Edexcel IGCSE Chemistry · Spec 1.3
Serial Dilution and the Particle Model
Covers carry out a serial dilution, Explain the colour change using particles and Distinguish dilution from diffusion.
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Serial Dilution and the Particle Model
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Explained
Serial dilution, and what the fading colour is telling you
A serial dilution is a sequence of dilutions, each one made from the last, so that the concentration falls by the same factor at every step.
It is used when you need a very dilute solution, or a set of solutions across a wide range of concentrations, and measuring a tiny volume accurately would be impossible. Diluting ten times by a factor of ten is easier and more accurate than trying to measure one part in ten billion directly.
The method
Start with a known concentration in the first tube.
Transfer a fixed volume of it into a second tube containing a fixed volume of solvent, usually distilled water. Mix thoroughly.
Take the same fixed volume from that second tube into a third containing the same volume of solvent, and carry on.
The two volumes must be the same at every transfer, and that is what makes the dilution factor constant. If you take 1 cm cubed into 9 cm cubed of water each time, the total is 10 cm cubed and each tube is a tenth of the one before it.
Mixing between steps matters more than it looks. Drawing from an unmixed tube takes a sample that is not representative, and the error carries into every tube after it.
Use a fresh pipette for each transfer, or rinse between them, since carrying solute across on the outside of the pipette raises the concentration of the next tube.
Reading the concentrations
If each step dilutes by ten, the concentrations run as a tenth, a hundredth, a thousandth of the original, and so on. Tube n has one over ten to the power of n minus one.
So a series of six tubes covers a range of a hundred thousand times, which no single measurement could manage.
To dilute by five, transfer 1 cm cubed into 4 cm cubed of solvent. The rule is that the dilution factor is the total volume divided by the volume transferred.
Explaining the colour with particles
Each tube is paler than the one before it, and the reason is a statement about number per volume.
Adding solvent does not destroy any solute particles. The same particles are now spread through a larger volume, so there are fewer coloured particles in any given amount of the liquid, and less light is absorbed.
That is what concentration means: amount of solute per unit volume. Saying there is less solute is imprecise, because in the tube as a whole there is less, and what has actually changed is how much there is per cubic centimetre.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks what two processes are involved when a coloured solid is dropped into water and the colour spreads through it. It credits dissolving and diffusion, and adds that the two can be given in either order.
Two words, two marks, and they describe different things. Dissolving is the solid separating into particles that mix with the solvent. Diffusion is those particles then spreading from where they are concentrated to where they are not. One has to happen before the other can, but the mark scheme does not insist you say so.
The next part asks why the process is faster at a higher temperature. It credits that the particles move faster, and then that they collide and react after a shorter period of time. Its notes allow that the particles have more kinetic energy, that they diffuse faster, and that there are more frequent collisions.
Notice that the first mark is never enough on its own. Particles move faster is where every answer starts, and the second mark is for saying what that does. The accepted alternatives all name a consequence.
A related mark scheme describes the particles in a state of matter and labels its four marking points in the margin: a comment on spacing, a comment on arrangement, a comment on movement, and a comment on energy.
That is a checklist worth carrying. When a question asks you to describe particles, you have four things to say about them, and most answers give two.
Dilution is not diffusion
The two words appear together in this topic and describe different processes.
Dilution lowers a concentration by adding more solvent. Something is added, and the number of solute particles per unit volume falls.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, and nothing is added at all. The particles spread out on their own because of their random motion, and the process stops when the concentration is even throughout.
A tube of dilute dye that has been mixed shows both. It was diluted by the solvent you added, and it became evenly coloured by diffusion once you stopped stirring.
Spec 1.3
What you need to know
- Carry out a serial dilution
- Explain the colour change using particles
- Distinguish dilution from diffusion
Active recall
Quick check
Answer each question before opening the answer.
What must remain the same at every transfer?
The transferred volume and solvent volume
Why does the colour become paler?
There are fewer coloured solute particles per unit volume
How does dilution differ from diffusion?
Dilution lowers concentration using solvent; diffusion is net particle movement down a concentration gradient
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