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: sugar dissolving in tea? (physical change / chemical change) → physical changePrompt: Is a new substance made?
A nail left outside turns orange-brown and flaky. What has happened? (The nail melted / Iron reacted with oxygen and water to make rust / The nail dissolved / Paint fell on it) → Iron reacted with oxygen and water to make rustPrompt: Rusting needs both air and water.
Physical and chemical changes: which statement is NOT true? (Burning is a chemical change. / Dissolving is a physical change. / All bubbles mean a chemical reaction. / In a chemical reaction, atoms are rearranged to make new substances.) → All bubbles mean a chemical reaction.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?
Ice melts into water. A student says this is a chemical reaction because it changes. Why is this wrong? (Melting releases a new gas / No new substance forms: it is still water, so it is a physical change / Ice and water are different elements / All changes are chemical reactions) → No new substance forms: it is still water, so it is a physical changePrompt: Is the substance after the change different from before?

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.

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 wax melts / The wick is white) → Carbon dioxide and water vapour are made, and energy is released as heat and lightPrompt: Melting alone is only physical.
What evidence of a chemical reaction is shown here: a glow stick gives out light when bent? (a gas is given off / a colour change / a solid (precipitate) forms / an energy change (heat or light)) → an energy change (heat or light)Prompt: Did you see bubbles, a new colour, a new solid or an energy change?

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
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)
2
Question 2
Which observation is the strongest evidence that burning a candle is a chemical change?
- A. The wick is white
- B. The candle gets shorter
- C. Carbon dioxide and water vapour are made, and energy is released as heat and light
- D. The wax melts
3
Question 3
Chemical reactions: which statement is NOT true?
- A. Atoms are destroyed in chemical reactions.
- B. Many chemical reactions are hard to reverse.
- C. Rusting is a chemical change.
- D. In a chemical reaction, new substances are made.
4
Question 4
Sugar dissolves in tea and seems to disappear. What kind of change is this?
- A. chemical, because the sugar is destroyed
- B. chemical, because the tea gets sweeter
- C. physical, because the sugar is still there and could be recovered
- D. the sugar turns into water
5
Question 5
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)
6
Question 6
Bread is toasted and goes brown and crisp. What kind of change is this?
- A. physical, because it is still bread
- B. chemical, because new substances form that cannot be turned back into bread
- C. no change at all
- D. physical, because it is only heated
7
Question 7
What evidence of a chemical reaction is shown here: fizzing when vinegar is poured on baking soda?
- 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
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. No new substance forms: it is still water, so it is a physical change
- D. All changes are chemical reactions

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Answer key: Spot the evidence that a chemical reaction happened

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