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C12 · 4.8Paper 2🔶 Yr9/10 — mostly covered

Chemical Analysis

Pure substances vs mixtures, chromatography (Rf values), flame tests, gas tests. Practical exam questions every year.

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Key facts

Pure substances and mixtures

  • Pure substance — single element or compound. Melts/boils at a single specific temperature.
  • Mixture — two or more substances not chemically joined. Melts/boils over a range of temperatures.
  • Formulation = a useful mixture deliberately designed to have specific properties (medicines, fuels, cleaning products).
  • Chromatography — separates mixtures of dissolved substances by how far they travel up paper/plate.
  • Rf = distance moved by substance ÷ distance moved by solvent.
  • Rf is characteristic for a substance in a given solvent — used for identification.

Flame tests and gas tests

  • Flame tests — identify metal ions. Li⁺ = crimson. Na⁺ = yellow. K⁺ = lilac. Ca²⁺ = orange-red. Cu²⁺ = green.
  • Gas tests:
  • Hydrogen — burning splint → squeaky pop.
  • Oxygen — glowing splint → relights.
  • Carbon dioxide — limewater → turns milky/cloudy.
  • Chlorine — damp litmus paper → bleaches (turns white).
  • Ammonia — damp red litmus paper → turns blue.
Exam questions — 4 questions · 10 marks
Extended questions included. Read the hint. Show all working. AI marks against the real AQA mark scheme.
1 markRf valueAQA 8462 P2 style

In a chromatography experiment, a substance travels 4.8 cm and the solvent front travels 8.0 cm. What is the Rf value?

A 1.67
B 0.6
C 3.2
D 0.4
1 markFlame testAQA 8462 P2 style

A solution gives a yellow flame in a flame test. Which ion does this indicate?

A Potassium (K⁺)
B Copper (Cu²⁺)
C Lithium (Li⁺)
D Sodium (Na⁺)
4 marksIdentify an unknown saltAQA 8462 P2 style

Describe a series of tests to identify whether an unknown white solid is sodium chloride or potassium chloride. Include the test, expected results, and conclusion for each test. (4 marks)

Hint: Flame test distinguishes Na (yellow) from K (lilac). Both are chlorides so the silver nitrate test cannot distinguish between them — use it to confirm chloride, flame test to confirm the metal.
+40 XP
1 markGas testAQA 8462 P2 style

What is the test for carbon dioxide and what result indicates its presence?

A A burning splint — it ignites with a pop
B A glowing splint — it relights
C Bubble through limewater — it turns milky/cloudy
D Damp litmus paper — it turns white

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