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USS · Paper II

Chemistry

Matter, elements and compounds, acids and bases, everyday chemistry.

7 of the 50 questionsWrite this topic →
What the paper asks

Seven questions. Acids and bases, physical and chemical change, the composition of air, common formulas and symbols, and simple separation methods. Symbols and everyday chemistry carry most of the marks.

Acids, bases and pH
  • pH below 7 — acidic. 7 — neutral. Above 7 — basic. The lower the number, the stronger the acid.
  • An acid turns blue litmus red; a base turns red litmus blue. The two tests are mirror images.
  • Acid + base → salt + water (neutralisation).
  • Acid + metal → salt + hydrogen — the gas burns with a pop.
  • Acid + carbonate → salt + water + carbon dioxide — the gas turns lime water milky.
AcidFound in
Acetic acidvinegar
Citric acidlemon, orange
Lactic acidsour milk, curd
Hydrochloric acidthe stomach
Formic acidan ant's sting
Tartaric acidtamarind
Physical or chemical change?
  • Physical — no new substance, usually reversible: melting ice, boiling water, dissolving sugar, breaking glass.
  • Chemical — a new substance forms, not easily reversed: rusting, burning, curdling milk, cooking.
  • The test is not 'did it look different' but 'is it a different substance now'.
Symbols and formulas worth knowing
NameSymbol / formula
GoldAu (aurum)
SilverAg (argentum)
IronFe (ferrum)
SodiumNa (natrium)
PotassiumK (kalium)
CopperCu (cuprum)
WaterH₂O
Common saltNaCl — sodium chloride
Baking sodaNaHCO₃ — sodium bicarbonate
Carbon dioxideCO₂
Limestone / chalkCaCO₃ — calcium carbonate
Air, and elements, compounds and mixtures
  • Air is about 78% nitrogen and 21% oxygen — the gas we use is not the gas there is most of.
  • Air is a mixture: the gases are not chemically joined and the proportions vary.
  • An element has one kind of atom; a compound has two or more chemically joined in a fixed ratio; a mixture has no fixed ratio and can be separated physically.
  • Burning needs oxygen. Carbon dioxide does the opposite, which is why it is used in extinguishers.
Solutions and separation
  • Solute dissolves; solvent does the dissolving; together they are a solution.
  • A saturated solution can dissolve no more at that temperature. 'Concentrated' only means a lot of solute.
  • Filtration separates an insoluble solid from a liquid. If it has dissolved, filtration will not work — use evaporation.
  • Distillation recovers the liquid as well as the solid.
  • Evaporation happens at the surface at any temperature; boiling happens throughout the liquid at one fixed temperature.
Alloys
  • Brass = copper + zinc.
  • Bronze = copper + tin.
  • Steel = iron + a small amount of carbon.
Worked through

A gas turns lime water milky. What is it, and what does that tell you?

  1. Turning lime water milky is the standard test for carbon dioxide.
  2. CO₂ forms insoluble calcium carbonate, which is the cloudiness.

Is rusting a physical or a chemical change? Why?

  1. Iron combines with oxygen and water to form iron oxide.
  2. Iron oxide is a different substance from iron.
  3. It cannot be reversed by cooling or by any simple physical means.
Where the marks go

These are the mistakes, not the rules — each one is something a student who knows the topic still does under time pressure.

  1. Answering oxygen for the most abundant gas in air. It is nitrogen, at 78%.
  2. Calling air a compound. Its gases are mixed, not chemically joined.
  3. Expecting carbon dioxide from an acid and a metal. That gives hydrogen; CO₂ comes from a carbonate.
  4. Confusing 'saturated' with 'concentrated'.
  5. Trying to filter out something that has dissolved.
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