Filtration and evaporation

Filtration separates a solid that hasn't dissolved; evaporation recovers a solid that has. Describe both, name the residue and filtrate, and choose the right one.

How do you get clean salt back out of sandy salt water? Two techniques, in the right order: filtration takes out what didn't dissolve, and evaporation takes the water away from what did.

Both techniques use a property difference from the last lesson: particle size for filtration, boiling point for evaporation. This lesson is about doing them correctly.

Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.

WidthMore contexts, same idea.

EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.

Engage: sandy salt water

Sandy salt water is poured through filter paper. What stays in the paper?Sand onlySand and saltNothingSalt only

ExploreTry it on the board and compare what happens with your prediction.

Explore: filter, then evaporate

The dashed line is the filter paper. Its holes are smaller than a sand grain.

The board loads the filtrate in an evaporating dish.

Try it yourselfNothing is saved: start again as often as you like.

Pour the sandy salt water through the filter paper.

Use the board. This ticks when you get there.

Heat the filtrate until the water has gone.

Use the board. This ticks when you get there.

Name what you got.

The sand left in the filter paper

The salty water that came through the paper

What was left in the dish after evaporating

ExplainUse the particle model to explain what you saw.

Explain: get the steps in order

Put the filtration steps in order.

Pieces (click to add)

Your program

Put the evaporation steps in order.

Pieces (click to add)

Your program

Filtration can't get the salt out of salt water, but evaporation can. Why?Heating destroys the water and leaves the saltDissolved salt has disappeared, and heating makes it come backDissolved salt particles are small enough to pass through the paper, but when heated only the water particles escape, because water boils far below saltSalt is heavier than water, so it sinks while the lighter water rises away

PractiseA few on your own, to check you have it.

Mix it up

Looking back at pure substances and mixtures: before it is filtered, is sandy salt water a pure substance?No, because it looks cloudyYes, because the salt has dissolvedYes, because it is all waterNo: it has three kinds of particle (water, salt and sand), so it is a mixture

PractiseA few on your own, to check you have it.

Which technique?

For each mixture, choose what you would use to get the part named. Start by asking: did it dissolve?

Try it yourselfNothing is saved: start again as often as you like.

Choose the right technique six times.

Use the board. This ticks when you get there.

Elaborate TypicalUse the same idea in a new context.

Elaborate: design a procedure

You have a dry mixture of sand and salt. Order the steps to end up with dry sand and salt crystals, separately.

Pieces (click to add)

Your program

During filtering, which particles pass through the paper?All the particlesOnly water particlesWater and salt particles

During evaporation, which particles leave the dish?Only water particlesWater and saltOnly salt

Elaborate WiderUse the same idea in a new context.

Elaborate: out in the world

Filtration or evaporation?

A coffee machine holds the grounds in a paper cone.

Shallow sea-salt ponds are left in the sun for weeks.

A pool pump pushes water through a sand or cartridge filter.

Wet clothes on a washing line dry out.

Elaborate WiderUse the same idea in a new context.

Elaborate: First Nations separation techniques

Many Aboriginal communities have long processed grass seeds into flour. A coolamon, a shallow wooden dish, is one of the tools used.

Which property difference does each technique use?

Winnowing: seeds are tossed in a coolamon so the breeze carries the husks away.

Yandying: the coolamon is rocked in a rhythm so seeds and heavier grit move to different parts of the dish.

Hand-picking: sticks and stones are picked out of the seeds by hand.

Elaborate WiderUse the same idea in a new context.

Elaborate: does the filter paper matter?

A class filtered 100 mL of the same muddy water through three grades of filter paper. (Sample results, not measured.)

Filter paper	Time to filter	Filtrate
Coarse	30 s	Cloudy
Medium	75 s	Slightly cloudy
Fine	180 s	Clear

What is the pattern in the results?Finer paper gives a clearer filtrate but takes longerCoarser paper is better in every wayThere is no pattern

Why is the fine paper's filtrate clearer?The water goes slower, so the mud is lighterIt soaks up the mudIts smaller holes trap smaller mud particles

Why does the fine paper take longer?The water gets heavierThe fine paper soaks up more of the waterSmaller holes let less water through each second

EvaluateLook back at your first ideas and what changed.

Evaluate: look back

What did you think would stay in the filter paper? Write one sentence: "At first I thought… Now I think… because…"

Revision summary

Filtration separates an undissolved solid from a liquid. The solid left in the paper is the residue; the liquid that passes through is the filtrate.
It works because the solid's particles are too big for the holes. Dissolved particles pass straight through.
Evaporation separates a dissolved solid from the liquid: the liquid's particles escape as a gas; the solid's particles stay and form crystals.
For a solid that dissolves plus one that doesn't: filter first, then evaporate the filtrate.

I compared my first prediction with what happened and can say what changed in my thinking.

For teachers

Students describe filtration and evaporation accurately, name the residue and filtrate, and choose the right technique with a property reason (AC9S7U06, node 3). About 25 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

I can describe how filtration and evaporation are done, in the right order.
I can name the residue and the filtrate.
I can choose filtration, evaporation or both for a mixture, and say why.

Before class

A concept tutorial for introducing or revising the techniques; it prepares students for the practical, it does not replace it. Set the width with ?width=minimum, typical (default) or wider.

The node's response mode is consolidation, so the step ordering and the Practise quiz (six correct) are part of the minimum path.

First Nations content is written at a general level: connect it to the Country your school is on and use local community knowledge and protocols where possible. The filter paper data in the last Wider step is sample data, not measured: collecting real data is a good extension.

Practical safety: heat the evaporating dish gently and stop before it is completely dry, as salt can spit.

Running the lesson

Engage: count how many predicted sand and salt. That is the misconception the filter board answers.
Explain: ordering the steps is the procedural core; read the orders aloud together.
Practise: the quiz includes 'neither' items; those test whether students ask 'did it dissolve?' or just pick a technique.

The hard part (on purpose)

Why dissolved salt passes through a filter. When a student says the salt stays in the paper, ask how big dissolved particles are compared with the holes.

When students get stuck

A student says the salt stays in the filter paper. Ask: How big are dissolved salt particles compared with the holes? Fix: Dissolved particles are single and tiny: they pass through with the water.
A student mixes up residue and filtrate. Ask: Which one stays behind? Which one passes through? Fix: Residue remains (in the paper). Filtrate flows through.
A student filters a dry sand and salt mixture. Ask: Can a filter separate two dry solids? Fix: Add water first so the salt dissolves; then filtering can split them.

Stretch and support

Stretch: Why is it hard to get pure water back by evaporation? What would you need to add to collect it?

Support: Give the two words with a picture cue: residue (stays in the paper), filtrate (flows through).

Where it leads

Next: node 4, distillation (which collects the water) and chromatography.

In class: filter and evaporate a sand and salt mixture; compare students' procedures with the one in the Typical step.

What each step is for

Engage: sandy salt water: The common error is thinking the filter catches dissolved salt too.
Explore: filter, then evaporate: Seeing the particles at each stage makes residue, filtrate and crystals concrete.
Explain: get the steps in order: The node's response mode is consolidation: procedural accuracy is the target, so order the steps exactly.
Mix it up: Counting kinds of particle is what tells a mixture from a pure substance, and it decides which technique to use on sandy salt water.
which technique?: Choosing between filtration, evaporation, both or neither, with feedback, builds the habit of asking 'did it dissolve?'
Elaborate: design a procedure: Designing the procedure for a dry mixture needs the dissolving step first, which students often miss.
Elaborate: out in the world: Spotting filtration and evaporation in everyday settings checks the techniques are understood, not memorised.
Elaborate: First Nations separation techniques: Traditional techniques use the same property differences as lab methods.
Elaborate: does the filter paper matter?: A variable investigation (filter paper grade) shows the trade-off between clarity and time, explained by particle size.

Now you can

Describe how filtration and evaporation are done, in the right order.
Name the residue and the filtrate.
Choose filtration, evaporation or both for a mixture, and say why.

Practise this skillNew questions, new numbers each timeGet 3 right in a row to mark it practised.Next lesson9. Distillation and chromatographyDistillation collects a liquid by boiling and condensing it; chromatography separates dissolved substances such as dyes. Explain each principle and when distillation beats evaporation.

Source: content/science/chemistry/filter-evaporate/lesson.md