Preparation, separation and testing
Preparing a substance
Plan the reaction, choose the apparatus and decide how to collect the product. Example: marble (calcium carbonate) + dilute hydrochloric acid → calcium chloride + water + carbon dioxide. CO₂ is heavier than air, so collect it by upward delivery (jar upright). A gas that is lighter than air (hydrogen) or does not dissolve can be collected over water or by downward delivery.
Separating a mixture
Pick the method from the property that differs:
- Solid not dissolved in liquid → filtration.
- Dissolved solid → evaporation or crystallisation.
- Liquids with different boiling points → distillation.
- Dyes or similar mixtures → chromatography.
Testing to identify
Use a test with a clear result: CO₂ turns limewater milky; a lit splint pops with hydrogen; flame tests (sodium yellow, copper green, potassium lilac); universal indicator red-orange for acids, green neutral, blue-purple for alkalis. Purity check: a pure solid melts at a sharp temperature.
Constant (exact) measurements
Some experiments are quantitative: we want a number. Good habits make the number reliable:
- Use the right tool: balance for mass, burette or pipette for exact volume, measuring cylinder for rough volume.
- Read the bottom of the curve (meniscus) at eye level.
- Repeat and keep readings that agree (concordant titres: within 0.1 mL). Average them. Ignore the first rough run.
- Keep other conditions constant: same temperature, same amounts.
Titration. Add acid from the burette to a known volume of alkali with an indicator until the colour just changes (the end point). At that point acid and alkali have exactly reacted: C₁V₁ = C₂V₂ for a 1 : 1 reaction.
Percentage yield = actual mass ÷ theoretical mass × 100.
Industrial-process simulation in the lab
Factories cannot be tried out in a school lab, but we can build a small model of the steps: feed → reactor with catalyst → cooler and separator → product, with unused reactant recycled back.
Why recycle? If only 30% reacts in one pass, throwing the rest away wastes 70%. After 3 passes the total is 1 − 0.7³ = 66%. After 8 passes it is about 94%.
A simple lab model: dissolve and crystallise to make a salt, filter the crystals, and reuse the leftover liquid (mother liquor) in the next batch. Look at what is saved: raw material, energy and waste. Compare with real examples such as the Haber process, where nitrogen and hydrogen that do not react are sent back.
Environmental testing: is this water safe?
Chemistry helps society (STSE: science, technology, society, environment). To check a water sample we measure:
- pH with universal indicator or a meter. Drinking water is about 6.5 to 8.5. Each pH step is 10 times in hydrogen-ion amount.
- Nitrate (from fertiliser and sewage) with a test strip. A common drinking limit is 50 mg/L as nitrate.
- Others: hardness, chloride, dissolved oxygen, germs.
Good practice: collect in a clean bottle, label place and time, test the same day, compare with a safe limit, and report. A reading outside the limit is a clue to find the source (a factory drain, farm run-off or sewage).
Try it: kitchen and garden chemistry
Predict, then check. At home, mix soil with water, filter it through a cloth, and look at the clear water. Is anything dissolved? Let a little evaporate on a plate. A red-cabbage juice indicator turns pink in lemon juice and green-blue with baking soda. Draw a table of what you predicted and what you saw. Do not taste or smell lab chemicals, and always ask an adult before heating.
Key formulas and definitions
- C₁V₁ = C₂V₂ (1 : 1 acid–alkali titration)
- Percentage yield = (actual ÷ theoretical) × 100
- Mean titre = sum of concordant readings ÷ number of readings
- Fraction reacted after n passes with recycling = 1 − (1 − p)ⁿ
- Drinking-water guide: pH 6.5 to 8.5; nitrate ≤ 50 mg/L
Worked examples
1. Concordant titres are 20.1, 20.0 and 20.1 mL. Find the mean titre.
(20.1 + 20.0 + 20.1) ÷ 3 = 60.2 ÷ 3 = 20.07 mL ≈ 20.1 mL.
2. 25.0 mL of NaOH needs 20.0 mL of 0.10 mol/L HCl. Find the NaOH concentration.
1 : 1 reaction, so C₁V₁ = C₂V₂. C(NaOH) = 0.10 × 20.0 ÷ 25.0 = 0.080 mol/L.
3. You get 8.4 g of a salt but the maths says 12.0 g is possible. Find the percentage yield.
8.4 ÷ 12.0 × 100 = 70%.
4. Each pass converts 30% of the feed, and unused gas is recycled. What share is converted after 3 passes?
Unreacted = 0.7³ = 0.343. Converted = 1 − 0.343 = 0.657, about 66%.
5. 5.0 g of CaCO₃ (M = 100 g/mol) reacts with excess acid. What volume of CO₂ forms at room conditions (24 L/mol)?
Moles = 5.0 ÷ 100 = 0.050 mol. CaCO₃ : CO₂ = 1 : 1. Volume = 0.050 × 24 = 1.2 L.
6. A river sample has pH 4.8 and nitrate 64 mg/L. Is it safe to drink? Which clue points to a factory drain?
No: pH is below 6.5 and nitrate is above 50 mg/L. The low pH (acidic) points to an acid-releasing factory drain; the high nitrate points to fertiliser or sewage as well.
Common mistakes
- Reading the burette from above or below instead of at eye level.
- Averaging every titre, including the rough first run, instead of the concordant ones.
- Collecting a heavy gas like CO₂ by downward delivery (jar upside down). It will not stay.
- Calling a nearly clear liquid "pure". Pure means one substance; test the melting or boiling point.