Saturated Solutions

Introduction

Keep adding sugar to a glass of water, stirring after each spoonful, and eventually the sugar stops disappearing and collects at the bottom. The water has taken all the sugar it can hold at that temperature: the solution is saturated . Saturated solutions are the starting point for making crystals, and understanding them helps you use the words concentrated, dilute and saturated correctly — three words that students often muddle.

Core explanation

Saturated solutions. A saturated solution contains the maximum mass of solute that can dissolve in the solvent at that temperature. If more solute is added, it simply stays undissolved. The amount in a saturated solution is the solubility: for sodium chloride at 20 °C, about 36 g in every 100 g of water.

Unsaturated solutions. An unsaturated solution contains less solute than the maximum, so more can still dissolve at that temperature. Most solutions you meet day to day, such as sweet tea, are unsaturated.

Recognising saturation. The clearest sign that a solution is saturated is undissolved solid remaining in contact with the solution even after thorough stirring. If every crystal dissolves, you cannot be sure the solution is saturated.

Saturation depends on temperature. For most solids, solubility rises with temperature. So:

- Heating a saturated solution (with no solid left) makes it unsaturated; it can now dissolve more. - Cooling a saturated solution makes it hold less, so the extra solute comes out of solution as crystals .

This is exactly how crystallisation works: a hot, nearly saturated solution is left to cool, and crystals grow as solubility falls.

Removing solvent. Evaporating some of the solvent from an unsaturated solution raises its concentration until it becomes saturated; further evaporation forces solid out.

Concentrated is not the same as saturated. "Concentrated" and "dilute" compare how much solute is in a given amount of solution. A concentrated solution has a lot of solute; a dilute one has little. "Saturated" means no more can dissolve. A saturated solution of a sparingly soluble substance, such as limewater (calcium hydroxide), is actually very dilute, because its maximum is tiny — only about 0.17 g per 100 g of water at 20 °C. Conversely, a very concentrated sugar syrup may still be unsaturated when hot.

A saturated solution with solid present is at a balance. Particles leave the crystal surface and dissolved particles rejoin it at equal rates, so the amount of solid stays the same even though particles are exchanging.

Step-by-step reasoning

To make a saturated solution at room temperature:

1. Add solute a little at a time to the solvent, stirring well. 2. Continue until some solid remains undissolved after stirring. 3. Allow time for the mixture to settle at a steady temperature. 4. The clear liquid above the solid is a saturated solution.

Visual explanation

Imagine three beakers of blue copper(II) sulfate solution. The first is pale and clear with no solid: dilute and unsaturated. The second is deep blue with no solid: concentrated, possibly unsaturated. The third is deep blue with crystals lying on the bottom that never shrink: saturated.

Real-world analogy

A saturated solution is like a car park with a "Full" sign. Every space is taken, so any more cars arriving must wait outside. If some spaces open up (heating raises the solubility), more cars can get in; if spaces are closed (cooling), some cars must leave.

Real-world example

Honey is a very concentrated sugar solution that is often saturated or even beyond saturation. Over time, especially in a cool cupboard, sugar crystallises out and the honey turns grainy. Gently warming the jar raises the solubility and dissolves the crystals again.

Why?

Why does solid stop dissolving once the solution is saturated? Dissolved particles are constantly colliding with the crystal and rejoining it. At saturation, particles rejoin as fast as they leave, so there is no overall change in the mass of solid.

Common misconception

"A saturated solution must be concentrated." Saturated means the maximum has dissolved; concentrated means there is a lot of solute. For substances with low solubility, such as calcium hydroxide, a saturated solution is still dilute.

Worked example

Question: The solubility of potassium chloride at 20 °C is about 34 g per 100 g of water. A student dissolves 25 g in 100 g of water at 20 °C. Is the solution saturated? How much more could dissolve?

Reasoning: 25 g is less than the maximum of 34 g, so the solution is unsaturated. The difference is the extra that could dissolve.

Answer: Unsaturated; about 34 − 25 = 9 g more could dissolve at 20 °C.

Quick check

1. What observation shows that a solution is saturated? Answer: Undissolved solute remains in the solution even after thorough stirring.

Exam focus

Define a saturated solution with "maximum mass of solute … at that temperature" — the temperature is essential. Keep saturated/unsaturated separate from concentrated/dilute. Link cooling a saturated solution to crystal formation.

Advanced insight

A solution can sometimes be carefully cooled so that it holds more solute than its solubility allows, without forming crystals. This is a supersaturated solution and is unstable: dropping in a tiny seed crystal can make large amounts of solid crystallise at once. Reusable hand warmers use a supersaturated sodium ethanoate solution, which releases heat as it crystallises.

Summary

A saturated solution holds the maximum solute that can dissolve at a given temperature, shown by undissolved solid remaining; an unsaturated one could dissolve more. Heating usually allows more to dissolve, while cooling or evaporating forces crystals out. Saturated describes the limit; concentrated and dilute describe the amount of solute — they are different ideas.

Practice questions

1. Define an unsaturated solution. Answer: A solution that could dissolve more solute at the same temperature. 2. A saturated solution of potassium nitrate is cooled. What will you observe and why? Answer: Crystals form, because the solubility falls as the temperature drops and the excess solute can no longer stay dissolved. 3. Explain why limewater can be both saturated and dilute. Answer: Calcium hydroxide has a very low solubility, so even the maximum dissolved amount is a small quantity of solute. 4. The solubility of a salt is 40 g per 100 g of water at 30 °C. Is a solution of 45 g in 100 g of water at 30 °C possible as a stable solution? Explain. Answer: No; only 40 g can dissolve, so about 5 g would remain undissolved (unless it is an unstable supersaturated solution).