Chemical analysis and lab work

Chemistry is an experimental science: the properties of substances are observed in the lab, mixtures are separated, and ions and gases are identified with qualitative tests. Exams regularly ask about safety rules, apparatus, separation methods, collecting gases and "which reagent detects it?". If you know the reason behind each rule, you will not need to memorise it.

§1. Safety rules

• Add acid to water, never the reverse. Mixing concentrated H₂SO₄ with water releases a lot of heat. If water is poured into acid, it boils on the surface and spatters the acid.
• Never taste substances or sniff them directly — waft the smell towards you with your hand.
• When heating a test tube, never point its mouth at yourself or others; first warm the whole tube evenly.
• Keep flammable liquids (alcohol, ether, petrol) away from flames. Put out a spirit lamp with its cap, not by blowing.
• Acid on the skin — plenty of water, then NaHCO₃ (baking soda) solution; alkali — plenty of water, then a weak acetic or boric acid solution.
• Work with toxic or smelly gases (Cl₂, H₂S, NO₂) in a fume cupboard; goggles and a lab coat are compulsory.

§2. Lab apparatus and preparing solutions

• Vessels: test tube, beaker, conical and flat-bottomed flasks, porcelain evaporating dish, crucible (for strong heating).
• Measuring apparatus: measuring cylinder, pipette, burette (for titration), volumetric flask, balance.
• Other equipment: stand, spirit lamp, funnel, separating funnel, Kipp's apparatus (for producing gases), thermometer.

When reading a volume, keep your eye level with the meniscus; for aqueous solutions read the bottom of the concave meniscus.
To prepare a solution, use the mass fraction:

Worked example

How much salt and water are needed to prepare 200 g of a 15% NaCl solution?

1. m(NaCl) = 200·0.15 = 30 g.
2. m(water) = 200 − 30 = 170 g (with density 1 g/ml, that is 170 ml).
3. Weigh the salt on a balance, measure the water with a measuring cylinder, and stir in a beaker with a glass rod.

§3. Methods of separating mixtures

The method is chosen according to differences in physical properties:

• Settling and decanting — a heavy insoluble solid sinks and the liquid is carefully poured off.
• Filtration — an insoluble solid + a liquid (sand + water). The residue stays on the filter paper; the filtrate passes through.
• Evaporation — recovering a dissolved solid (salt from salt solution).
• Crystallisation — uses the dependence of solubility on temperature.
• Distillation — liquids with different boiling points; distilled water is made this way.
• Separating funnel — immiscible liquids (oil + water).
• Magnet — removing iron filings; chromatography — separating a mixture of dyes.

Worked example

How can a mixture of table salt, sand and iron filings be separated?

1. Remove the iron filings with a magnet.
2. Dissolve the rest in water — the salt dissolves, the sand does not.
3. Filter: the sand stays on the filter.
4. Evaporate the filtrate — table salt is obtained.

§4. Producing, collecting and identifying gases

The collection method depends on the gas's density relative to air (M(air) = 29) and its solubility in water:

Laboratory preparation:

• Zn + 2HCl → ZnCl₂ + H₂↑
• 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂↑ (heating); 2H₂O₂ → 2H₂O + O₂↑ (MnO₂ catalyst)
• CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
• 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃↑ + 2H₂O (heating)

Identifying gases:

• CO₂ — turns limewater milky: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O;
• O₂ — relights a glowing splint;
• H₂ — burns with a squeaky "pop" near a flame;
• NH₃ — pungent smell, turns damp red litmus paper blue;
• Cl₂ — yellow-green colour, turns starch-iodide paper blue.

Worked example

How is chlorine collected?

1. M(Cl₂) = 71, D(air) = 71/29 ≈ 2.45 > 1 — heavier than air.
2. The vessel is held mouth up; chlorine is toxic, so the work is done in a fume cupboard.

§5. Qualitative tests for ions and indicators

Flame colours: Na — yellow, K — lilac, Ca — brick-red, Li — crimson, Ba — yellow-green, Cu — green.
Indicators:

• Litmus: red in acid, purple in neutral solution, blue in alkali.
• Phenolphthalein: colourless in acidic and neutral solutions, crimson (pink) in alkali.
• Methyl orange: red (pink) in acid, orange in neutral, yellow in alkali.

Worked example

Three test tubes contain solutions of NaCl, Na₂SO₄ and Na₂CO₃. How can they be identified?

1. Add HCl to each: gas is given off — Na₂CO₃.
2. Add BaCl₂ to the other two: white precipitate — Na₂SO₄.
3. Add AgNO₃ to the last one: white curdy precipitate — NaCl.

The order matters: AgNO₃ can also precipitate with carbonate and sulfate, so it is used last.

§6. Lab calculations: dilution and gas volume

Two kinds of calculation dominate practical work. The first is dilution: when water is added to a concentrated solution, the mass of the dissolved substance stays the same and only the mass of the solution grows. The second is the volume of gas released: find the amount of substance (mol) and multiply it by the molar volume at standard conditions (STP).

Worked example

How much water must be added to 50 g of 20% NaCl solution to obtain a 10% solution?

1. Mass of salt: 50·0.20 = 10 g — it does not change.
2. Mass of the new solution: 10/0.10 = 100 g.
3. Water to add: 100 − 50 = 50 g.

Worked example

13 g of zinc dissolves completely in hydrochloric acid. What volume of hydrogen (STP) is released?

1. Zn + 2HCl → ZnCl₂ + H₂↑, n(Zn) = 13/65 = 0.2 mol.
2. From the equation n(H₂) = n(Zn) = 0.2 mol.
3. V(H₂) = 0.2·22.4 = 4.48 L.

In practice the volume is slightly lower: some gas stays in the apparatus and the zinc carries an oxide film. That is why labs also calculate the practical yield η = V(actual)/V(theoretical).

Check yourself

1. Which of H₂, CO₂, O₂ is collected in an upside-down vessel?

2. Which reagent detects the sulfate ion?

3. How can salt be recovered from a table-salt solution?

4. What colour is phenolphthalein in NaOH and in HCl solutions?

5. How many grams of salt and water are needed for 150 g of a 10% solution?

6. Which gas turns limewater milky? Write the equation.

7. What volume of oxygen (STP) is released when 31.6 g of KMnO₄ is heated?

8. 100 g of 30% solution is mixed with 200 g of 15% solution. What is the mass fraction?

Common mistakes

• Thinking phenolphthalein turns red in acid — in acid it is colourless.
• Collecting NH₃ mouth up or over water (it is lighter than air and very soluble in water).
• Pouring water into concentrated acid.
• Trying to separate a dissolved substance by filtration — the whole solution passes through the filter.
• Adding AgNO₃ first in qualitative analysis and misreading the result.
• Confusing the mass of water with the mass of solution in solution calculations.
• Assuming the mass of the solute changes on dilution — only water is added.