KS3 · Chemistry

Precipitation reactions

Pour one clear liquid into another and a bright yellow cloud appears. Nothing solid went in, yet a solid came out — evidence that a reaction has happened.

Chemistry · Is there a solid?

Hold it up to the light

Pick a tube, then decide: is it only a solution, or has a precipitate formed?

Still to sort

Solution only (0)

A substance dissolved in water — an aqueous solution. You can see through it, whatever its colour.

Where the line is: Transparent means dissolved. A solution can be any colour, or no colour at all, and still be see-through.

Precipitate formed (0)

An insoluble solid — one that won't dissolve in the water — has formed. The mixture goes cloudy or opaque.

Where the line is: Cloudy or opaque means a solid is there, even if it's only tiny specks and never one big lump.

7 of 7 still to sort.

Some of these tubes are just solutions. Some have made a brand-new solid. Colour won't tell you which — so what will?

Watch out: Colour on its own tells you nothing. A bright blue liquid you can see through is still a solution. The test is: can you see through it?

From two clear solutions to a solid you can hold

Follow one precipitation reaction from the moment of mixing to the moment you collect the solid.

Chemistry · Writing it as an equation

Let the state symbols find the solid

Copper sulfate solution is mixed with sodium hydroxide solution. They make copper hydroxide and sodium sulfate, and a blue precipitate appears. Write the word equation with state symbols.

  1. One done for you: lead nitrate (aq) + potassium iodide (aq) → lead iodide (s) + potassium nitrate (aq)(aq) means dissolved in water. (s) means solid. The yellow solid, lead iodide, gets the (s) — so the (s) tells you which product is the precipitate.
  2. Now yours. Words first: copper sulfate + sodium hydroxide → copper hydroxide + sodium sulfateGet the names right before you add any symbols.
  3. missing step
Which line is step 3?

Chemistry · Using precipitates to identify

Which metal is hiding in there?

You've added the sodium hydroxide solution. Choose what you see.

What you see

  • Look for a precipitate — cloudiness you can't see through — and check its colour. A coloured solution you can see through isn't a result.

4 possible results.

On Add sodium hydroxide solution to the unknown solution. 4 branches to choose from.

Add sodium hydroxide solution to a solution of a metal compound. What you see tells you which metal could be there.

Watch out: This is a qualitative test: it tells you whether a substance is present, not how much of it is there.

The same trick, two water problems

Making drinking water safevsSoftening hard water

Both jobs turn something dissolved into a solid, then filter it off. Look at what's different.

Focus

The unwanted dissolved substances

Making drinking water safe

Lead and copper compounds

Softening hard water

Calcium and magnesium compounds

The insight

Dissolved substances go straight through filter paper with the filtrate — so they have to be turned into a solid first.

The problem they cause

Making drinking water safe

They can make water unsafe to drink.

Softening hard water

Limescale in kettles, pipes and taps; a different taste from soft water; soap scum, so you need more soap or detergent to get a lather.

The precipitate made

Making drinking water safe

For example, lead hydroxide (s)

Softening hard water

Calcium carbonate (s) and magnesium hydroxide (s)

How it's removed

Making drinking water safe

Filtration

Softening hard water

Filtration

Is the job finished?

Making drinking water safe

Not yet — the water still needs more processing, such as adding chlorine to sterilise it, before it's safe to drink.

Softening hard water

Removing the precipitates reduces the hard-water problems.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • A precipitate is an insoluble solid formed when two solutions react; precipitation is the process of it forming.
  • Aqueous means dissolved in water, shown by (aq). Insoluble means it will not dissolve in a particular solvent.
  • A precipitate makes the mixture cloudy or opaque so you can't see through it, and over time it settles.
  • In a word equation with state symbols, the product with (s) is the precipitate.
  • Adding sodium hydroxide solution identifies some metals by the colour of the precipitate.

The big picture

A precipitation reaction happens when two solutions combine to make an insoluble solid, called a precipitate. You know it has formed because the mixture turns cloudy or opaque — you can't see through it — and the solid later settles. In an equation, the precipitate is the product with (s). Precipitates can be collected by filtration, and precipitation is used to treat water, soften hard water and identify metals.

Key points

1Transparency, not colour, tells you whether a precipitate has formed: a coloured solution is still see-through.
2A white precipitate is not the same as a colourless solution, and a precipitate can be tiny specks rather than one lump.
3The appearance of a precipitate is evidence that a chemical reaction has happened.
4Filtration collects the precipitate in the filter paper; the filtrate passes through.
5Precipitation removes lead and copper compounds from drinking water and calcium and magnesium compounds from hard water.
6A qualitative test tells you whether a substance is present, not how much.

Worked example

Problem

One way to soften hard water uses this reaction: calcium chloride (aq) + sodium carbonate (aq) → calcium carbonate (s) + sodium chloride (aq). Which substance is the precipitate, what would you see when the solutions are mixed, and how would you collect the precipitate?

⚠ Watch out

Thinking a coloured liquid must contain a solid. Colour tells you nothing on its own — a bright blue liquid you can see through is a solution. Look for cloudiness: if you can't see through the mixture, a precipitate has formed.

🧠

Memory hook

See-through? Solution. Can't see through? A solid's formed. And in the equation, (s) spots the solid.

✓

Check yourself

Lead nitrate solution reacts with sodium hydroxide solution to make insoluble lead hydroxide and dissolved sodium nitrate. Write the word equation with state symbols. What ends up in the filter paper?

Flashcards

(15)
What is a precipitate?
An insoluble solid formed when two solutions react.
What does 'insoluble' mean?
The substance will not dissolve in a particular solvent.
What is an aqueous solution, and what is its state symbol?
A solution made by dissolving a substance in water. State symbol: (aq).
How can you tell that a precipitate has formed?
The mixture turns cloudy or opaque so you can't see through it, and over time the solid settles. It's evidence a reaction has happened.
Coloured precipitate or coloured solution — how do you tell them apart?
A coloured solution is transparent: you can see through it. A coloured precipitate makes the mixture cloudy or opaque.
A mixture turns white. Why isn't that the same as a colourless solution?
White and opaque means a solid has formed. A colourless solution has no colour and you can see straight through it.
Does a precipitate have to be a big lump of solid?
No. It can be tiny bits spread through the liquid, making it cloudy — it's still a solid.
In a word equation with state symbols, how do you spot the precipitate?
It's the product with (s). The reactant solutions and any dissolved product have (aq).
What happens when colourless lead nitrate and potassium iodide solutions are mixed?
A yellow precipitate of lead iodide forms, plus a solution of potassium nitrate.
What is the filtrate?
The liquid that gets through the filter paper during filtration.
Sodium hydroxide test: what do a blue and a green-or-brown precipitate show?
Blue: a copper compound. Green or brown: an iron compound.
Sodium hydroxide solution gives a white precipitate. What does that show?
It could be an aluminium, calcium, magnesium or lead compound — further tests are needed to tell which.
What does a qualitative test tell you — and what doesn't it?
Whether a substance is present. It doesn't tell you how much is there.
How is precipitation used to make drinking water safer?
Unwanted dissolved lead and copper compounds are turned into insoluble solids and filtered out. The water still needs more processing, such as chlorine to sterilise it.
What causes hard water, and what problems does it cause?
Calcium and magnesium compounds. Limescale in kettles, pipes and taps, a different taste, and soap scum — you need more soap to get a lather.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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