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 · 6 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: cutting paper? (physical change / chemical change) → physical changePrompt: Is a new substance made?
Which observation shows that a chemical reaction has happened? (Ice turns to water / Salt dissolves / The substance changes shape / A gas is given off and the colour changes) → 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? (All bubbles mean a chemical reaction. / Rusting needs both oxygen and water. / In a chemical reaction, atoms are rearranged to make new substances. / Burning is a chemical change.) → All bubbles mean a chemical reaction.Prompt: Does each change make a new substance?

Hook · 6 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 · 12 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 · 12 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: limewater turns cloudy? (a gas is given off / a colour change / a solid (precipitate) forms / an energy change (heat or light)) → a solid (precipitate) formsPrompt: Did you see bubbles, a new colour, a new solid or an energy change?
Which observation is the strongest evidence that burning a candle is a chemical change? (The candle gets shorter / Carbon dioxide and water vapour are made, and energy is released as heat and light / The wick is white / The wax melts) → Carbon dioxide and water vapour are made, and energy is released as heat and lightPrompt: Melting alone is only physical.

You do: independent practice · 17 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 · 7 min

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

Sugar dissolves in tea and seems to disappear. What kind of change is this? (physical, because the sugar is still there and could be recovered / 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 recoveredPrompt: Evaporating the water would leave something behind.
Chemical reactions: which statement is NOT true? (Atoms are destroyed in chemical reactions. / In a chemical reaction, new substances are made. / Rusting is a chemical change. / Many chemical reactions are hard to reverse.) → Atoms are destroyed in chemical reactions.Prompt: Is a new substance always made when bubbles appear?

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. Rusting is a chemical change.
- B. Many chemical reactions are hard to reverse.
- C. Dissolving salt in water is a physical change.
- D. Boiling water is a chemical reaction because bubbles form.
2
Question 2
Bread is toasted and goes brown and crisp. What kind of change is this?
- A. physical, because it is only heated
- B. physical, because it is still bread
- C. no change at all
- D. chemical, because new substances form that cannot be turned back into bread
3
Question 3
What evidence of a chemical reaction is shown here: limewater turns cloudy?
- 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
Sugar dissolves in tea and seems to disappear. What kind of change is this?
- A. chemical, because the tea gets sweeter
- B. chemical, because the sugar is destroyed
- C. the sugar turns into water
- D. physical, because the sugar is still there and could be recovered
5
Question 5
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)
6
Question 6
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
7
Question 7
Ice melts into water. A student says this is a chemical reaction because it changes. Why is this wrong?
- A. Ice and water are different elements
- B. Melting releases a new gas
- C. All changes are chemical reactions
- D. No new substance forms: it is still water, so it is a physical change
8
Question 8
What evidence of a chemical reaction is shown here: a hand warmer gets hot when opened?
- A. a gas is given off
- B. a colour change
- C. a solid (precipitate) forms
- D. an energy change (heat or light)

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Answer key: Spot the evidence that a chemical reaction happened

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