Factors Affecting Solubility
In 1888, French chemist Henri Le Chatelier showed that cooling water to 4 °C and raising pressure 5-fold forced 5× more CO₂ to dissolve — the same principle used to carbonate every fizzy drink sold today.
Printable Worksheets
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Q1 · You drop one sugar cube into Cup A and the same mass of caster sugar into Cup B. Both are at room temperature, both unstirred. Which will dissolve faster, and why?
Q2 · A friend says "Stirring makes more sugar dissolve in a cup." Are they right that stirring increases the total amount that can dissolve? Or does stirring only change something else?
● Know
- The three factors: temperature, surface area, stirring
- Solid solutes dissolve MORE in hot water; gases dissolve LESS
- Surface area and stirring change the RATE, not the maximum amount
● Understand
- Why hot water makes fizzy drinks go flat faster
- The difference between independent, dependent and controlled variables
- Why a "fair test" only changes one variable at a time
● Can do
- Design a fair test to investigate one solubility factor
- Identify the IV, DV and control variables in a given experiment
- Read a simple solubility data table and describe the trend
- Solubility
- Independent variable
- Dependent variable
- Controlled variable
- Surface area
- The variable you measure
- How much solute can dissolve at a given temperature
- The total outside area of a solid touching the solvent
- The one variable you deliberately change
- A variable you keep the same
Fill two glasses — one with cold water, one with hot water — and add a teaspoon of salt to each. The salt vanishes faster in the hot glass, but more importantly, if you keep adding salt, the hot water will dissolve noticeably more before grains start sitting on the bottom. Temperature is the only factor that changes the maximum amount of solute that will dissolve — not just the speed.
For most solid solutes in water:
- Higher temperature → more dissolves.
- Particle reason: hot water particles move faster, knocking solute loose more easily and leaving bigger gaps.
For most gas solutes in water:
- Higher temperature → less dissolves.
- Particle reason: gas particles need calm spaces to sit between water molecules. Hot, jittery water shakes them back out.
That's why a warm cola goes flat fast: the dissolved CO₂ escapes much more quickly when the water is hot. It's also why rivers in hot summers carry less dissolved oxygen, which is bad news for fish.
| Temperature | Sugar in 100 mL water (g) | Oxygen in 1 L water (mg) |
|---|---|---|
| 10°C | ~191 | ~11.3 |
| 20°C | ~204 | ~9.1 |
| 40°C | ~238 | ~6.4 |
| 80°C | ~362 | ~3.0 |
Hot water dissolves sugar than cold water. But hot water dissolves oxygen than cold water. That is why warm fizzy drinks go faster than cold ones — the escapes more easily.
Take 5 g of sugar as a single cube. Now take 5 g as caster sugar (powder). Same mass, but the powder has thousands of times more outside surface exposed to the water.
Water particles can only attack the surface of the solid. More surface = more places to be attacked at the same time = faster dissolving.
BUT — surface area only changes the rate (how fast it dissolves). It does NOT change the maximum amount that can dissolve. A sugar cube and a spoon of caster sugar (same mass) will end up at the same saturation point. The powder just gets there much faster.
This is why recipes say "use caster sugar" or "use crushed ice" when you need things to mix quickly.
When sugar dissolves, the water particles right next to the grain become full of dissolved sugar. They become "tired" — they have less room to grab any more sugar.
Stirring sweeps these full water particles away from the grain and brings in fresh, empty water particles. Empty water particles can keep dissolving more sugar quickly.
Like surface area, stirring only changes the rate. It does not change the maximum amount that will dissolve. Stir an unsaturated solution and it dissolves faster. Stir a saturated solution and the extra solute still sits at the bottom — there's just nowhere for it to go.
If you want to find out which factor really controls dissolving, you have to change only one thing at a time. That's the heart of a fair test.
- Independent variable (IV) — the one thing you change.
- Dependent variable (DV) — what you measure to see if the change had an effect.
- Controlled variables — everything else you keep the same.
Example experiment: Does temperature affect how fast sugar dissolves?
| Role | Variable |
|---|---|
| IV | Temperature of water (10°C, 20°C, 40°C, 60°C) |
| DV | Time taken for 5 g sugar to fully dissolve (seconds) |
| Controlled | Mass of sugar (5 g), volume of water (100 mL), particle size (caster), stirring speed (none), type of cup |
You then repeat each temperature 3 times (replicates) and take the average. The repeats make the data more reliable.
| Temp (°C) | Trial 1 (s) | Trial 2 (s) | Trial 3 (s) | Average (s) |
|---|---|---|---|---|
| 10 | 180 | 175 | 185 | 180 |
| 20 | 120 | 115 | 125 | 120 |
| 40 | 60 | 55 | 65 | 60 |
| 60 | 30 | 32 | 28 | 30 |
Trend: As temperature increases, the time taken to dissolve decreases. That answers the question — temperature does affect the rate of dissolving.
Wrong: "Stirring lets me dissolve more sugar in my drink." Stirring only makes it dissolve faster. The max amount is set by the temperature.
Right: Stirring affects rate, not maximum. To dissolve more, you need to heat the water (for solid solutes).
Wrong: "Hot water dissolves more of EVERYTHING." Solid solutes — yes. Gas solutes — the opposite. Hot water releases dissolved gases, which is why warm fish tanks need pumps and warm fizzy drinks go flat.
Right: Solid solutes: more dissolves when hot. Gas solutes: less dissolves when hot.
Wrong: "If I change the temperature AND the surface area, my fair test still works." If you change two things at once, you can't tell which one caused the effect. That's not a fair test.
Right: A fair test changes only one variable at a time. Everything else must be controlled.
A student investigates the effect of surface area on rate of dissolving. They time how long it takes 5 g of sugar to dissolve in 100 mL of water at 20°C, no stirring. Their data: cube (sugar lump) = 240 s, granulated sugar = 90 s, caster sugar = 35 s, icing sugar = 12 s. Predict: based on this data, what is the trend, AND what would happen if the experiment was repeated with the cup STIRRED?
How close was your prediction?
Nice — you spotted that surface area and stirring stack up to speed things up.
Good — keep this idea: surface area and stirring change the rate, temperature changes the limit.
At the start of this lesson you were asked: Open a warm fizzy drink and it goes flat fast — why does heat kill the fizz when it helps sugar dissolve? That seemed like a contradiction, right?
Now explain why heat affects dissolved gases differently from dissolved solids. Use the words rate, maximum, temperature and solubility in your answer.
Q1. Name the three main factors that affect dissolving. For each, state whether it changes the rate, the maximum amount, or both. (3 marks)
Q2. A student wants to find out whether smaller sugar pieces dissolve faster. Design a fair test by stating the IV, DV and four controlled variables. (4 marks)
Q3. Use the temperature/solubility table from the lesson. Describe the trend for sugar AND the trend for oxygen as temperature increases. Then explain at the particle level why the two trends are opposite. (4 marks)
Answers
▾MCQ 1
B — Only temperature changes the maximum amount that can dissolve. Surface area and stirring change rate; the cup material doesn't matter.
MCQ 2
D — Warm water dissolves LESS gas, so dissolved CO₂ leaves the cola more easily as bubbles.
MCQ 3
C — The DV is what you measure: time taken to dissolve. Temperature is the IV; mass and cup type are controlled.
MCQ 4
A — Smaller pieces have more total outside surface in contact with the solvent. More surface means more places to be attacked at once, so dissolving is faster.
MCQ 5
B — Repeats and averages reduce random error and improve reliability. They don't prove the result, but they make it less likely to be a fluke.
Short Answer 1
Model answer: The three factors are temperature (changes both rate AND maximum amount that dissolves), surface area (changes rate only) and stirring (changes rate only). Only temperature increases the saturation limit.
Short Answer 2
Model answer: IV = particle size (e.g. cube, granulated, caster, icing sugar). DV = time taken for the sugar to fully dissolve, in seconds. Controlled variables: mass of sugar (e.g. 5 g), volume of water (e.g. 100 mL), water temperature (e.g. 20°C), no stirring (or the same number of stirs per minute), the same type of cup used each time. Repeat each particle size 3 times and take the average for reliability.
Short Answer 3
Model answer: As temperature increases, the mass of sugar that dissolves increases (from ~191 g at 10°C to ~362 g at 80°C). The mass of oxygen that dissolves decreases (from ~11.3 mg/L at 10°C to ~3.0 mg/L at 80°C). The reason is that hot water particles move faster — they knock solid sugar particles loose more easily (so more solid dissolves), but they also shake dissolved gas particles back out of solution (so less gas stays dissolved).