GCSE · Chemistry · AQA · Spec 8462

Test for carbon dioxide

You've collected a gas. Is it carbon dioxide? Don't guess — test it: get the gas into limewater, watch, and decide.

Walk the test

Is it carbon dioxide? Ask the limewater.

Start with the unknown gas. First choose how it meets the limewater, then choose what you see happen. Try every route — and notice which choice actually changes the answer.

How the gas meets the limewater → What the limewater does

Your reagent is limewater. Instead of guessing, you let the limewater answer. First, get the gas into it.

4 ways this test can go.

On An unknown gas. 2 branches to choose from.

Quick check

Wait — what actually went milky?

Picture the tube: a gas has been bubbled through limewater, and now it looks milky. Two substances were involved — the gas and the limewater.

Which of these is closest to what you'd write?

The gas turned milky, so the gas is carbon dioxide.The limewater turned milky, so the gas is carbon dioxide.The limewater turned milky, so the gas is not carbon dioxide.

How sure are you?

I'm sureI think soI'm guessing

The gas turned milky, so the gas is carbon dioxide.

This is a common misconception

Right verdict, wrong substance. The gas is what you're testing; the limewater is what you watch, and it's the limewater that turns milky (cloudy). Say which substance changed.

The limewater turned milky, so the gas is carbon dioxide.

This matches the evidence

Spot on. The limewater turned milky (cloudy), and that is the positive result: carbon dioxide is present. Naming the limewater as the thing that changed is exactly what a good answer does.

The limewater turned milky, so the gas is not carbon dioxide.

This is a common misconception

You've read the result backwards. Milky limewater is the positive result — it shows carbon dioxide is present, not absent.

The gas is tested; the limewater changes. Milky limewater means carbon dioxide.

What's in the tube?

So what is limewater, really?

You've now seen limewater do the deciding in every route. The name sounds friendly — maybe a bit too friendly. It's easy to use it without knowing what it is.

Which of these is closest to what you think limewater is?

It's water with lime juice in it — like the fruit.It's just ordinary water. 'Lime' is only a name.It's water with carbon dioxide dissolved in it.It's calcium hydroxide dissolved in water.

How sure are you?

I'm sureI think soI'm guessing

It's water with lime juice in it — like the fruit.

This is a common misconception

The name leads a lot of people here, but there's no fruit involved. Limewater is an aqueous solution of calcium hydroxide. 'Lime' is part of its everyday name, not a clue to a fruit.

It's just ordinary water. 'Lime' is only a name.

This is a common misconception

Close in one way — it is a solution in water. But it isn't plain water: it has calcium hydroxide dissolved in it. That dissolved calcium hydroxide is what makes it the reagent for this test.

It's water with carbon dioxide dissolved in it.

This is a common misconception

This one swaps the two roles. Carbon dioxide is the gas you're testing FOR; limewater is the reagent you test WITH. Limewater is an aqueous solution of calcium hydroxide.

It's calcium hydroxide dissolved in water.

This matches the evidence

Yes. Limewater is an aqueous solution of calcium hydroxide — 'aqueous' just means dissolved in water. So one reagent, two names: limewater, or calcium hydroxide solution.

Limewater = an aqueous solution of calcium hydroxide. Two names, one reagent.

Your turn to write it

Describe the test in your own words

Describe the test for carbon dioxide. Include what you would see if the gas is carbon dioxide. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Names the reagent: limewater (an aqueous solution of calcium hydroxide). (1 mark)
Says how the gas meets it: shake the gas with the limewater, or bubble the gas through it. (1 mark)
Gives the result: the limewater turns milky (cloudy) if carbon dioxide is present. (1 mark)

Model answer

Shake the gas with limewater, or bubble the gas through limewater. (Limewater is an aqueous solution of calcium hydroxide.) If the limewater turns milky (cloudy), the gas is carbon dioxide.

Reagent, contact, result. If your answer names all three — and says it's the limewater that goes milky — you've described the whole test.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One reagent, one thing to watch: does the limewater turn milky?

What you need to know

The test for carbon dioxide uses limewater.
Limewater is an aqueous solution of calcium hydroxide.
The gas is shaken with the limewater or bubbled through it.
If the limewater turns milky (cloudy), carbon dioxide is present.

The big picture

The test for carbon dioxide uses limewater, which is an aqueous solution of calcium hydroxide. You bring the gas into contact with the limewater, either by shaking the gas with it or by bubbling the gas through it. If the limewater turns milky (cloudy), carbon dioxide is present. The method you use doesn't change the rule: it's what the limewater does that decides the answer.

Key points

1Reagent: limewater, also called calcium hydroxide solution.

2Two ways in, one test: shaking and bubbling are both fine.

3Positive result: the limewater turns milky (cloudy).

4It's the limewater that changes, not the gas.

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Worked example

Problem

A student tests two gases, A and B. Gas A is shaken with limewater and the limewater turns milky. Gas B is bubbled through limewater and the limewater stays clear. What can the student conclude about each gas?

⚠ Watch out

Writing that 'the gas turns milky'. The gas is what you're testing; it's the limewater that turns milky (cloudy). Say which substance changes.

🧠

Memory hook

Milky means yes. If the limewater clouds over, carbon dioxide is there.

✓

Check yourself

Without looking back: what is limewater, what are the two ways of getting the gas into it, and what do you see if the gas is carbon dioxide?

Flashcards

(5)

Which reagent is used to test for carbon dioxide?

Limewater.

What is limewater, chemically?

An aqueous solution of calcium hydroxide — calcium hydroxide dissolved in water.

What are the two ways of bringing the gas into contact with limewater?

Shake the gas with the limewater, or bubble the gas through it.

What do you see if the gas is carbon dioxide?

The limewater turns milky (cloudy).

In the test, which substance turns milky: the gas or the limewater?

The limewater. The gas is the thing being tested.

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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Is it carbon dioxide? Ask the limewater.

Wait — what actually went milky?

So what is limewater, really?

Describe the test in your own words

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