Chemical Analysis Questions P2

What is a pure substance in chemistry?

A substance made of a single element or compound, not mixed with anything else

How can you test if a substance is pure?

Check melting/boiling point — pure substances have a sharp melting/boiling point

What is a formulation?

A mixture designed for a specific purpose, made with precise quantities of components (e.g. paint, medicine)

What is chromatography used for?

To separate mixtures and help identify substances

What is the Rf value formula?

Rf = Distance moved by substance ÷ Distance moved by solvent

What must the baseline be drawn in during chromatography?

Pencil (so it doesn’t dissolve in the solvent)

Why is a lid used in chromatography experiments?

To stop the solvent evaporating

What is a flame test used for?

To identify certain metal ions by the colour of the flame

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

Crimson red

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

Yellow

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

Lilac

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

Orange-red

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

Green

What precipitate forms when Cu²⁺ reacts with NaOH?

Blue precipitate (Cu(OH)₂)

What precipitate forms when Fe²⁺ reacts with NaOH?

Green precipitate (Fe(OH)₂)

What precipitate forms when Fe³⁺ reacts with NaOH?

Brown precipitate (Fe(OH)₃)

What precipitate forms when Al³⁺ reacts with NaOH?

White precipitate that dissolves in excess NaOH

What precipitate forms when Ca²⁺ reacts with NaOH?

White precipitate that does not dissolve in excess NaOH

What precipitate forms when Mg²⁺ reacts with NaOH?

White precipitate that does not dissolve in excess NaOH

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

Add dilute acid → fizzing → bubble gas through limewater → turns cloudy

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

Add dilute HCl, then barium chloride → white precipitate

How do you test for chloride ions (Cl⁻)?

Add dilute nitric acid then silver nitrate → white precipitate

How do you test for bromide ions (Br⁻)?

Add dilute nitric acid then silver nitrate → cream precipitate

How do you test for iodide ions (I⁻)?

Add dilute nitric acid then silver nitrate → yellow precipitate

How do you test for hydrogen gas?

Lit splint → squeaky pop

How do you test for oxygen gas?

Glowing splint → relights

How do you test for carbon dioxide gas?

Bubble through limewater → turns cloudy

How do you test for chlorine gas?

Damp litmus paper → turns red briefly then bleaches white

How do you test for ammonia gas (NH₃)?

Damp red litmus paper → turns blue

How do you test for water vapour?

Cobalt chloride paper → turns from blue to pink OR anhydrous copper sulfate → turns from white to blue

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

To remove carbonate ions that could form false positive precipitates

What happens to aluminium hydroxide in excess NaOH?

White precipitate dissolves → colourless solution

What is Flame Emission Spectroscopy used for?

To identify metal ions and determine their concentrations

How does Flame Emission Spectroscopy work?

Sample is heated, emits light → passed through spectroscope → produces line spectrum

What does each line in a flame emission spectrum represent?

A specific wavelength of light emitted by an ion

What does the intensity of the lines in a flame emission spectrum show?

The concentration of the metal ion

Why is Flame Emission Spectroscopy better than flame tests?

More accurate, detects mixtures, works with small samples