Get tutored by the creator of Mooramo

As well as making Mooramo, I also tutor Science and Maths in London and online.

To find out more, visit my website, georgewellertutoring.co.uk

Concentration

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

Diagram illustrating the concept of concentration. The diagram consists of two rectangles, one of the left labelled "Low Concentration" and one on the right labelled "High Concentration". Both rectangles contain red circles representing the particles of a substance. In the low concentration rectangle, the circles are highly spaced apart. There is a label under it which reads, "Particles highly spaced apart". In the high concentration rectangle, the circles are closely packed together. There is a label under it which reads, "Particles closely packed together".

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.

Diagram illustrating the relationship between concentration and volume. Two cubes are shown - one on the left labelled "Low Concentration" and one on the right labelled "High Concentration". The low concentration cube contains a small number of red circles, with large spaces between them. Underneath it, there is a label that says "Small number of particles in a volume of 1 litre". The high concentration cube contains a large number of red circles closely packed together. Underneath it, there is a label that says "Many particles in a volume of 1 litre".

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.

Diagram showing the process of making two different sugar solutions and the concentrations of the different solutions formed. The diagram starts with a drawing of a beaker of water with "1L" written on it (1 litre). Then there is a plus sign and a drawing of a spoon heaped with sugar. Then there is an arrow and a drawing of the beaker now containing a sugar solution. Sugar particles are drawn spread out within the water in the beaker. The beaker is labelled "Solution A". Under this whole things is the same diagram again but this time with two spoonfuls of sugar. The solution formed is labelled "Solution B" and it has twice as many sugar particles in it as solution A. Underneath all of this, there is the following writing: "The concentration of solution B is double the concentration of solution A".

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.

Diagram of a more dilute solution and more concentrated solution. The solution on the left is labelled 'More dilute'. It is drawn a beaker containing water and within the water there are small hexagons representing sugar molecules. The solution on the right is labelled 'More concentrated'. It is drawn the same as the first solution but with twice as many sugar molecules in the same volume of water.

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).

Diagram about concentration and volume. The title at the top is "Dissolving solutes in a liquid does not usually have much effect on its volume". Under this there is an illustration of the formation of a solution. This starts, on the left, with a diagram of a beaker containing water. This is labelled, "1 litre of water". Then there is a plus sign. Then a drawing of two spoonfuls of sugar, labelled "Sugar". Then an arrow, then drawing of the same beaker but now with sugar molecules spread out within the water. It is labelled "1 litre of solution".

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.

Concentration gradients

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.

Diagram illustrating the concept of a concentration gradient. The diagram shows a unicellular organism floating in a pond. The title at the top of the diagram says, "A Concentration Gradient". Within the cell there are many small circles, representing solute molecules. The inside of the cell is labelled "High Concentration". The water outside the cell also contains solute molecules but they are much more spread out. This area is labelled "Low Concentration".

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.

Diagram of a substance moving down a concentration gradient. The title at the top says "Moving down the concentration gradient". The diagram shows a unicellular organism floating in water. There are small circles representing solutes dissolved in the cell's cytoplasm and in the water around the cell. The concentration is much higher inside the cell than outside. There are arrows showing solute molecules moving out of the cell.

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.

Diagram of a substance moving against a concentration gradient. The title at the top says "Moving against the concentration gradient". The diagram shows a unicellular organism floating in water. There are small circles representing solutes dissolved in the cell's cytoplasm and in the water around the cell. The concentration is much higher inside the cell than outside. There are arrows showing solute molecules moving into the cell.

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.

1/5

What is concentration?

2/5

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

3/5

What is a concentration gradient?

4/5

How do we describe a substance moving from an area of higher concentration to an area of lower concentration?

5/5

How do we describe a substance moving from an area of lower concentration to an area of higher concentration?

Donate

Please consider donating to support Mooramo. I am one person doing this whole project on my own - including building the site, writing the content, creating illustrations and making revision resources. By making a one-time or repeating donation you will buy me time to work on Mooramo, meaning that I can get new content on here more quickly.

Donate