GCSE Biology - AQA

3.1.2 - Concentration

# Concentration

Concentration is a measure of how closely packed together the particles of a substance are.

The word 'concentration' refers to how closely packed together the particles of a substance are.

The concept of concentration applies to many different kinds of substances. However, in this course we are mainly going to focus on the concentrations of two kinds of substances:

• Gases
• Substances in a solution (that is, solutes or solvents)

The concentration of a substance is usually defined as the number of particles of the substance in a particular volume - usually a litre.

For a gas, concentration is the number of particles of the gas per litre of gas.

For a substance in a solution, concentration is the number of particles of the substance per litre of the solution.

Concentration is often defined as the number of particles of a substance per litre.

For example, imagine a solution made by dissolving a teaspoon of sugar in a litre of water, and a second solution made by dissolving two teaspoons of sugar in a litre of water. The concentration of sugar in the second solution would be double the concentration of sugar in the first solution.

Solution A is made by adding a spoonful of sugar to a litre of water. Solution B is made by adding two spoonfuls of sugar to a litre of water. The concentration of solution B is double the concentration of solution A.

## Dilute and concentrated solutions

A solution with a lower concentration is described as being more dilute, and a solution with a higher concentration is described as being more concentrated. So in the example above, we could say that the first sugar solution is more dilute and the second sugar solution is more concentrated.

A solution with a lower concentration is described as being 'more dilute' and a concentration with a higher concentration is described as being 'more concentrated'.

## The volume of a solution is roughly equal to the volume of the solvent

When you dissolve a solute in a solvent, the volume of the solvent does not change very much, because the solute particles fit in between the solvent particles.

The new bonding arrangements formed as the solute particles dissolve in the solvent can either cause the solvent particles to move very slightly closer together or very slightly further apart (depending on what the solute and the solvent are).

However, either way, the change in volume is tiny, so the volume of the solution formed is essentially the same as the volume of the solvent was before it had the solute dissolved in it.

So in the example above, when the sugar was dissolved in 1 litre of water, we can assume that the volume of the solution formed was also 1 litre (regardless of the amount of sugar added).

The volume of a solution is usually roughly the same as the volume of the solvent. Dissolving solutes in a solvent does not usually change its volume by a significant amount.

When there is a difference in concentration between two areas, this is described as a concentration gradient.

For example, imagine a unicellular organism floating in a pond. The organism's cytoplasm has a relatively high salt concentration, and the pond water has a relatively low salt concentration (that is, the cell's cytoplasm is more concentrated and the pond water is more dilute). We can therefore say that there is a concentration gradient between the organism's cytoplasm and the pond water.

A concentration gradient is a difference in concentration between two areas. In this example, there is a concentration between the cell's cytoplasm (which has the higher concentration) and the surrounding pond water (which has the lower concentration).

If a substance moves from an area of higher concentration to an area of lower concentration, it is described as moving down the concentration gradient. For example, if salt moved out of the cell into the pond water, we would say that salt was moving down the concentration gradient.

When a substance moves from an area of higher concentration to an area of lower concentration, this is described as moving down the concentration gradient.

If a substance moves from an area of lower concentration to an area of higher concentration, it is described as moving against the concentration gradient. For example, if salt from the pond water moved into the cell, we would say that salt was moving against the concentration gradient.

When a substance moves from an area of lower concentration to an area of higher concentration, this is described as moving against the concentration gradient.

## Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

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What is concentration?

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What do the terms 'dilute' and 'concentrated' mean?

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