C4.2 - Identifying the products of chemical reactions

How do you test for carbon dioxide gas?

Bubble the gas through limewater (calcium hydroxide solution). If CO₂ is present, the limewater turns milky/cloudy due to the formation of insoluble calcium carbonate

How do you test for hydrogen gas?

Hold a lit splint at the mouth of a test tube containing the gas. If hydrogen is present it produces a squeaky pop as it ignites with oxygen in the air

How do you test for oxygen gas?

Insert a glowing splint into a test tube of the gas. If oxygen is present the splint relights

How do you test for chlorine gas?

Hold damp litmus paper in the gas. Chlorine first turns the paper red (acidic) then bleaches it white. Chlorine must always be handled in a fume cupboard due to its toxicity

What is a precipitation reaction in the context of testing for cations?

When NaOH solution is added to a solution containing a metal cation, an insoluble metal hydroxide precipitate forms. The colour of the precipitate identifies the cation

What is the colour of the precipitate formed when NaOH is added to Al³⁺?

White precipitate of aluminium hydroxide. It is soluble in excess NaOH (redissolves), which distinguishes it from calcium and magnesium

What is the colour of the precipitate formed when NaOH is added to Mg²⁺?

White precipitate of magnesium hydroxide. Insoluble in excess NaOH — this distinguishes it from aluminium

What is the colour of the precipitate formed when NaOH is added to Ca²⁺?

White precipitate of calcium hydroxide. Also insoluble in excess NaOH — can be distinguished from Mg²⁺ using a flame test (Ca²⁺ gives orange-red flame)

What is the colour of the precipitate formed when NaOH is added to Cu²⁺?

Light blue precipitate of copper(II) hydroxide. Insoluble in excess NaOH

What is the colour of the precipitate formed when NaOH is added to Fe²⁺?

Green precipitate of iron(II) hydroxide. Insoluble in excess NaOH

What is the colour of the precipitate formed when NaOH is added to Fe³⁺?

Red-brown precipitate of iron(III) hydroxide. Insoluble in excess NaOH

How do you test for carbonate ions (CO₃²⁻)?

Add dilute acid to the sample — if carbonate is present, CO₂ gas is produced which turns limewater milky. Equation: CO₃²⁻ + 2H⁺ → H₂O + CO₂

How do you test for sulfate ions (SO₄²⁻)?

Acidify the sample with dilute HCl, then add barium chloride solution. A white precipitate of barium sulfate forms if sulfate ions are present. The acid is added first to remove any carbonate ions that might give a false positive

How do you test for halide ions (Cl⁻, Br⁻, I⁻)?

Acidify the sample with dilute nitric acid, then add silver nitrate solution. The colour of the precipitate identifies the halide present

What are the colours of silver halide precipitates?

Chloride (Cl⁻) → white precipitate of AgCl. Bromide (Br⁻) → cream precipitate of AgBr. Iodide (I⁻) → yellow precipitate of AgI

Why is the sample acidified before adding silver nitrate in the halide test?

Dilute nitric acid is added to remove any carbonate ions that might also form a white precipitate with silver nitrate, which would interfere with the results

What are flame tests used for?

To identify metal ions in a sample. A wire loop is cleaned, dipped in the sample and held in a roaring blue Bunsen flame — the colour produced identifies the metal cation present

What is the flame colour for lithium (Li⁺)?

Red

What is the flame colour for sodium (Na⁺)?

Yellow/orange — sodium produces such a strong yellow that it can mask other colours

What is the flame colour for potassium (K⁺)?

Lilac

What is the flame colour for calcium (Ca²⁺)?

Orange-red

What is the flame colour for copper (Cu²⁺)?

Blue-green

What is a common problem with flame tests and how is it addressed?

Sodium contamination is very common and produces a strong yellow flame that can mask other colours. Blue cobalt glass is used to filter out the yellow so other colours can be seen

What is flame photometry and how does it work?

An instrumental technique that analyses flame emission. The sample is burned and the emitted light is passed through a spectroscope to produce a line emission spectrum — a unique pattern of coloured lines for each element

What is a line emission spectrum?

A pattern of distinct coloured lines produced when electrons in excited atoms fall back to lower energy levels, releasing energy as specific wavelengths of light. Each element produces a unique spectrum which acts as a fingerprint

How do you use reference spectra to identify an unknown element?

Compare the line emission spectrum of the unknown sample to known reference spectra. Matching lines at the same positions indicates the unknown contains that element

What is mass spectrometry used for?

To accurately measure the relative atomic mass of elements and the relative molecular mass of molecules. It can also identify isotopes and help identify unknown compounds

What does the mass spectrum of an element show?

Peaks at different m/z values corresponding to each isotope. The height of each peak shows the relative abundance of that isotope — used to calculate relative atomic mass

What is the molecular ion peak (M⁺) in a mass spectrum of a molecule?

The peak at the highest m/z value — it corresponds to the full molecule that has lost one electron. Its m/z value gives the relative molecular mass of the compound

What are fragment ions in a mass spectrum?

Molecules can break apart during mass spectrometry, producing smaller fragment ions that also show up as peaks. The pattern of fragments helps identify the structure of the molecule

What are the advantages of instrumental analysis over chemical tests?

More accurate and sensitive. Faster — results obtained quickly. Can detect and identify very small sample sizes. Can perform multiple simultaneous analyses. Automated and less prone to human error