AQA GCSE Combined Science Chemistry - Chemical Analysis Continued...

How do you test for carbonates?

Add dilute acid – if it fizzes and the gas turns limewater cloudy, it's CO₂.

How do you test for sulfates?

Add dilute hydrochloric acid, then barium chloride – a white precipitate forms.

What does a white precipitate in the sulfate test mean?

Sulfate ions are present (BaSO₄ forms).

Why is hydrochloric acid added before barium chloride in the sulfate test?

To remove any other ions that could interfere with the test.

What is the ionic equation for the sulfate test?

SO₄²⁻ + Ba²⁺ → BaSO₄ (s)

How do you test for halide ions?

Add nitric acid, then silver nitrate solution.

What precipitate forms with chloride ions?

White precipitate (AgCl)

What precipitate forms with bromide ions?

Cream precipitate (AgBr)

What precipitate forms with iodide ions?

Yellow precipitate (AgI)

Why is nitric acid added before silver nitrate in halide tests?

To remove carbonate ions that could form unwanted precipitates.

What are the common errors in qualitative tests?

Contamination, using the wrong acid, misidentifying precipitate colours.

What is the difference between a pure substance and a mixture?

Pure has a fixed melting point; mixture melts over a range.

What is a chromatogram?

The result of a chromatography experiment showing separated substances.

What does it mean if a dye stays at the baseline?

It is not soluble in the solvent.

Why do we draw the baseline in pencil for chromatography?

Ink would dissolve and interfere with the results.

What is qualitative analysis?

Testing to find out what substances are present.

What is quantitative analysis?

Testing to find out how much of a substance is present.

Why are flame tests sometimes unreliable?

Colours can be masked if metal ions are mixed.

How can you get more reliable flame test results?

Use a clean nichrome wire and only one sample at a time.

Why are instrumental methods better than traditional ones?

Faster, more sensitive, and more accurate.

What type of chemistry are these tests used in?

Forensics, environmental testing, and healthcare.

Can you name a common flame test error?

Contaminated wire loop giving false colours

Which ions form coloured precipitates with sodium hydroxide?

Copper(II) – blue, Iron(II) – green, Iron(III) – brown.