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6.1Chemistry 03. Changes To Materials

Identify whether a chemical reaction has taken place through… — Cambridge Checkpoint Science 0893 Revision Notes & Topical Questions

5 min read 8 parts Lesson 1 of 9 1 quiz

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6.1 Identifying whether a chemical reaction has taken place — CHECKPOINT - Lower Secondary Science 0893 notes: Identify whether a chemical reaction has taken place from observations such as gas given off, colour change, heat, change in pH or a precipitate.Chemical reaction — a ch…

  • Examples of reactions
  • Carbon dioxide
  • Limewater test
  • Properties of products
  • Hydrogen — a lit splint gives a squeaky pop
  • Oxygen — a glowing splint relights

Section 1 of 8Chemical reaction

Chemical reaction
  • Chemical reaction a change in which new substances (products) are made from the starting substances (reactants).
  • In a physical change no new substance forms.
Examples of reactions
  • Examples of reactions burning, rusting, digestion and decay.

Section 2 of 8Signs that a reaction has taken place

ClueExample
A gas is given offBubbles when magnesium reacts with hydrochloric acid
A reactant disappearsMagnesium ribbon is used up in acid
A colour changeBlack copper oxide and sulfuric acid make blue copper sulfate
Heat is producedZinc in hydrochloric acid makes the test tube feel hot
A change in pHNeutralisation of an alkali by an acid
A precipitate formsSilver nitrate and calcium chloride form solid silver chloride

Section 3 of 8Tests for gases

Hydrogen
  • Hydrogen a lit splint gives a squeaky pop.
Carbon dioxide
  • Carbon dioxide limewater turns cloudy.
Oxygen
  • Oxygen a glowing splint relights.

Section 4 of 8Precipitate

Precipitate
  • Precipitate an insoluble solid formed when two solutions react.
  • Silver chloride forms from silver nitrate and calcium chloride.
Limewater test
  • Limewater test carbon dioxide reacts with calcium hydroxide in limewater to form a precipitate of calcium carbonate that turns it cloudy.

Section 5 of 8Burning as a reaction

Burning
  • Burning a substance reacts with oxygen in the air to form oxides.
  • Carbon burns to make carbon dioxide.
  • Magnesium burns with a bright flame to make white magnesium oxide.
Properties of products
  • Properties of products they differ from the reactants, for example magnesium oxide has a melting point of 2800 degrees Celsius.

Section 6 of 8Safety in reaction tests

Safety in reaction tests
Safety glasses must be worn.
Magnesium burns brightly so do not look directly at the flame.
Potassium and water reaction is done only as a teacher demonstration.

Section 7 of 8Key terms

TermMeaning
ReactantA substance that takes part in a chemical reaction.
ProductA substance made in a chemical reaction.
Chemical reactionA change in which new substances form.
PrecipitateAn insoluble solid formed in a reaction between solutions.
LimewaterSolution that turns cloudy with carbon dioxide.

Section 8 of 8Exam technique

1
A change of state is a chemical reaction — no new substance forms.
2
Bubbles always show a reaction — boiling also makes bubbles, so the gas must be identified.
3
The reactant disappearing means it is destroyed — its atoms are in the products.
4
A colour change shows a physical change — a colour change is a sign of a chemical reaction.

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