Spot the evidence that a chemical reaction happened

Chemical reactions, acids and bases

CCSS NGSS.MS-PS1-2England: KS3 Chemistry: Chemical reactions

Objective and success criteria

Students will: Spot the evidence that a chemical reaction happened.

I can answer questions on today's skill on my own, without help.
I can explain each step and say why it works.
I can check my answer and spot a mistake.

Before and after

Needs first: Physical and chemical changes

Leads to: Conservation of mass in reactions

Materials: Teacher demo or video, Indicator color chart, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 5 min

Quick retrieval from earlier learning (Physical and chemical changes). Students answer on mini whiteboards.
Show answers together and note who needs support in the main task.

Is this a physical or a chemical change: fireworks exploding? (physical change / chemical change) → chemical changePrompt: Is a new substance made?
Which observation shows that a chemical reaction has happened? (A gas is given off and the colour changes / Salt dissolves / Ice turns to water / The substance changes shape) → A gas is given off and the colour changesPrompt: Look for signs of something new being made.
Physical and chemical changes: which statement is NOT true? (Atoms are destroyed in chemical reactions. / Rusting needs both oxygen and water. / In a chemical reaction, atoms are rearranged to make new substances. / Burning is a chemical change.) → Atoms are destroyed in chemical reactions.Prompt: Does each change make a new substance?

Hook · 5 min

Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
Share the objective: Spot the evidence that a chemical reaction happened. Read the success criteria together.

I do: model it · 10 min

Work through the example out loud, saying why you do each step, not just what you do.
Example: What evidence of a chemical reaction is shown here: a shiny iron nail slowly turns orange-brown in damp air?
Step 1: A new colour can show a new substance has formed.
Step 2: So the answer is: a colour change
Answer: a colour change
Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 min

Do these together. Students try each step on whiteboards before you reveal it.
Use the hints as prompts rather than giving the answer.

What evidence of a chemical reaction is shown here: a shiny iron nail slowly turns orange-brown in damp air? (a gas is given off / a colour change / a solid (precipitate) forms / an energy change (heat or light)) → a colour changePrompt: Did you see bubbles, a new colour, a new solid or an energy change?
Sugar dissolves in tea and seems to disappear. What kind of change is this? (chemical, because the tea gets sweeter / the sugar turns into water / chemical, because the sugar is destroyed / physical, because the sugar is still there and could be recovered) → physical, because the sugar is still there and could be recoveredPrompt: Evaporating the water would leave something behind.

You do: independent practice · 10 min

Students work on the student sheet. Circulate and check the first two answers of every student early.
Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

What evidence of a chemical reaction is shown here: bubbles form when an antacid tablet is dropped in water? (a gas is given off / a colour change / a solid (precipitate) forms / an energy change (heat or light)) → a gas is given offPrompt: Did you see bubbles, a new colour, a new solid or an energy change?
Ice melts into water. A student says this is a chemical reaction because it changes. Why is this wrong? (No new substance forms: it is still water, so it is a physical change / Melting releases a new gas / All changes are chemical reactions / Ice and water are different elements) → No new substance forms: it is still water, so it is a physical changePrompt: Is the substance after the change different from before?

Watch out for

Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

Review first: Physical and chemical changes.
Use concrete materials: teacher demo or video.
Give the first step of each question, or a partly worked example to finish.
Pair with a confident partner for the guided practice.

Stretch

Move on to: Conservation of mass in reactions.
Explain a common mistake in writing and how to fix it.
Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-6.

Free lesson plan from eurekami.com/lesson-plan-builder

eurekami

Spot the evidence that a chemical reaction happened

Name

Date

Score/ 8

1
Question 1
Chemical reactions: which statement is NOT true?
- A. Dissolving salt in water is a physical change.
- B. In a chemical reaction, new substances are made.
- C. Boiling water is a chemical reaction because bubbles form.
- D. Many chemical reactions are hard to reverse.
2
Question 2
Which observation is the strongest evidence that burning a candle is a chemical change?
- A. The wax melts
- B. The candle gets shorter
- C. The wick is white
- D. Carbon dioxide and water vapour are made, and energy is released as heat and light
3
Question 3
What evidence of a chemical reaction is shown here: a cut apple turns brown?
- A. a gas is given off
- B. a colour change
- C. a solid (precipitate) forms
- D. an energy change (heat or light)
4
Question 4
Ice melts into water. A student says this is a chemical reaction because it changes. Why is this wrong?
- A. Melting releases a new gas
- B. No new substance forms: it is still water, so it is a physical change
- C. Ice and water are different elements
- D. All changes are chemical reactions
5
Question 5
What evidence of a chemical reaction is shown here: an instant cold pack gets cold when squeezed?
- A. a gas is given off
- B. a colour change
- C. a solid (precipitate) forms
- D. an energy change (heat or light)
6
Question 6
Sugar dissolves in tea and seems to disappear. What kind of change is this?
- A. physical, because the sugar is still there and could be recovered
- B. chemical, because the tea gets sweeter
- C. chemical, because the sugar is destroyed
- D. the sugar turns into water
7
Question 7
What evidence of a chemical reaction is shown here: two clear solutions are mixed and a yellow solid appears?
- A. a gas is given off
- B. a colour change
- C. a solid (precipitate) forms
- D. an energy change (heat or light)
8
Question 8
Bread is toasted and goes brown and crisp. What kind of change is this?
- A. chemical, because new substances form that cannot be turned back into bread
- B. physical, because it is only heated
- C. no change at all
- D. physical, because it is still bread

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Answer key: Spot the evidence that a chemical reaction happened

1. C. Boiling water is a chemical reaction because bubbles form.
2. D. Carbon dioxide and water vapour are made, and energy is released as heat and light
3. B. a colour change
4. B. No new substance forms: it is still water, so it is a physical change
5. D. an energy change (heat or light)
6. A. physical, because the sugar is still there and could be recovered
7. C. a solid (precipitate) forms
8. A. chemical, because new substances form that cannot be turned back into bread