Factors Affecting the Rate of Dissolving
The rate of dissolving is how quickly a solute dissolves in a solvent, and it is increased by higher temperature, stirring and a larger surface area of the solute. In brief: powdered sugar dissolves faster than a sugar cube, and a cube dissolves faster in hot tea that is being stirred than in cold tea left alone.
The rate of dissolving is different from solubility. Solubility tells how much solute can dissolve at most, whereas the rate tells how fast it dissolves. This article explains the main factors, why each works on the particle level, how to design a fair experiment, a worked example with data and the usual errors that students make.
What is the difference between rate and solubility?
Solubility is a limit: the maximum mass of solute that dissolves in a given amount of solvent at a given temperature. The rate of dissolving is a speed: how quickly the solute disappears into the solvent. A factor that speeds up dissolving does not necessarily change the final amount that can dissolve. Stirring, for instance, does not increase the solubility of sugar, but it helps sugar reach its limit sooner.
How does temperature change the rate?
At higher temperatures, particles of the solvent move faster and collide with the surface of the solute with more energy and more often. They pull particles out of the solid more quickly, so dissolving is faster. Hot tea dissolves sugar faster than iced tea for this reason.
For most solids temperature also increases solubility, so both effects work together. For gases the effect is the opposite: warming makes gases dissolve less.
How does stirring change the rate?
Without stirring, a layer of solution that is nearly saturated forms around the solid. This layer slows the dissolving because the solvent next to the solid cannot take up more solute. Stirring (or shaking) moves this layer away and brings fresh solvent to the surface, so dissolving continues quickly.
How does particle size change the rate?
Dissolving takes place at the surface of the solid. Crushing a solid into smaller pieces increases the total surface area that touches the solvent, so more particles can leave at the same time. Fine granules dissolve faster than large crystals of the same mass.
The effect can be large. A cube with sides of 2 cm has a surface area of 6 × 2 × 2 = 24 cm2. If it is cut into eight cubes with sides of 1 cm, the total surface area is 8 × 6 × 1 × 1 = 48 cm2, twice as much.
Summary of the factors
| Factor | Change | Effect on rate | Reason |
|---|---|---|---|
| Temperature | Increase | Faster | Faster particle motion, more energetic collisions |
| Stirring | Stir | Faster | Replaces saturated layer with fresh solvent |
| Particle size | Crush | Faster | Larger total surface area |
| Amount already dissolved | Near saturation | Slower | Solution can accept less solute |
How do you test the factors fairly?
A fair test changes only one variable at a time and keeps all others constant.
- Choose the variable to test, for example temperature.
- Use the same mass of solute, the same volume of solvent and the same form of solid in every trial.
- Start a timer when the solid is added, and stop it when no solid is visible.
- Repeat each condition at least three times and calculate the average time.
Worked example: reading experimental data
A student dissolves 5.0 g of sugar in 100 mL of water under four conditions and records the time.
| Trial | Sugar form | Water | Stirred? | Time (s) |
|---|---|---|---|---|
| A | Cube | Cold (10 °C) | No | 600 |
| B | Cube | Hot (60 °C) | No | 240 |
| C | Cube | Hot (60 °C) | Yes | 90 |
| D | Powder | Hot (60 °C) | Yes | 20 |
- Effect of temperature (A and B, only temperature differs): 600 s becomes 240 s, a speed-up of 600 ÷ 240 = 2.5 times.
- Effect of stirring (B and C): 240 s becomes 90 s, a speed-up of 240 ÷ 90 ≈ 2.7 times.
- Effect of particle size (C and D): 90 s becomes 20 s, a speed-up of 90 ÷ 20 = 4.5 times.
- Overall. Between A and D, the dissolving was 600 ÷ 20 = 30 times faster.
These numbers are illustrative, not measured values for every kitchen, but they show how the factors combine. Each of the three factors makes dissolving faster, and together they have an even greater effect.
How did chemists study reaction rates?
The study of how quickly processes happen is called kinetics. In the 1800s, scientists such as Wilhelm Ostwald and Svante Arrhenius linked the rate of changes to temperature and the energy of collisions between particles. Arrhenius proposed in 1889 an equation that shows how rates grow rapidly with temperature, and the same ideas of collision frequency and energy explain why heating makes dissolving faster.
Which mistakes are common?
Mistake 1: Treating rate and solubility as the same thing
Stirring speeds up dissolving but does not allow more sugar to dissolve at the same temperature.
Mistake 2: Changing several variables at once
If a student uses hot water and a powder at the same time, it is impossible to know which factor was responsible.
Mistake 3: Believing that a larger piece has a larger surface in proportion
When mass is kept the same, many small pieces have more total surface than one large piece.
Mistake 4: Assuming that stirring heats the solution
Stirring does not warm the solution in any significant way. It only moves the liquid.