3.2 A solution is a solute dissolved in a solvent

3.2

A solution is a solute dissolved in a solvent

Learning intentions

By the end of this topic, you will be able to: • contrast the terms ‘soluble’ and ‘insoluble’,

‘solute’ and

‘solvent’, and

‘concentrated’ and ‘dilute’ • describe the properties of a saturated solution • explain the importance of water as a solvent.

Key ideas

• A solvent can be used to dissolve a soluble solute. • A dilute solution has very little solute in the solvent. • The more solute that is dissolved in the solvent, the more concentrated the solution. • A solution is saturated when no more solute will dissolve.

soluble can be dissolved in a liquid

solute a substance that dissolves in a liquid (solvent)

solvent a liquid in which other substances dissolve

dilute containing a small number of solute particles in the volume of solution

concentrated containing a large number of solute particles in the volume of solution

Solvent particles

Solute particles

Figure 1 A solute dissolves in a solvent to create a solution. Figure 2 The concentration of salt in the Dead Sea is so high that when people try to swim in it, they float instead because their body is less dense than all that salt!

Solubility and insolubility Working with solutions

In some places in Australia, the water from You have seen that a solution is a solute the local water supply has an unpleasant taste, dissolved in a solvent (Figure 1). Solutions or washing with soap is diffi cult because the can be compared in terms of their water forms a scum instead of a foamy lather. concentration: how much solute is in the In these cases, the water contains metal salts solvent. If just a little solute is dissolved, that affect its taste and behaviour. Because they the solution is described as dilute (low are so small, these metal salts do not fall to the concentration). If a lot of a solute is dissolved, bottom or fl oat on the top; instead, they remain the solution is described as concentrated evenly spread through the liquid. The resulting (high concentration). This is shown in mixture (a solution) is clear – light will shine Figures 2 and 3. through it. We say that the metal salts have dissolved in the water. A substance that is able to dissolve in a liquid is considered to be soluble , whereas a solid that cannot dissolve is described as insoluble. The substance dissolving is called the solute , and the liquid into which it dissolves is called the solvent . An example of this is salty water. The salt is the solute, and the water is the solvent. Sometimes it is necessary to help a solute such as salt to dissolve. Warming the solvent (water) is the most common way of making a solute dissolve faster.

Solute particle Dilute solution Concentrated solution Figure 3 A concentrated solution contains more solute particles in a given volume than a dilute solution. DRAFT

It is only possible to dissolve a certain amount of a particular solute in a solvent. If no more solute can dissolve into a solution, the solution is described as saturated . What sort of cordial drink do you prefer: dilute, concentrated or saturated? We often work with solutions in our everyday lives. By adding solutes to pure liquids, the properties of the pure liquids may change. An example is adding bath crystals to a bath to give the water a pleasant smell.

Water as a solvent

Water is a good solvent. This is one of its most important properties. Our digestive system uses water to dissolve our solid and liquid food, and to break up the food into nutrients that our body needs to build new cells, grow and repair.

Our bodies are more than 60 per cent water. Our blood, which is mainly water, transports oxygen to every cell and carries away dissolved carbon dioxide gas (a waste product). Humans are not the only living things that depend on water as a solvent. Without water’s ability to dissolve gases, there would be no underwater life in our oceans and lakes and no fi sh in the rivers. These creatures all live by extracting dissolved oxygen gas from the water (Figure 4). Imagine that you found a colourless and see-through liquid and were really thirsty. Is it water? There are many other colourless and clear liquids, and you do not know what substances might be dissolved in them. Tasting may be dangerous. There are more scientifi c ways of working out whether a liquid is pure. This is explored further in Topic 3.3. DRAFT 3.2 Check your learning

Retrieve

1 Defi ne the following ‘s’ words used in this section: solute, solvent, solution, soluble and saturated. 2 Someone asks for a dilute glass of cordial.

Identify if you would add a lot of cordial or only a little.

Comprehend

3 Describe how you could increase the amount of a solute that will dissolve in a solvent. 4 Explain if you can see the particles of a solute in a solution. 5 Explain why water cannot be a solute for all substances. 6 Explain what happens to sugar particles when they dissolve in water.

Analyse

7 Compare the solubility of particles in suspensions, colloids, emulsions and solutions.

Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level. saturated describes a solution in which no more solute can be dissolved

Figure 4 Oxygen dissolved in water is essential for aquatic organisms.