Experimental techniques and chemical analysis

Apparatus

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

When you choose apparatus, think about accuracy: a burette (read to the nearest 0.05 cm³) or a volumetric pipette is far more accurate than a measuring cylinder; a balance that reads to 0.01 g is better than one that reads to 0.1 g for small masses. A gas can also be collected over water in an upturned measuring cylinder, but not if it dissolves in water (such as ammonia or hydrogen chloride).

Planning an experiment. Change only one variable (the independent variable), measure the effect on one other (the dependent variable), and keep everything else the same (the control variables). For example, to find how the temperature of an acid affects how fast magnesium reacts with it: change the temperature; measure the time for the magnesium to disappear, or the volume of hydrogen made each minute; keep the same volume and concentration of acid and the same mass and size of magnesium. Repeat each experiment and find the mean.

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 measures the volume of one solution needed to react with a measured amount of 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 observed indicator colour change. Choose a suitable indicator so this occurs close to the equivalence point, where the reactants have combined in the required ratio. The two points need not be exactly identical. Read the burette to find the volume added.

Litmus is not used for titrations, because its colour change is gradual, not sharp. Universal indicator is not used either, because it shows many colours.

Doing a titration accurately.

1. Rinse the burette with a little of the solution it will hold, and the pipette with its solution (rinsing with water would dilute them). Rinse the conical flask with distilled water only.
2. Fill the burette, open the tap briefly to fill the tip below the tap, and read the starting volume at the bottom of the meniscus, with your eye level with it.
3. Add the solution from the burette while swirling the flask, standing on a white tile so the colour change is easy to see. Near the end-point, add it drop by drop.
4. Read the final volume. The volume added is the titre 滴定量 = final reading − starting reading.
5. The first titration is a rough one. Repeat until two titres are concordant (within 0.10 cm³ of each other), and find the mean of the concordant titres only.

Worked example. Titres: rough 25.40 cm³, then 24.85, 24.90 and 25.20 cm³. The concordant titres are 24.85 and 24.90 (within 0.10 cm³); 25.20 is left out. Mean titre $= (24.85 + 24.90)/2 = 24.875$, given as 24.88 cm³.

A titration can also be used to prepare a soluble salt: find the volume of acid that exactly neutralises a measured volume of alkali, then repeat with the same volumes but no indicator, so the salt solution is pure. Evaporate some of the water and crystallise the salt.

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:

Crystallisation, step by step. Heat the solution in an evaporating basin to boil off some of the water, until the solution is saturated: test this by dipping in a cold glass rod — if crystals form on it, stop heating. Leave the solution to cool: the solid is less soluble in cold water, so crystals form. Filter off the crystals, wash them with a little cold distilled water, and dry them between filter papers or in a warm oven. Do not heat the solution to dryness if the salt contains water of crystallisation (such as hydrated copper(II) sulfate), or the crystals will break down.

Simple distillation, in detail. The bulb of the thermometer is level with the side arm, so it measures the temperature of the vapour going into the condenser. Cold water enters the condenser at the bottom and leaves at the top, so the condenser stays full of cold water. Anti-bumping granules make the liquid boil smoothly. In fractional distillation, for example of ethanol (boiling point 78 °C) and water (100 °C), the column has a range of temperatures, hot at the bottom and cooler at the top. The liquid with the lower boiling point reaches the top first and distils over while the thermometer reads 78 °C; when the reading rises, the next fraction is collected in a fresh container. Fractional distillation is used to separate crude oil and liquid air.

Choosing a method. Use the information you are given:

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. Pure substances matter in everyday life: medicines and food must not contain harmful impurities, even in small amounts.

Tests for water. Water turns white anhydrous copper(II) sulfate blue, and blue anhydrous cobalt(II) chloride paper pink. These show that water is present, but not that it is pure. To show that water is pure, check that it boils at exactly 100 °C and freezes at exactly 0 °C. Distilled water is used in practical work instead of tap water, because tap water contains dissolved ions that could react with the chemicals or give false test results.

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:

Identifying a compound

Worked example. A white solid X dissolves in water. (1) Aqueous sodium hydroxide gives a white precipitate that dissolves in excess; aqueous ammonia gives a white precipitate that also dissolves in excess. (2) Dilute nitric acid and aqueous barium nitrate give a white precipitate. Identify X. Test 1: a white precipitate that dissolves in excess of both sodium hydroxide and ammonia is the zinc ion, $\text{Zn}^{2+}$ (aluminium's precipitate does not dissolve in excess ammonia; calcium's does not dissolve in excess sodium hydroxide). Test 2: a white precipitate with barium nitrate shows sulfate, $\text{SO}_4^{2-}$. So X is zinc sulfate, $\text{ZnSO}_4$.

Nitric acid is added first in the halide and sulfate tests to remove any carbonate ions, which would also give a precipitate. Do not use hydrochloric acid in the halide test (it adds chloride ions), or sulfuric acid in the sulfate test (it adds sulfate ions).

• 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.
• Titration: use methyl orange or thymolphthalein, not litmus; read the bottom of the meniscus; average only concordant titres (within 0.10 cm³).
• Crystallisation: heat to the point of crystallisation (test with a glass rod), cool, filter, wash, dry.
• Water turns anhydrous copper(II) sulfate from white to blue; pure water boils at 100 °C.