Signs of a Chemical Reaction — Drills · Grade 8 Science

Chapter: Chemistry

What these drills covers

By the end of Signs of a Chemical Reaction you should be able to:

8 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: chemical reaction, reactant, product, precipitate, model, evidence, system boundary, assumption.

Problem 1

Which is the clearest sign that a chemical reaction has happened?
  1. The mixture got colder
  2. A gas was produced that was not there before
  3. The liquid was stirred
  4. The container became wet
Answer and explanation

A gas was produced that was not there before

A new gas indicates a new substance. Temperature change alone can accompany physical changes too.

Problem 2

A solid that forms when two solutions are mixed is called
  1. a precipitate
  2. a solvent
  3. a solute
  4. a catalyst
Answer and explanation

a precipitate

A precipitate is an insoluble solid produced by the reaction.

Problem 3

Bubbles alone do not prove a chemical reaction because
  1. bubbles are always air
  2. boiling also produces bubbles without a new substance
  3. gases are not substances
  4. bubbles cannot be measured
Answer and explanation

boiling also produces bubbles without a new substance

Boiling is a physical change that produces bubbles of the same substance in gas form.

Problem 4

A reaction's temperature falls from 22.0 °C to 14.5 °C. What is the temperature change, as a positive number of degrees?
Answer and explanation

7.5 °C

22.0 − 14.5 = 7.5 °C; the fall indicates energy was absorbed.

Problem 5

Which pairing of sign and meaning is correct?
  1. Colour change — always physical
  2. Light released — always physical
  3. Precipitate formed — evidence of a new substance
  4. Odour change — never relevant
Answer and explanation

Precipitate formed — evidence of a new substance

A precipitate is a new solid that was not present in either solution.

Problem 6

Name three signs that suggest a chemical reaction and explain why none of them is conclusive on its own.
Answer and explanation

Colour change, gas production and a temperature change all suggest a reaction. None is conclusive because each also occurs in physical changes: mixing paints changes colour, boiling produces gas, and dissolving can warm or cool a solution. A confident conclusion needs evidence that the substances present afterwards differ in their properties from those present before.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 7

Vinegar is added to baking soda and the mixture fizzes and cools slightly. Which sign is the stronger evidence, and why?
Answer and explanation

The fizzing is stronger evidence, because a gas is being produced from two substances that were liquid and solid, so a new substance has formed. The cooling only shows that energy was absorbed, which can also happen when a solid simply dissolves, so it supports the conclusion without establishing it.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

A reaction that releases energy to its surroundings is
  1. endothermic
  2. exothermic
  3. neutral
  4. physical
Answer and explanation

exothermic

Exothermic reactions release energy, which is usually detected as a temperature rise in the surroundings.

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