Introduction and key concepts

Introduction

We are surrounded by different types of solutions in our daily lives. Any solution is made up of a solute and a solvent . A solute is a substance that dissolves in a solvent. In the case of a salt (NaCl) solution, the salt crystals are the solute. A solvent is the substance in which the solute dissolves. In the case of the NaCl solution, the solvent would be the water. In most cases, there is always more of the solvent than there is of the solute in a solution.

Solutes and solvents

A solute is a substance that is dissolved in another substance. A solute can be a solid, liquid or gas. A solvent is the liquid that dissolves a solid, liquid, or gaseous solute.

Types of solutions

When a solute is mixed with a solvent, a mixture is formed, and this may be either heterogeneous or homogeneous . If you mix sand and water for example, the sand does not dissolve in the water. This is a heterogeneous mixture. (Hetero is Greek for different). When you mix salt and water, the resulting mixture is homogeneous because the solute has dissolved in the solvent. (Homo is Greek for the same).

Solution

In chemistry, a solution is a homogeneous mixture that consists of a solute that has been dissolved in a solvent.

A solution then is a homogeneous mixture of a solute and a solvent. Examples of solutions are:

• A solid solute dissolved in a liquid solvent e.g. sodium chloride dissolved in water.
• A gas solute dissolved in a liquid solvent e.g. carbon dioxide dissolved in water (fizzy drinks) or oxygen dissolved in water (aquatic ecosystems).
• A liquid solute dissolved in a liquid solvent e.g. ethanol in water.
• A solid solute in a solid solvent e.g. metal alloys.
• A gas solute in a gas solvent e.g. the homogeneous mixture of gases in the air that we breathe.

While there are many different types of solutions, most of those we will be discussing are liquids .

Forces and solutions

An important question to ask is why some solutes dissolve in certain solvents and not in others. The answer lies in understanding the interaction between the intramolecular and intermolecular forces between the solute and solvent particles.

Experiment : solubility

Aim:

To investigate the solubility of solutes in different solvents.

Apparatus:

Salt, vinegar, iodine (CAUTION! Iodine stains the skin.) , ethanol

Method:

1. Mix half a teaspoon of salt in 100cm ${}^3$ of water
2. Mix half a teaspoon of vinegar (acetic acid) in 100cm ${}^3$ of water
3. Mix a few grains of iodine in ethanol
4. Mix a few grains of iodine in 100cm ${}^3$ of water

Results:

Record your observations in the table below:

You should have noticed that in some cases, the solute dissolves in the solvent, while in other cases it does not.

Conclusions:

In general, polar and ionic solutes dissolve well in polar solvents, while non-polar solutes dissolve well in non-polar solvents. An easy way to remember this is that 'like dissolves like', in other words, if the solute and the solvent have similar intermolecular forces, there is a high possibility that dissolution will occur. This will be explained in more detail below.

• Non-polar solutes and non-polar solvents (e.g. iodine and ether) Iodine molecules are non-polar, and the forces between the molecules are weak van der Waals forces. There are also weak van der Waals forces between ether molecules. Because the intermolecular forces in both the solute and the solvent are similar, it is easy for these to be broken in the solute, allowing the solute to move into the spaces between the molecules of the solvent. The solute dissolves in the solvent.
• Polar and ionic solutes and polar solvents (e.g. salt and water) There are strong electrostatic forces between the ions of a salt such as sodium chloride. There are also strong hydrogen bonds between water molecules. Because the strength of the intermolecular forces in the solute and solvent are similar, the solute will dissolve in the solvent.