Dissolving

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Key points

  • A solution is made when a dissolves into a .
  • If a substance can into a solvent, it is . If it cannot dissolve, it is described as insoluble.
  • Heating, stirring and using fine powders are all ways to speed up dissolving.

When sugar is added to hot water and stirred, it seems to disappear. What has really happened to it?

It has dissolved. The sugar crystals break into tiny particles which are too small to see and spread out through the hot water.

Video

Watch this video to find out about how are made from a solute and a solvent.

Solubility with Jon Chase

What happened to the solubility of sugar when the temperature of the tea was hottest?

The solubility increased with temperature. The hotter the water, the more sugar can dissolve.

What is a solution?

A solution is made when a substance into a liquid. The liquid is called the solvent. The substance that has been dissolved is called the solute.

A solution can also be called a mixture.

When a solid dissolves, it breaks down into smaller particles that spread out through the solvent.

Example

Salt dissolves when it is stirred into water. In sea water, the water is the solvent and the salt is the solute.

In sea water, the water is the solvent and salt is the solute

Coloured and colourless solutions

When a white solid dissolves, it makes a colourless , for example, salt or sugar in water.

When a coloured solid dissolves, it makes a coloured solution, for example, copper sulphate dissolves to makes a blue solution.

Saturation

There is a limit to the mass of that will dissolve in a particular volume of the . When no more solute dissolves, the solution is saturated.

copper sulfate solution

When copper sulfate is added to water to make copper sulfate, which substance is the solvent, and which is the solute?

Copper sulfate is the solute. Water is the solvent.

How do particles behave in a solution?

What happens to particles of a solid solute when it dissolves?

Figure 1 - The solvent is water shown here as blue particles. The solute is solid shown here as a square of red particles.
Figure 2 - The solute is added to the solvent.
Figure 3 - The individual solute particles break apart and spread out.
Figure 4 - The solute is now fully dissolved. The solute particles are widely spaced throughout the solvent.

A red solute is dissolved into a colourless solvent making a pink solution. Why do you think it is pink?

Because the red solute has broken apart and the particles are now spread throughout the solvent.

Insoluble or soluble?

If a solid does not dissolve in a specific solvent, it is in that solvent. Some examples include:

  • Wax is insoluble in water.
  • Chalk is insoluble in water.
  • Salt is insoluble in propanone.
  • Polystyrene is insoluble in water, but soluble in propanone.

When an insoluble white powder is stirred into a colourless liquid it does not dissolve. At first the liquid will appear to turn white, but after a few minutes the liquid will return to colourless as the powder settles on the bottom of the container.

Did you know?

Chalk is insoluble in water. If chalk is added to water and stirred, the chalk will eventually settle at the bottom of the glass.

The solubility of gas

measures how much solute can dissolve in a volume of solvent at a specific temperature. For example, sugar has a higher solubility in water than salt. Therefore, more sugar than salt can dissolve in a volume of water at a specific temperature.

You can speed up a solid by:

  • Heating up the solvent. The solubility of a solid solute usually increases when the temperature increases.
  • Stirring.
  • Using fine powder rather than large pieces of solute.

The solubility of a gas usually decreases when the temperature increases.

For example, fizzy drinks have lots of carbon dioxide dissolved in them when they are cold. Fizzy drinks warm up quickly in your mouth and release bubbles of carbon dioxide gas.

Working scientifically

Planning an experiment

Often experiments always involve things that can change, known as variables. Variables need to be identified, so they can then either be changed or controlled.

Plan an experiment to investigate one of the factors that affects solubility.

Make sure that you only change one variable, and keep the others constant for a valid investigation.

What will you measure, known as the ?

Write your method as a numbered list of brief instructions.

When investigating how temperature affects the solubility of salt in 100 cm of water, what would be the independent variable, the dependent variable and the control variables?

  • The independent variable would be the temperature of the water.
  • The dependent variable would be the amount of salt you can dissolve in 100 cm³ of water - ideally measured in grams.
  • The control variables that would need to be kept the same are the size of the salt crystals and the speed of stirring.

Find out more about planning an experiment.

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