Solubility

Introduction

Stir sugar into tea and it disappears; stir sand into water and it settles to the bottom. Whether a substance dissolves — and how much of it can dissolve — is its solubility . Solubility is a physical property with huge practical importance, from making medicines and drinks to understanding how pollutants travel through rivers.

Core explanation

The vocabulary.

- The solute is the substance that dissolves (sugar). - The solvent is the liquid it dissolves in (water). - Together they form a solution — a mixture in which the solute is spread evenly as individual particles too small to see. - A substance that dissolves is soluble ; one that does not is insoluble .

Water dissolves so many substances that it is often called the universal solvent, but other solvents matter too: nail varnish dissolves in propanone (acetone), and grease dissolves in hydrocarbon solvents rather than in water.

Solubility as a quantity. Solubility is the maximum mass of solute that will dissolve in a fixed amount of solvent — usually 100 g of water — at a stated temperature. At 20 °C:

Substance Solubility (g per 100 g water) --- --- Sucrose (table sugar) about 200 Sodium chloride (salt) about 36 Copper(II) sulfate (anhydrous) about 20 Calcium carbonate (chalk) about 0.001 (practically insoluble)

Saturated solutions. If you keep adding salt to water while stirring, a point is reached where no more dissolves and extra crystals remain at the bottom. The solution is saturated . A solution that could dissolve more solute is unsaturated .

Effect of temperature. For most solids, solubility increases as temperature rises: more sugar dissolves in hot tea than in iced tea. When a hot saturated solution is cooled, some solute comes out of solution as crystals — the basis of crystallisation. Sodium chloride is an exception whose solubility changes only a little with temperature. For gases , solubility decreases as temperature rises: warm fizzy drinks go flat faster.

Dissolving is a physical change. The solute's particles spread out among the solvent particles, but no new substance is formed. Evaporating the water from salt solution gives back the salt.

Step-by-step reasoning

To find whether 50 g of salt will dissolve completely in 100 g of water at 20 °C:

1. Look up the solubility: 36 g per 100 g water. 2. Compare: 50 g is more than 36 g. 3. Only 36 g will dissolve; the solution will be saturated. 4. The remaining 50 − 36 = 14 g stays undissolved.

Visual explanation

A solubility curve plots solubility (g per 100 g water) against temperature. Potassium nitrate's line rises steeply from about 32 g at 20 °C to about 110 g at 60 °C, while sodium chloride's line is almost flat near 36–37 g. A particle diagram beside it shows solute particles spread evenly among water particles, with extra undissolved crystals at the bottom of a saturated solution.

Real-world analogy

A bus has a fixed number of seats (the solvent's capacity). Passengers (solute particles) keep boarding until every seat is full — the bus is "saturated". Any extra passengers have to wait at the stop (undissolved solid). On a bigger or "warmer" bus, more seats open up, so more passengers fit.

Real-world example

Fish in rivers and lakes need dissolved oxygen. Because oxygen, a gas, is less soluble in warm water, discharge of warm water from power stations or long heatwaves can lower dissolved oxygen levels and cause fish deaths. Environmental agencies monitor water temperature and dissolved oxygen for this reason.

Why?

Why does dissolving not create a new substance? The solute particles are only separated and surrounded by solvent particles; they are not chemically changed. The original substance can be recovered by removing the solvent, which shows that it was there all along.

Common misconception

"When sugar dissolves, it disappears and its mass is lost." The sugar particles are still present, spread among the water particles. The mass of the solution equals the mass of the water plus the mass of the sugar, and the sugar can be recovered by evaporating the water.

Worked example

Question: Potassium nitrate has a solubility of about 110 g per 100 g water at 60 °C and about 32 g at 20 °C. A saturated solution in 100 g of water is cooled from 60 °C to 20 °C. What mass of crystals forms?

Reasoning: At 60 °C, 110 g is dissolved. At 20 °C only 32 g can stay dissolved. Crystals formed = 110 − 32.

Answer: About 78 g of potassium nitrate crystals.

Quick check

1. Name the solute and the solvent in salt water. Answer: Salt is the solute; water is the solvent.

Exam focus

Use the terms solute, solvent, solution, saturated and solubility precisely. Always state temperature with a solubility value. Solubility-curve questions ask you to read values, compare substances and calculate the mass of crystals formed on cooling (subtract the lower solubility from the higher).

Advanced insight

"Like dissolves like": solvents dissolve substances with similar types of attraction between their particles. Water, with its polar molecules, dissolves ionic and polar substances; non-polar solvents such as hexane dissolve non-polar substances such as oils. This principle guides the choice of solvents in chemistry and in dry cleaning.

Summary

A solute dissolves in a solvent to form a solution. Solubility is the maximum mass of solute that dissolves in a given amount of solvent at a stated temperature; a solution holding this much is saturated. Most solids become more soluble at higher temperatures, while gases become less soluble. Dissolving is a physical change and mass is conserved.

Practice questions

1. Define a saturated solution. Answer: A solution containing the maximum amount of dissolved solute at a given temperature, so that no more will dissolve. 2. Why do fizzy drinks lose their fizz faster when warm? Answer: Gases such as carbon dioxide are less soluble at higher temperatures, so more gas escapes. 3. 10 g of sugar dissolves in 90 g of water. What is the mass of the solution? Answer: 100 g, because mass is conserved when dissolving. 4. Is chalk soluble in water? Use the table to explain. Answer: It is practically insoluble; only about 0.001 g dissolves in 100 g of water.