Aerosols and pressure. Solutions and suspensions.

2. Particle matters

Solutions and suspensions

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	What does dissolve mean?
		A solid dissolves in a liquid when it mixes completely with the liquid. The solid has broken down into pieces so small that its particles spread all through the new mixture. The solid and liquid will stay mixed without shaking. The original solid breaks apart into pieces not much bigger than a few atoms or ions.

Picture 2.8 Sodium chloride in water: The moving water particles attach themselves to the solid particles and drag the ions away to form a solution.

What’s this?

	What special words are used when talking about dissolving?
		The substance that is dissolved is the solute. It dissolves into the solvent (often a liquid), to form a solution.

	What happens when salt dissolves?
		Salt is an ionic compound - sodium chloride (Na⁺ ions and Cl^- ions). Water is a polar liquid. Step 1. The moving water particles attach themselves to the solid particles (because they are polar). Step 2. The water molecules drag the ions away - because they are moving. Step 3 The ions go into solution.

What sorts of substances dissolve?

Usually we think of liquid solvents and solid solutes. However, gases or liquids can be solutes too. Carbon dioxide gas is dissolved in a fizzy drink and is only released when you open the bottle; the alcohol (liquid ethanol) that is in wine is dissolved in water.

Solubility

Instant coffee dissolves easily in warm water. However, it is difficult to get a spoonful to dissolve in cold water. Only a certain amount will dissolve. It has a limited solubility. The solubility of solids increases with temperature.

When gases (like oxygen or carbon dioxide) dissolve in water, their solubility decreases with temperature. So when you heat water, you often see bubbles forming as the gases come out of solution.

Water as a solvent

Water is a good solvent - especially of ionic compounds like salt (sodium chloride). However, often it will not dissolve molecular materials, especially liquids like oil. We say that the two liquids are not miscible. They do not mix together or dissolve - they form two separate layers.

Picture 2.9 Oil and water don't mix. They are immiscible. They form two separate layers. Shaking them will mix them for a short time.

What is a suspension?

A suspension is different from a solution. The solid particles have not dissolved but are suspended in the liquid. Suspended literally means that they are being held up or supported.

The individual pieces of solid are much smaller than the eye can see. However, unlike a dissolved solid, each piece has loads of atoms. They mix completely with the liquid. The viscosity of the liquid supports the solid pieces and stops them falling to the bottom. However, unlike a solution, the suspension may not stay mixed without shaking – after a while, the solid pieces might fall to the bottom.

The solid pieces can be separated from the suspension by passing it through a filter.

Picture 2.10 Deoderant aerosols contain a suspension. The deoderant is a solid suspended in propellant. The particles will begin to settle and eventully fall to the bottom. It needs to be shaken to reform the suspension.

Summary Close

a solution is a complete mixture of (usually) a solid in (usually) a liquid
the solid particles are broken down into pieces a few atoms big
a suspension is larger lumps of solid floating in a liquid