Signs of a Chemical Reaction — Practice · Grade 8 Science

Chapter: Chemistry

What this practice covers

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

12 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 of these is NOT necessarily a sign of a chemical reaction?
  1. Formation of a precipitate
  2. Gas bubbles forming
  3. A change of shape
  4. A color change
Answer and explanation

A change of shape

A change of shape alone (like bending or cutting) is a physical change, not evidence of a chemical reaction.

Problem 2

Mixing two clear solutions produces a cloudy solid that settles to the bottom. This solid is called a…
  1. reactant
  2. catalyst
  3. precipitate
  4. solvent
Answer and explanation

precipitate

A solid that forms and settles out of a solution during a reaction is called a precipitate.

Problem 3

A reaction releases energy, raising the temperature of a solution from 20°C to 35°C. What is the temperature change (ΔT)?
Answer and explanation

15 °C

ΔT = 35 − 20 = 15°C.

Problem 4

Name two signs that suggest a chemical reaction has occurred, and explain why each suggests a new substance formed.
Answer and explanation

Gas bubbles suggest a new gaseous substance is escaping that wasn't there before, and a color change suggests new compounds have formed with different light-absorbing properties than the original substances.

Problem 5

Which of these is an example of a chemical reaction?
  1. ice melting
  2. iron rusting
  3. water boiling
  4. sugar dissolving in tea
Answer and explanation

iron rusting

Rusting forms a new substance, iron oxide, so it is a chemical reaction — unlike the other physical changes listed.

Problem 6

Why isn't a temperature change alone considered definite proof of a chemical reaction?
Answer and explanation

Physical processes can also change temperature — for example, dissolving some substances absorbs or releases heat even though no new substance forms — so a temperature change is only a clue, not final proof.

Problem 7

A clear liquid forms bubbles when warmed. Give two possible explanations and one useful follow-up observation. Explain the reasoning in your own words.
Answer and explanation

Both physical gas release and chemical gas production are possible; bubbles alone are insufficient evidence.

One possibility is boiling or dissolved gas leaving the liquid.

Another possibility is a reaction producing gas; checking whether new substances formed helps distinguish them.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

Water bubbles when boiling. Explain why bubbles alone do not prove a reaction. Show the reasoning, not just the conclusion.
Hints

Use more than one observation and distinguish a precipitate from material present initially.

Answer and explanation

Water vapor forms during a physical phase change. Gas bubbles can arise without producing a new chemical substance.

Problem 9

A student reports only that a mixture became warm. Suggest a comparison that would strengthen a reaction claim.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Record temperatures and compare with separate mixing controls, then look for evidence of new composition. Warming alone can also accompany physical mixing or dissolving.

Problem 10

Compare supplied descriptions of cutting iron, forming rust and mixing an alloy. Give an appropriate mechanism and explain why “all chemicals are dangerous” and “every mixture is a new compound” fail.
Hints

Use the illustrated study titled Chemical interactions in daily life and history. Keep the observations, model and conditions separate.

Answer and explanation

Cutting can retain iron identity while rust formation produces different substances. An alloy can be a mixture rather than one fixed-ratio compound. Chemical risk cannot be decided from the word alone; identity, amount, exposure and use need evidence. Relate the chosen process to a practical effect while keeping ordinary atom rearrangement distinct from nuclear element formation.

Cutting can retain iron identity while rust formation produces different substances. An alloy can be a mixture rather than one fixed-ratio compound. Chemical risk cannot be decided from the word alone; identity, amount, exposure and use need evidence. Relate the chosen process to a practical effect while keeping ordinary atom rearrangement distinct from nuclear element formation.

Response criteria

  • Distinguish composition from appearance.
  • Explain a daily/historical consequence with a mechanism.
  • Evaluate blanket chemical/mixture claims with conditions.

Problem 11

New substances form when the same atoms regroupNew substances form when the same atoms regroupReactants: 2H₂ + O₂Products: 2H₂OBefore and after: 4 H atoms and 2 O atoms. New combinations form.Blue = H; red = O. Counting model only; no hydrogen ignition activity.
This supplied water-forming reaction contains four hydrogen atoms and two oxygen atoms on each side. Their combinations change from H₂ and O₂ to H₂O. Colored circles distinguish atom types rather than literal colors, and the bonds are counting aids. It is a different example from a bath-bomb reaction; the figure does not claim that a bath bomb contains hydrogen gas or oxygen gas.
For this unit’s illustrated case, what evidence in the diagram supports a chemical change without supporting creation of new atoms?
Answer and explanation

The product molecules have a different composition/combination from the reactant molecules, while each element’s atom count stays four H and two O.

A reactant is a starting substance and a product is a resulting substance. Different combinations can give different characteristic properties. The drawing is an explanation for an already specified reaction, not proof that every visible bubble is a new substance.

The product molecules have a different composition/combination from the reactant molecules, while each element’s atom count stays four H and two O.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 12

An unknown mixture forms bubbles, but a control shows trapped air can also form bubbles. What conclusion is justified now and what additional evidence should distinguish the explanations?
Answer and explanation

Bubbles are observed; a new chemical substance is not yet established. Obtain appropriate supplied gas-identity or characteristic-property evidence and compare the control/conditions.

Record appearance without assigning an unsupported identity.

Use a discriminating comparison rather than one visual sign.

Bubbles are observed; a new chemical substance is not yet established. Obtain appropriate supplied gas-identity or characteristic-property evidence and compare the control/conditions.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Browse all courses