Chemical Analysis

What is a pure substance in chemistry?

A single element or compound, not mixed with anything else

How can purity be tested?

Check melting/boiling point compared to known value

What effect do impurities have on melting point?

Lower it and broaden melting range

What effect do impurities have on boiling point?

Raise it and broaden boiling range

What is a formulation?

A mixture designed as a useful product (e.g., medicines, paints)

Give examples of formulations.

Pharmaceuticals, paints, fuels, cleaning agents

Why are precise ratios important in formulations?

Ensure correct properties and effectiveness

What is chromatography?

Technique to separate substances in a mixture

What is the stationary phase in paper chromatography?

Chromatography paper

What is the mobile phase in paper chromatography?

Solvent (liquid that moves up the paper)

How does chromatography separate mixtures?

Substances travel different distances depending on solubility and attraction to paper

How is Rf value calculated?

Distance travelled by substance ÷ distance travelled by solvent

What is an Rf value?

Ratio describing how far a substance moves on a chromatogram

Why is pencil used to draw the baseline in chromatography?

Pencil is insoluble and doesn't affect results

What does a pure substance show in chromatography?

One spot on chromatogram

What does a mixture show in chromatography?

Multiple spots

What is the test for hydrogen gas?

Lit splint causes a "pop" sound

What is the test for oxygen gas?

Glowing splint relights

What is the test for carbon dioxide gas?

Turns limewater milky/cloudy

What is the test for chlorine gas?

Turns damp blue litmus paper red, then bleaches it white

What is a flame test?

Heating a substance in a flame to observe its colour for metal ions

What colour is the flame for lithium ions?

Crimson red

What colour is the flame for sodium ions?

Yellow/orange

What colour is the flame for potassium ions?

Lilac

What colour is the flame for calcium ions?

Brick red/orange-red

What colour is the flame for copper ions?

Green

What is a limitation of flame tests?

Colours can be masked by others in a mixture, difficult with low concentrations

How do you test for ammonium ions?

Add sodium hydroxide, gas released turns damp red litmus paper blue

What is the ionic equation for ammonia released by ammonium ions?

NH₄⁺ + OH⁻ → NH₃ + H₂O

How do you test for metal hydroxides?

Add sodium hydroxide, observe coloured precipitate

What colour precipitate does iron(II) produce with NaOH?

Green

What colour precipitate does iron(III) produce with NaOH?

Brown

What colour precipitate does copper(II) produce with NaOH?

Blue

What is a precipitate?

An insoluble solid formed in a solution

How do you test for carbonate ions?

Add dilute acid, look for fizzing, test gas with limewater for CO₂

How do you test for sulfate ions?

Add hydrochloric acid then barium chloride, white precipitate forms

What does the white precipitate in the sulfate test mean?

Barium sulfate formed, confirms sulfate ions

How do you test for halide ions?

Add nitric acid then silver nitrate

What colour is the precipitate for chloride ions?

White (silver chloride)

What colour is the precipitate for bromide ions?

Cream (silver bromide)

What colour is the precipitate for iodide ions?

Yellow (silver iodide)

Why is nitric acid added before silver nitrate?

Removes interfering carbonates

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

Removes interfering carbonates

What is qualitative analysis?

Identifying substances present in a sample

What is quantitative analysis?

Measuring how much of a substance is present

What technique is used for separating coloured compounds?

Paper chromatography

What is chromatography used for in forensics?

Identify substances in samples (e.g., inks, drugs)

What is a mobile phase?

Phase that moves in chromatography (usually liquid or gas)

What is a stationary phase?

Phase that does not move in chromatography (solid or liquid supported on solid)

What does a high Rf value mean?

Substance moves far up the paper (high solubility in mobile phase)

Why doesn’t the solvent front reach the top?

To allow better separation of spots

Why use covered beaker in chromatography?

Prevents evaporation of solvent

What is the solvent front?

The furthest point reached by the solvent

How can chromatography identify unknown substances?

Compare Rf values and spots with known samples

What causes flame colours in flame tests?

Metal ions excited by heat release energy as coloured light

How do you clean a wire loop for flame tests?

Dip in dilute acid, then rinse with distilled water and heat to clean

Why do some ions not produce a flame colour?

Low concentration, or colours masked by other ions

What needs to be controlled in a flame test?

Clean wire loop, avoid contamination

What is gas chromatography?

A technique for separating and analysing compounds in a sample

What is a chromatogram?

Output (paper or digital) showing separated spots/peaks of substances

What does a single spot in a chromatogram mean?

Pure substance

What happens if two substances have the same Rf value in chromatography?

Difficult to distinguish; use another solvent for better separation

What is instrumental analysis?

Methods using machines (e.g., mass spectrometry, gas chromatography)

Give one advantage of instrumental analysis.

Faster, more sensitive, more accurate

What is retention time in gas chromatography?

Time taken for a substance to travel through the column

What are reference materials in analysis?

Known substances for comparison with sample results

Why must water used in tests be distilled/deionised?

Avoid contamination from ions

What is the white precipitate formed in the sulfate test?

Barium sulfate

What is the principle behind mass spectrometry?

Measures mass-to-charge ratio of ions to identify substances

What is a control in chemical analysis?

Sample known to give a negative or positive result, ensures reliability

Why is qualitative analysis important?

Identifies substances, e.g., in drugs, forensics, water testing

How do you record chromatography results?

Mark spots, measure distances, calculate Rf values

In what type of reaction is a precipitate formed?

Precipitation reaction (double displacement)

How do you separate a precipitate?

Filtration

What is an indicator?

A substance that changes colour in acids or bases

How is chromatography used in food analysis?

Identify food colours/flavours/additives

What practical safety measure is necessary in chemical testing?

Wear goggles, handle chemicals with care

What is the filter paper used for in flame tests?

Not used—flame tests use a clean wire loop, chromatography uses filter paper

How are solvents in chromatography chosen?

Must dissolve the analysed sample, differ for samples

What does a systematic error in chemical analysis mean?

Repeats same error in every reading, e.g., contaminated rinsing water

What is the main difference between qualitative and quantitative analysis?

Qualitative: what is present; Quantitative: how much is present

What is the colour of phenolphthalein in acid?

Colourless

What is the colour of phenolphthalein in alkali?

Pink

What is the main use of silver nitrate in ion testing?

Detects halide ions by forming a precipitate

How might mixtures affect chromatography?

Overlapping spots, unclear separation

What does a “trail” or “tail” on chromatogram suggest?

Mixture or slow movement of compound

Why is it important to dry the chromatogram before measuring Rf?

Solvent continues to spread, changes result

What happens if the baseline is submerged in chromatography solvent?

Samples may dissolve into solvent, not travel up paper

What type of analysis is pH testing?

Qualitative (identifies acidity/alkalinity)

What is the main confirmation in a carbonate test?

CO₂ detected, confirmed by limewater

What two ions combine to make barium sulfate?

Ba²⁺ + SO₄²⁻

Why do you need to perform tests in a set order?

Some tests (like for carbonates) produce products that would interfere with others

When would you use a spectroscope?

To analyse light emitted, e.g., flame emission spectroscopy

What is a standard solution?

Solution with a known concentration used for titration/analysis

What is the best way to compare chromatograms?

Run reference and unknown samples side by side