Experimental techniques and chemical analysis

Apparatus

You should know which piece of apparatus to use for each measurement:

Key words

These words are used throughout practical chemistry:

• A solvent 溶剂 is a substance that dissolves a solute.
• A solute 溶质 is the substance that dissolves in the solvent.
• A solution 溶液 is a mixture of one or more solutes dissolved in a solvent.
• A saturated solution 饱和溶液 holds the maximum amount of solute that will dissolve at a given temperature.
• A residue 残渣 is the substance left behind after evaporation, distillation or filtration.
• A filtrate 滤液 is the liquid that has passed through a filter.

A titration 滴定 finds the exact volume of one solution that reacts with another.

• Use a volumetric pipette to measure a fixed volume of one solution into a flask.
• Add a few drops of a suitable indicator 指示剂.
• Add the other solution from a burette, slowly, until the colour just changes.

The end-point 终点 is the moment the indicator changes colour, which shows the reaction is exactly complete. You read the burette to find the volume added.

Solution is run from the burette into the flask until the indicator just changes colour (the end-point)

A real titration: solution from the burette is added to the flask, where the indicator has turned orange

Paper chromatography 纸色谱法 separates a mixture of soluble substances. You put a spot of the mixture near the bottom of the paper, then stand the paper in a solvent. As the solvent rises up the paper, the substances move different distances, so they separate.

As the solvent rises, the substances travel different distances and separate; $R_f$ is the spot distance ($a$) divided by the solvent distance ($b$)

On real chromatograms, each substance in the mixture rises a different distance, leaving a separate coloured spot

• For coloured substances you can see the spots directly.
• For colourless substances you must spray a locating agent 显色剂 to make the spots show up.

The finished paper is a chromatogram 色谱图. You can use it to:

• identify an unknown substance by comparing it with known substances;
• tell if a substance is pure (a pure substance gives only one spot).

You can also calculate the $R_{\text{f}}$ value of a spot:

Worked example. On a chromatogram the solvent front has risen 8.0 cm from the baseline, and a spot has moved 6.0 cm. Find its $R_{\text{f}}$ value.

Both distances are measured from the baseline (the pencil line), and the spot's distance is taken to the centre of the spot. An $R_{\text{f}}$ has no unit, and it is always less than 1, because a spot can never travel further than the solvent carrying it - an answer above 1 means the two distances have been divided the wrong way round.

You choose a method based on the mixture:

Filtration: the insoluble solid stays on the filter paper (residue) and the liquid passes through (filtrate)

Simple distillation: the solvent boils off, the condenser cools it back to a liquid, and the pure distillate is collected

Fractional distillation adds a fractionating column, so liquids with different boiling points separate cleanly

You can check the purity of a substance using its melting point 熔点 and boiling point 沸点: a pure substance melts and boils at sharp, fixed temperatures, while impurities lower the melting point and raise the boiling point.

Tests for anions

An anion 阴离子 is a negative ion.

Tests for cations

A cation 阳离子 is a positive ion. Add aqueous sodium hydroxide, or aqueous ammonia, and look at the precipitate 沉淀 formed.

Adding sodium hydroxide forms a coloured precipitate for some metal ions

Tests for gases

Testing gases: hydrogen pops, oxygen relights a glowing splint

Flame tests

A flame test 焰色试验 identifies some metal cations by the colour they give to a flame:

A flame test: the metal ion gives the flame a colour

• Pick apparatus by the measurement: a burette or pipette for accurate volumes, a measuring cylinder for a rough volume, a gas syringe for a gas volume, a balance for mass.
• $R_f = \dfrac{\text{distance moved by the substance}}{\text{distance moved by the solvent}}$, so it is always less than 1. A pure substance gives a single spot.
• Learn the anion tests: a carbonate fizzes and the gas turns limewater milky; halides with silver nitrate give a chloride white, bromide cream, iodide yellow precipitate; a sulfate gives a white precipitate with barium nitrate.
• Learn the gas tests: hydrogen gives a squeaky pop, oxygen relights a glowing splint, carbon dioxide turns limewater milky, chlorine bleaches damp litmus, ammonia turns damp red litmus blue.
• For cations, add sodium hydroxide and read the precipitate colour: copper(II) light blue, iron(II) green, iron(III) red-brown.