Solutions and their concentration

The parts of a solution

When salt dissolves in water, we get a solution. A solution is a homogeneous mixture of two or more substances.

• The substance that is dissolved is the solute. Salt is the solute.
• The substance in which it dissolves is the solvent. Water is the solvent. The solvent is usually present in the larger amount.

Solutions are not only a solid in a liquid. Soda water is a gas (carbon dioxide) in a liquid. Air is a mixture of gases. A solution of a liquid in a liquid is also possible, such as vinegar in water. Brass is a solution of a solid in a solid.

The properties of a solution

• It is homogeneous. The composition is the same everywhere.
• The particles of the solute are very small, less than $1\ \text{nm}$ across, so we cannot see them with the eye or with a microscope.
• It is stable. The solute does not settle at the bottom if we leave the solution still.
• A solution does not scatter a beam of light. The beam cannot be seen as it goes through.
• The solute cannot be separated from the solution by filtering. The particles pass through the holes of filter paper.

How much is dissolved? The concentration

Two glasses of salty water can be salty in different degrees. The concentration of a solution is the amount of solute in a certain amount of solution. There are three common ways to give it.

1. Mass by mass percentage.

The mass of the solution is the mass of the solute plus the mass of the solvent.

Example. $5\ \text{g}$ of salt is dissolved in $95\ \text{g}$ of water. The solution is $100\ \text{g}$. The concentration is $\dfrac{5\ \text{g}}{100\ \text{g}} \times 100 = 5\ \%$.

2. Mass by volume percentage. It is the mass of solute (in grams) in $100\ \text{mL}$ of solution. It is often used for a solid in a liquid.

3. Volume by volume percentage. It is the volume of solute in $100\ \text{mL}$ of solution. It is used for a liquid in a liquid, such as $20\ \text{mL}$ of alcohol in $80\ \text{mL}$ of water, which is $20\ \%$.

Saturated and unsaturated solutions

Add salt to water, a little at a time, and stir. At first, it all dissolves. Later, no more dissolves, and extra salt stays at the bottom.

• A solution that can still dissolve more solute, at that temperature, is unsaturated.
• A solution that cannot dissolve any more solute at that temperature is saturated.

The solubility of a substance is the greatest mass of it that dissolves in $100\ \text{g}$ of the solvent, at a given temperature, to make a saturated solution. For some solutes, in water at $293\ \text{K}$ ($20\ ^\circ\text{C}$):

The effect of temperature

For most solids, the solubility rises when the temperature rises. We can dissolve more sugar in hot tea than in cold tea. For a gas, the solubility falls when the temperature rises. This is why a warm soft drink loses its gas faster than a cold one.

You should find that heating water from $20\ ^\circ\text{C}$ to $60\ ^\circ\text{C}$ more than triples the potassium nitrate it can hold, while the salt hardly changes.

Practise with new numbers

Try it in Python

This program finds the concentration of a salt solution in three ways.

Remember these

• A solution has a solute that is dissolved in a solvent. It is homogeneous, stable, and it does not scatter light.
• Mass by mass $\% = \dfrac{\text{mass of solute}}{\text{mass of solution}} \times 100$.
• A saturated solution cannot dissolve more solute at that temperature. The solubility of most solids rises with temperature.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. $8\ \text{g}$ of salt is dissolved in $152\ \text{g}$ of water. What is the mass by mass percentage?

2. What are the solute and the solvent in sea water?

3. At $20\ ^\circ\text{C}$, $36\ \text{g}$ of salt dissolves in $100\ \text{g}$ of water. Is a solution of $30\ \text{g}$ of salt in $100\ \text{g}$ of water saturated?

4. Why does a cold drink stay fizzy longer than a warm one?