differs from melting, which needs no solvent, and from reactions with acids, which make new substances.

Practice questions

1. Define solute, solvent and solution. Answer: The solute is the substance that dissolves, the solvent is the liquid it dissolves in, and the solution is the mixture formed. 2. Describe how you could show that dissolved sugar is still sugar. Answer: Evaporate the water; the solid left behind is sugar with the same appearance and taste as before. 3. Explain the difference between dissolving and melting. Answer: Melting is one substance changing from solid to liquid by heating; dissolving is a solute spreading into a solvent to form a solution. 4. Magnesium fizzes and vanishes in acid. Why is this not simply dissolving? Answer: A gas is produced and evaporating the liquid leaves a new salt, not magnesium, so a new substance has formed and the change is chemical.