7 mistakes learners make with separating mixtures, each one named and explained.
Choose the method by what is holding the mixture together, not by what it looks like.
A method is chosen that does not act on whatever is holding this particular mixture together.
The methods are learned as a list of names rather than as answers to different questions.
Question. Separating the dyes in a black ink
A common answer. filter it
The answer. use chromatography
Why. The dyes are all dissolved, so only their different travel speeds separate them.
Name what is holding the mixture together first. The method follows from that.
You choose filtration to get salt out of salt solution.
Salt is a solid you can see before it dissolves, so it feels like a solid the paper could catch.
Question. Salt solution poured through filter paper
A common answer. the salt stays on the paper
The answer. the salt passes straight through with the water
Why. Dissolved particles are single particles, far smaller than the holes in the paper.
Filtering separates by particle size. Once something has dissolved, size no longer separates it.
You say the salt has gone or disappeared, rather than that it is still present in the water.
It genuinely cannot be seen any more, and disappearing is what that looks like.
Question. Salt stirred into water until it is clear
A common answer. the salt has gone
The answer. the salt is still there, in particles too small to see
Why. The solution weighs exactly what the water and the salt weighed separately.
Weighing is the test. Nothing disappears in a physical change.
Distillation is chosen for a mixture that has not dissolved at all, such as sand in water.
Distillation is the most elaborate method, so it feels like the most powerful one.
Question. Sand mixed with water
A common answer. distil it
The answer. filter it
Why. The sand has not dissolved, so a filter paper separates it in one step.
Check first whether anything dissolved. If not, you never need to boil anything.
You pick evaporation to get pure water from seawater, which throws the water away.
Evaporation genuinely does separate the two, so it looks like the right answer.
Question. Getting drinking water from seawater
A common answer. evaporate it
The answer. distil it
Why. Evaporation leaves the salt and loses the water into the air.
Ask which part you are being asked to collect. That decides between the two.
The solid on the paper is called the filtrate and the liquid underneath the residue.
The two words are learned together and sound equally technical, so which is which does not stick.
Question. Sand and water poured through a filter
A common answer. the sand is the filtrate
The answer. the sand is the residue
Why. Filtrate is what filtered through; residue is what was left behind.
Both words contain the answer. Residue stays, filtrate travels.
Solvent, solute and solution are swapped, so the liquid you added is called the solution.
The three words look almost identical and are always taught in the same breath.
Question. Salt stirred into water
A common answer. the water is the solution
The answer. the water is the solvent
Why. The solution is the whole mixture, once the salt has dissolved into the water.
Solution is the pair of them together. Two ingredients, one result.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.