Chemical Solutions

Level 1

What is a solution?

In chemistry, a solution is a homogeneous mixture between two chemical species. This means that the components cannot be distinguished with the naked eye: everything is perfectly mixed. 
The term "solution" is mainly used for liquid mixtures. When a solution's solvent is water, it is called an aqueous solution. 

  Example: When you pour syrup into a glass of water and mix, you get a uniform liquid; it is impossible to distinguish the water from the syrup.

Different types of solutions

There are several ways to form a solution:

  • Liquid + liquid: limited by miscibility (ability of two liquids to mix). Example: water and vinegar.
  • Solid + liquid: limited by the solubility of the solid. Example: salt in water.
  • Gas + liquid: limited by the solubility of the gas. Example: carbon dioxide dissolved in a fizzy drink.

Miscibility

Miscibility is the ability of two liquids to mix together. 

  • If they are miscible → the mixture is homogeneous, it is impossible to distinguish the two liquids.
  • If they are immiscible → they remain separate, often in distinct layers. 

  Example : 
- Water + vinegar: miscible (homogeneous mixture). 
- Water + oil: immiscible (two visible layers).

Why do some liquids mix?

This depends on the chemical nature of the substances. Generally, two liquids with a similar chemical nature are miscible. Otherwise, they do not mix.

Dissolution

Dissolution is the process by which a substance (the solute) disperses in a liquid (the solvent), forming a homogeneous solution.

  • The solvent: majority species, often a liquid. E.g.: water.
  • The solute: minority species, which is dissolved. E.g.: salt, sugar, CO₂.

  Example: When sugar is put into coffee, the grains disappear and spread uniformly: this is dissolution.

Solubility

Solubility indicates the maximum amount of solute that can be dissolved in one litre of solvent, at a given temperature. 

  • It is expressed in grams per litre (g/L).
  • If this value is exceeded, the excess solute does not dissolve. The solution is said to be saturated. (this is saturation)

   Example: 
- Solubility of salt in water: 365 g/L → in 1 L of water, up to 365 g of salt can be dissolved. 
- Solubility of CO₂ in water: 1.688 g/L → much lower, hence the rapid loss of bubbles in an open drink.

Insoluble substances

Some substances do not dissolve at all in a given solvent; they are said to be insoluble. Example: sand in water.

Dissolution and solubility

Add a solid to water, stir the beaker and observe what dissolves or remains at the bottom.

Unsaturated solution

Amount dissolved 0 grains
Solid at the bottom 0 grains
Capacity before saturation 0 %

Stirring speeds up dissolution, but does not change the maximum amount that can be dissolved.

What is concentration?

The concentration of a solution indicates the amount of solute dissolved in a given amount of solvent. The higher the concentration, the more solute the solution contains. Concentration can be expressed in several ways:

  • Mass concentration: it is the most commonly used in middle school. It corresponds to the mass of solute dissolved in 1 litre of solution.

It is expressed in grams per litre (g/L).

Mass concentration formula

Mass concentration is calculated using the formula:

$$C =\frac{m}{V}$$

where: 

  • $C$ = mass concentration (in g/L)
  • $m$ = mass of dissolved solute (in g)
  • $V$ = volume of solution (in L) 

  Example: 20 g of sugar is dissolved in 500 mL of water. 
We know that 
$m = 20 g$ 
$V = 0.5 L$
$C= \frac{m}{V}$

So $C = \frac{20}{0.5} = 40 g/L.$ 

The sugar concentration of the solution is 40 g/L.

Dilution and concentration

Solutions of different concentrations can be prepared: 

  • By adding more solute, the solution becomes more concentrated.
  • By adding more solvent, the solution becomes less concentrated (this is a dilution). 

  Example: If you add water to a very sweet fruit juice, it becomes less sweet. Therefore, you have diluted the solution.