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.

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.

• 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:

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.

Tests for gases

Flame tests

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

• 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.