Solubility and Temperature

Introduction

You can stir far more sugar into hot tea than into iced tea before crystals start collecting at the bottom. The amount of a substance that can dissolve is not fixed: it depends on temperature. Chemists record this with solubility curves , graphs that show how much solute dissolves at each temperature. Reading them is a key exam skill, and they explain how crystallisation works and why warm rivers hold less oxygen for fish.

Core explanation

Defining solubility. The solubility of a substance is the maximum mass that will dissolve in 100 g of solvent at a particular temperature. For solids in water it is usually given in g per 100 g of water . Because it changes with temperature, a solubility value is meaningless unless the temperature is stated.

Solids usually become more soluble when heated. For most solids, solubility increases as temperature rises. The size of the effect varies greatly:

Temperature (°C) Potassium nitrate (g/100 g water) Sodium chloride (g/100 g water) --- --- --- 20 about 32 about 36 40 about 64 about 36.5 60 about 110 about 37 80 about 170 about 38 100 about 245 about 39

Potassium nitrate's solubility rises steeply, while sodium chloride's barely changes.

Solubility curves. Plotting solubility (vertical axis) against temperature (horizontal axis) gives a solubility curve. A steep curve means solubility depends strongly on temperature; a nearly flat curve means it hardly changes. Where two curves cross, the two substances have the same solubility at that temperature.

Reading a curve. To find the solubility at a given temperature, go up from the temperature on the horizontal axis to the curve, then across to the vertical axis. To find the temperature at which a given mass dissolves, go across from the solubility to the curve, then down.

Points above, on and below the curve. A point on the curve represents a saturated solution. A point below the curve means less than the maximum has dissolved, so more could dissolve. If a solution is cooled so that its point would lie above the curve, the excess solute comes out as crystals.

Gases behave the opposite way. Gases become less soluble as temperature rises. A warm fizzy drink goes flat faster than a cold one, and warm water holds less dissolved oxygen than cold water.

Scaling to other masses of water. Solubility is per 100 g of water, so for 50 g of water halve the value, and for 200 g double it.

Step-by-step reasoning

To calculate the mass of crystals formed on cooling a saturated solution:

1. Read the solubility at the higher temperature. 2. Read the solubility at the lower temperature. 3. Subtract: the difference is the mass that crystallises per 100 g of water. 4. Scale if the mass of water is not 100 g.

Visual explanation

Imagine a graph with temperature from 0 to 100 °C along the bottom and solubility up the side. The potassium nitrate line starts low and sweeps steeply upwards like a ski jump. The sodium chloride line runs almost flat, just below 40. The two lines cross at roughly 22 °C.

Real-world analogy

A solubility curve is like the maximum capacity of a lift that changes with the weather: on cold days it can carry only a few people, on hot days many more. The curve tells you the limit at each "weather" (temperature); any extra passengers have to get out.

Real-world example

Power stations and factories that release warm water into rivers can harm aquatic life. Warmer water holds less dissolved oxygen, so fish downstream may struggle to breathe. Environmental rules therefore limit how warm discharged water may be.

Why?

Why do most solids dissolve more in hot water? The solvent particles have more energy, so they are better at pulling solute particles away from the crystal, and the solution can hold more solute. Gas particles, by contrast, gain enough energy to escape from the liquid, so less gas stays dissolved.

Common misconception

"Heating always makes things more soluble." This is true for most solids but not for gases, which become less soluble when warmed. A few solids, such as calcium hydroxide, also become slightly less soluble at higher temperature.

Worked example

Question: A saturated solution of potassium nitrate in 100 g of water is cooled from 60 °C to 20 °C. Using the table, what mass of crystals forms?

Reasoning: Solubility at 60 °C is about 110 g; at 20 °C it is about 32 g. The difference no longer fits in solution.

Answer: 110 − 32 = about 78 g of potassium nitrate crystallises.

Quick check

1. What is the solubility of sodium chloride at 20 °C, according to the table? Answer: About 36 g per 100 g of water.

Exam focus

Always quote units (g/100 g water) and the temperature. Read graphs carefully from the axes, and show subtractions when asked for the mass of crystals formed. Remember that scaling is needed when the mass of water is not 100 g.

Advanced insight

Some substances, such as sodium sulfate, show a kink in their solubility curve: the solubility rises and then falls. This happens because a different form of the solid (with or without water of crystallisation) is stable above and below a certain temperature, and each form has its own solubility.

Summary

Solubility is the maximum mass of solute that dissolves in 100 g of solvent at a stated temperature. Most solids become more soluble as temperature rises, some steeply and some hardly at all; gases become less soluble. Solubility curves let you read solubilities and calculate the mass of crystals that form when a saturated solution cools.

Practice questions

1. Define solubility. Answer: The maximum mass of a solute that dissolves in 100 g of solvent at a particular temperature. 2. Using the table, find the solubility of potassium nitrate at 40 °C and state how much would dissolve in 50 g of water. Answer: About 64 g per 100 g; so about 32 g in 50 g of water. 3. Why is crystallisation by cooling a poor way to obtain sodium chloride? Answer: Its solubility changes very little with temperature, so cooling produces very few crystals. 4. Explain why fizzy drinks go flat faster when warm. Answer: Carbon dioxide, like other gases, is less soluble at higher temperatures, so it escapes from warm drinks more quickly. 5. A saturated potassium nitrate solution in 200 g of water is cooled from 80 °C to 40 °C. Estimate the mass of crystals formed. Answer: Per 100 g: 170 − 64 = 106 g; for 200 g of water, about 212 g.