GCSE Chemistry: Chemical Analysis (paper 2)

What is a pure substance?

A single element or compound, not mixed with any other substance

What does a pure substance have?

A specific melting and boiling point

What do impure substances have?

A range of melting and boiling points.

What is a formulation?

A complex mixture designed as a useful product

How do formulations have the properties we need?

The quantity of each component is carefully measured.

What are examples of formulations?

Fuels, cleaning products, paints, medicines, alloys, fertilisers and foods.

What does paper chromatography do?

Allows us to separate substances based on their different solubilities

Can give information to help identify substances

What does solubility mean?

ability to dissolve

What is paper described as in paper chromatography?

The stationary phase- as it doesn't move

What is the solvent described as in paper chromatography?

The mobile phase- as it moves

Describe paper chromatography

Method:

• 1. First draw a pencil baseline across a piece of chromatography paper.

• 2. Then place a dot of the first colour on the pencil line and next to that the dot of the other colours (making sure there’s an appropriate amount of space between the colours)

• 3. Place the chromatography paper is put in a jar with the solvent. Make sure that the solvent level is below the level of the spots otherwise the dye will wash off into the solvent. As the solvent moves up the paper, the dyes in the mixture separate from each other.

How can you tell if the compounds pure during chromatography?

It will produce a single spot in all solvents whereas the compounds in a mixture may separate into different spots depending on the solvent.

What happens in paper chromatography to the substance if it's more soluble?

It travels further up the paper the more soluble it is.

Why do we draw our starting line in pencil?

Because if we drew the line in pen, the pen ink would move up the paper, with the solvent.

How can you identify unknown substances using paper chromatography?

• Carry out chromatography.

• Use formula (distance travelled by substance/distance travelled by solvent) to calculate Rf value -Look this value up on a database in order to identify the substance

What is the Rf value formula?

Distance travelled by substance / distance travelled by solvent

What’s an issue regarding the identification of an unknown substance using paper chromotography?

Several different substances could have the same Rf value so you should repeat the experiment using different solvents in order to narrow it down further.

What's another issue regarding the identification of unknown substances experiment?

If the substance has never been analysed before then there will not be an Rf value on the database so you need to carry out further analysis.

Using paper chromatography how can you tell if certain chemicals are in the mixture.

They line up.

What is the test for hydrogen?

Put a lit splint where the gas is. If hydrogen is present then the gas will burn rapidly with a pop sound.

What is the test for oxygen?

Putting a glowing splint into a test tube containing a gas. If the splint relights (bursts into flames) then oxygen is present.

What is limewater?

An aqueous solution of calcium hydroxide

What is the test for carbon dioxide?

Shake the gas with limewater or bubble it through limewater and if the solution turns cloudy, carbon dioxide is present.

What is the test for chlorine?

Place damp litmus paper in gas and if it bleaches the litmus paper white, chlorine is present.

What colour does lithium go in a flame test?

crimson (flame)

What colour does sodium go in a flame test?

yellow (flame)

What colour does potassium go in a flame test?

lilac (flame)

What colour does calcium go in a flame test?

orange-red (flame)

What colour does copper go in a flame test?

green (flame)

What are problems with using flame tests to identify metal ions?

• Colour can be difficult to distinguish. (especially if there's only a low concentration of the metal compound).

• Sometimes a sample contains a mixture of metal ions which can mask the colour of the flame.

What do scientists use rather than flame tests?

flame emission spectroscopy

Describe flame emission spectroscopy

• Instrumental method used for identifying metal ions in solution or measuring their concentration

• A sample of metal ion is placed into a flame, the light given out from this is passes into a spectroscope, which converts the light into a line spectrum.

• The position of lines in the spectrum are specific for each metal ion, which allows for definite identification (which is good as sometimes colours are difficult to distinguish).

• Concentrations are found by measuring the intensity of light emitted. The more intense light, the greater the concentration of the metal ion in a solution.

• From the intensity vs concentration graph, you can read off a relevant concentration value at a given intensity.

What is an instrumental method?

A method carried out by a machine

What are advantages of instrumental methods?

accurate, sensitive (will work on even a tiny sample of a metal ion), rapid

Name two ways of identifying metal ions (cations)

• Flame tests

• Using sodium hydroxide solution

Which metal ions form a white precipitate when sodium hydroxide solution is added?

Solutions of aluminium, calcium and magnesium ions form white precipitates but only the aluminium hydroxide precipitate redissolves in excess sodium hydroxide solution.

You would need to carry out flame tests to work out which on is calcium.

What colour precipitate do calcium ions produce? (with sodium hydroxide)

white

What colour precipitate do magnesium ions produce? (with sodium hydroxide)

white

What colour precipitate do aluminium ions produce? (with sodium hydroxide)

white at first, then redissolves in excess NaOH to form a colourless solution

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

blue

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

green

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

brown

How do you test for carbonate ions?

• Carbonates can react with dilute acids to form carbon dioxide

• To test this add dilute acid to your sample

• We will see effervescence (fizzing) but that does not prove that we have carbon dioxide gas so we bubble the gas through limewater. If the limewater goes cloudy then this proves that we have carbon dioxide. This means that we started with carbonate ions.

What are halide ions?

Group 7 (halogen) ions e.g. fluoride -

How do you test for halide ions?

Add dilute nitric acid

Add silver nitrate solution

This is because halide ions in solution produce precipitates of the silver nitrate solution in the presence of dilute nitric acid. Each halide makes a different colour precipitate.

What colour precipitate do chloride ions produce?

White precipitate of silver chloride

What colour precipitate do bromide ions produce?

Cream precipitate of silver bromide

What colour precipitate do iodide ions produce?

Yellow precipitate of silver iodide

How do you test for sulfate ions?

Add dilute HCl to the sample-preventing any precipitation reactions not involving sulfate ions from taking place

Then add barium chloride solution, if sulfate ions are present a white precipitate will be formed.