topic 8 - Chemical Analysis

What is a pure substance in chemistry?

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

How can melting point data be used to test purity?

A pure substance has a specific melting point. An impure substance usually melts over a range of temperatures and often at a lower temperature.

How can boiling point data be used to test purity?

A pure substance has a specific boiling point. An impure substance usually boils over a range of temperatures.

What is a mixture?

Two or more substances that are not chemically combined and can be separated by physical methods.

What is a formulation?

A mixture designed as a useful product with carefully measured amounts of different substances.

Why are formulations useful?

They are designed to have particular properties for a specific purpose.

Give examples of formulations.

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

What is chromatography used for?

Separating mixtures and helping identify substances in a mixture.

What are the two phases in chromatography?

The stationary phase, which does not move, and the mobile phase, which moves through the stationary phase.

In paper chromatography, what is the stationary phase?

The paper.

In paper chromatography, what is the mobile phase?

The solvent that moves through the paper.

How does paper chromatography separate a mixture?

Different substances have different attractions to the stationary phase and different solubilities in the mobile phase, so they travel at different speeds and separate.

What is the origin line in paper chromatography?

The pencil line where the sample is placed at the start.

Why must the origin line be drawn in pencil?

Pencil is insoluble in the solvent, so it does not travel up the paper and interfere with the results.

Why must the solvent level start below the origin line?

If the spots are submerged, the samples may dissolve directly into the solvent instead of travelling up the paper.

Why should the sample spots be small and concentrated?

This helps produce clear, separate spots that are easier to measure and compare.

Describe how to carry out paper chromatography.

Draw a pencil origin line near the bottom of the paper. Place a small spot of the sample on the line. Stand the paper in a suitable solvent with the solvent below the line. Allow the solvent to travel up the paper, remove the paper before the solvent reaches the top, and mark the solvent front in pencil.

Why should the solvent front be marked immediately?

The solvent may evaporate, making its final position difficult to identify.

What does a single spot on a chromatogram usually indicate?

The sample contains one substance in that solvent, suggesting it is pure.

What do several spots on a chromatogram indicate?

The sample contains several substances, so it is a mixture.

Can a pure substance always be identified by a single spot in one solvent?

One spot suggests purity, but mixtures can sometimes produce overlapping spots. Using different solvents can help distinguish substances.

How can chromatography help identify an unknown substance?

Compare the positions or Rf values of its spots with those of known substances using the same solvent and conditions.

What is the formula for Rf value?

Rf = distance travelled by substance ÷ distance travelled by solvent.

How do you measure the distance travelled by a substance in chromatography?

Measure from the origin line to the centre of the substance's spot.

How do you measure the distance travelled by the solvent?

Measure from the origin line to the solvent front.

Does an Rf value have units?

No. It is a ratio of two distances measured in the same units.

Can an Rf value be greater than 1?

No, because the substance cannot travel further than the solvent front in a correctly measured chromatogram.

A spot travels 3 cm and the solvent front travels 6 cm. What is the Rf value?

Rf = 3 ÷ 6 = 0.50.

A spot travels 4.5 cm and the solvent front travels 9 cm. What is the Rf value?

Rf = 4.5 ÷ 9 = 0.50.

A spot has an Rf value of 0.25 and the solvent travels 8 cm. How far did the spot travel?

Distance travelled by substance = 0.25 × 8 = 2 cm.

Why must chromatography results be compared using the same solvent and conditions?

Rf values depend on the solvent and experimental conditions, so comparisons are meaningful when these are kept the same.

What is instrumental analysis?

Using scientific instruments to identify substances or measure how much of a substance is present.

Why can instrumental methods be better than simple chemical tests?

They can be faster, more sensitive and more accurate, especially when only a small amount of a substance is available.

What is flame emission spectroscopy used for?

It can identify metal ions by the characteristic wavelengths of light they emit when excited in a flame.

What does a spectrum show in flame emission spectroscopy?

Lines at particular wavelengths that are characteristic of the element present.

How can flame emission spectroscopy identify an element?

Compare its characteristic spectrum with reference spectra from known elements.

How can instrumental methods identify very small amounts of substances?

They can detect characteristic signals at very low concentrations, often below the detection limit of simple chemical tests.

What is the test for hydrogen gas?

Hold a burning splint at the open end of a test tube containing the gas.

What is the positive result for hydrogen gas?

It burns with a squeaky pop.

What is the test for oxygen gas?

Insert a glowing splint into the gas.

What is the positive result for oxygen gas?

The glowing splint relights.

What is the test for carbon dioxide gas?

Bubble the gas through limewater, which is aqueous calcium hydroxide.

What is the positive result for carbon dioxide gas?

Limewater turns milky or cloudy.

Why does limewater turn milky when carbon dioxide is present?

An insoluble calcium carbonate precipitate forms.

What is the test for chlorine gas?

Hold damp litmus paper in the gas.

What is the positive result for chlorine gas?

The damp litmus paper is bleached white.

What does it mean when a gas bleaches damp litmus paper?

The litmus loses its colour and turns white.

Which gas relights a glowing splint?

Oxygen.

Which gas gives a squeaky pop with a burning splint?

Hydrogen.

Which gas turns limewater milky?

Carbon dioxide.

Which gas bleaches damp litmus paper white?

Chlorine.

What is a precipitate?

An insoluble solid formed when two solutions react.

What does qualitative analysis mean?

Finding out which substances or ions are present, rather than measuring their amounts.

What does quantitative analysis mean?

Measuring how much of a substance is present.

Why is it important to use the correct gas test?

Different gases have different characteristic test results, allowing them to be identified.

What safety precautions should be followed during gas tests?

Wear eye protection, use small quantities, avoid directly inhaling gases, and follow the teacher's instructions and laboratory risk assessment.

What should you write in an exam when describing a positive gas test?

State the test and the precise observation, such as 'a glowing splint relights' or 'limewater turns milky'.

What is the difference between identifying a substance and measuring its concentration?

Identification determines what the substance is; concentration measurement determines how much is present in a given volume.