Chapter 8 Chemical Analysis

What is a pure substance?

A single element or compound that is not mixed with any other substance.

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

A pure substance has only one element or compound, while a mixture contains two or more substances.

How can melting and boiling points be used?

They can be used to distinguish pure substances from mixtures because pure substances have fixed melting and boiling points.

What is a formulation?

A mixture designed as a useful product.

Why are formulations made using carefully measured quantities?

To ensure the product has the required properties.

Give six examples of formulations.

Fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods.

What is chromatography used for?

To separate mixtures and help identify substances.

What are the two phases in chromatography?

A stationary phase and a mobile phase.

What does chromatography separate substances based on?

Their distribution between the stationary and mobile phases.

What is the Rf value?

The distance moved by the substance divided by the distance moved by the solvent.

State the formula for Rf value.

Rf = Distance moved by substance ÷ Distance moved by solvent.

Why do different compounds have different Rf values?

They have different solubilities in different solvents.

How many spots does a pure substance produce on a chromatogram?

One spot.

What does more than one spot on a chromatogram indicate?

A mixture.

What is paper chromatography?

An analytical technique that separates compounds by their relative speeds in a solvent as it moves through paper.

What happens to a substance that is more soluble during paper chromatography?

It travels further up the paper.

What is a pigment?

A solid coloured substance.

How do you test for hydrogen gas?

Hold a burning splint at the mouth of the test tube.

What is the positive test for hydrogen?

A squeaky pop.

How do you test for oxygen gas?

Insert a glowing splint into the gas.

What is the positive test for oxygen?

The glowing splint relights.

How do you test for carbon dioxide?

Bubble the gas through limewater.

What is the positive test for carbon dioxide?

The limewater turns milky/cloudy.

How do you test for chlorine gas?

Use damp litmus paper.

What is the positive test for chlorine?

The damp litmus paper is bleached white.

What colour flame does lithium produce?

Crimson.

What colour flame does sodium produce?

Yellow.

What colour flame does potassium produce?

Lilac.

What colour flame does calcium produce?

Orange-red.

What colour flame does copper produce?

Green.

What is one limitation of flame tests?

Some flame colours can be masked if more than one metal ion is present.

Which metal ions form a white precipitate with sodium hydroxide?

Aluminium, calcium and magnesium.

Which white precipitate dissolves in excess sodium hydroxide?

Aluminium hydroxide.

What colour precipitate does copper(II) produce with sodium hydroxide?

Blue.

What colour precipitate does iron(II) produce with sodium hydroxide?

Green.

What colour precipitate does iron(III) produce with sodium hydroxide?

Brown.

How do you test for carbonate ions?

Add a dilute acid and test the gas produced with limewater.

What is the positive result for carbonate ions?

Limewater turns cloudy because carbon dioxide is produced.

How do you test for halide ions?

Add dilute nitric acid, then silver nitrate solution.

What colour precipitate indicates chloride ions?

White.

What colour precipitate indicates bromide ions?

Cream.

What colour precipitate indicates iodide ions?

Yellow.

What mnemonic helps remember the halide precipitate colours?

Cats With Brains Can Ideally Yodel (Chloride-White, Bromide-Cream, Iodide-Yellow).

How do you test for sulfate ions?

Add dilute hydrochloric acid, then barium chloride solution.

What is the positive result for sulfate ions?

A white precipitate forms.

What are the advantages of instrumental methods?

They are accurate, sensitive and rapid.

What instrumental method is used to analyse metal ions in solution?

Flame emission spectroscopy.

How does flame emission spectroscopy work?

The sample is placed in a flame and the emitted light passes through a spectroscope.

What does flame emission spectroscopy produce?

A line spectrum.

What can a line spectrum be used for?

To identify metal ions and measure their concentrations.