Evidence for Chemical Change
In 1886, American chemist Charles Hall passed electricity through molten aluminium oxide and watched it fizz, glow orange, and produce 28 kg of silvery aluminium metal — 5 simultaneous signs that a chemical change had occurred.
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Q1 · Think of two things that change at home (one in the kitchen, one outside). For each, write down anything you can see, hear, smell or feel that tells you a change is happening.
Q2 · If you mix two clear liquids and the mixture stays clear and the same temperature, can you tell that a chemical change has happened? Why or why not?
● Know
- The five common signs of a chemical change
- That a chemical change makes a new substance
- Everyday examples of each sign
● Understand
- Why one sign alone is not always proof of a chemical change
- Why more signs at once gives stronger evidence
- Why some signs (like temperature change) can also happen in physical changes
● Can do
- List the five signs of chemical change
- Decide whether an observation is good evidence of a chemical change
- Describe a real-world example for each sign
- Chemical change
- Precipitate
- Reactant
- Product
- Evidence
- A solid that forms from two clear liquids
- Observations that support a claim
- A change that makes a new substance
- A new substance made by a reaction
- A substance you start with
Drop a small piece of steel wool into a jar and seal the lid. Come back the next day and the steel wool looks orange-brown and the jar feels slightly warm — two clues that something new was made overnight. When a chemical change happens, one or more brand-new substances form. You can't always see them directly, but they usually leave clues. Scientists look for these five common signs:
| # | Sign | Example you can see |
|---|---|---|
| 1 | Colour change | A clean iron nail turns rusty orange after weeks in damp air. The grey iron has turned into orange iron oxide. |
| 2 | Gas produced | Vinegar and bicarb soda fizz and bubble. A new gas (carbon dioxide) is being made. |
| 3 | Precipitate forms | Two clear liquids are mixed and the mixture suddenly looks cloudy. A new solid has formed in the liquid. |
| 4 | Temperature change | The mixture gets warmer (or colder) on its own. Steel wool in vinegar gets warm; vinegar and bicarb gets cold. |
| 5 | Light or sound | A sparkler throws sparks; magnesium ribbon burns with a bright white flash; fireworks bang and flash. |
If you see several of these together (for example, bubbling AND a temperature drop AND a colour change), the evidence is very strong that a chemical change is happening.
The five signs of chemical change are change, produced, forming, change, and light or .
Wrong: "Any bubbles mean a chemical change." Boiling water makes bubbles too — that is just water changing from liquid to gas. It is a physical change, not a new substance.
Right: Bubbles are a sign only when a new gas is being made. Look for other signs too — colour change, temperature change — before deciding.
Wrong: "Temperature change always means a chemical change." Ice melting absorbs heat from the room (so the surroundings cool), but it is still just water changing state.
Right: A temperature change on its own is weak evidence. Combined with bubbling, colour change or a precipitate it becomes strong evidence.
Wrong: "If the colour changes, it must be a chemical change." Mixing red and yellow paint changes colour but no new substance is made — it is just a mixture of two paints.
Right: Colour change in a single substance reacting (like a nail rusting) is strong evidence. Mixing coloured liquids is not.
An iron nail left outside slowly turns from shiny grey to flaky orange. The iron is reacting with oxygen in the air (and water) to form a new substance called iron oxide — what we call rust.
- Grey iron + oxygen + water → orange iron oxide.
- The product is brittle and flakes off, not strong like iron.
- You cannot turn rust back into iron just by drying it.
Slow chemical changes like rusting can take weeks, so the colour change is the main sign you notice.
Bubbles from a real reaction: When vinegar (an acid) meets bicarb soda, a brand-new gas (carbon dioxide, CO2) bubbles out. The bubbles aren't water vapour — they are an entirely different substance to either starting material.
Precipitate (cloudy from clear): If you mix two clear liquids and the mixture suddenly looks milky, a new solid has appeared. We call this solid a precipitate. A common school example is mixing lead nitrate solution with potassium iodide — both clear and colourless to begin with, but the mixture turns bright yellow.
If you see bubbles AND a cloudy precipitate AND a temperature change all at once — that's three signs together. Very strong evidence.
Temperature change: Some reactions release energy and the mixture gets warmer (steel wool soaked in vinegar warms up). Other reactions absorb energy and the mixture gets colder (vinegar + bicarb cools down).
Light or sound: Magnesium ribbon burns with a dazzling white light. A sparkler throws hot sparks. Fireworks bang and crack. These reactions release so much energy that you can see or hear it directly.
Burning is the most familiar example — it almost always shows up to four signs at once: light, heat, colour change and a new gas given off.
A spoon of bicarb soda is added to a beaker of vinegar. You see bubbles rising, the mixture goes a little cloudy and the beaker feels noticeably colder when you touch it. Predict: how many of the five signs of chemical change are happening, and which ones?
How close was your prediction?
Excellent — spotting three signs at once shows you can stack evidence.
Good — remember that cooling is just as valid a sign as warming.
At the start of this lesson you were asked: Vinegar and bicarb soda meet in a glass — it bubbles, gets colder and the liquid clouds. How many signs of chemical change is that? Can you count them now?
Write out each sign from the vinegar-and-bicarb experiment and name it correctly. Then explain why seeing multiple signs at once is strong evidence that a chemical change has occurred.
Q1. List the five common signs of chemical change. Give one short everyday example for each. (3 marks)
Q2. A student adds a fizzing tablet (like an antacid) to a glass of water. The tablet bubbles, the glass gets a bit colder and the liquid turns cloudy. Identify which signs of chemical change are present and explain how each sign supports the claim. (4 marks)
Q3. A student claims that "if the temperature changes, a chemical change must have happened". Evaluate this claim. Use at least one example that supports it and at least one that does not. (4 marks)
Answers
▾MCQ 1
C — A change in shape alone is a physical change. The five signs of chemical change are colour change, gas, precipitate, temperature change, and light/sound.
MCQ 2
A — Rusting is a slow chemical change. The most obvious sign is the colour change from shiny grey iron to dull orange iron oxide.
MCQ 3
D — Boiling water also makes bubbles, but boiling is a physical change. You should look for other signs (colour change, temperature change) before deciding.
MCQ 4
B — A new solid (precipitate) has formed in the liquid, and there is also a colour change. Two signs at once is strong evidence of a chemical change.
MCQ 5
C — Burning magnesium gives off bright light, lots of heat and changes colour (silver ribbon → white powder). At least three signs happen at once.
Short Answer 1
Model answer: (1) Colour change — a nail rusting orange; (2) Gas produced — vinegar and bicarb fizzing; (3) Precipitate — two clear liquids mixing and turning cloudy; (4) Temperature change — steel wool in vinegar getting warmer; (5) Light or sound — a sparkler throwing sparks. Accept any sensible example.
Short Answer 2
Model answer: Three signs are present. (1) Gas produced — the bubbles are a new gas (carbon dioxide) being made from the tablet reacting with water. (2) Temperature change — the glass getting colder shows energy is being absorbed by the reaction. (3) Precipitate / cloudiness — a new substance is forming that is not fully dissolved. Three signs together is strong evidence that a chemical change has occurred.
Short Answer 3
Model answer: The claim is partly correct. Many chemical changes do cause temperature changes — for example, vinegar reacting with bicarb soda gets colder, and burning magnesium gets extremely hot, so temperature change can be evidence. However, the claim is too strong: ice melting in a warm drink causes the drink to cool, but that is only a physical change. Temperature change alone is weak evidence — you need other signs (colour, gas, precipitate, light) before you can be confident that a chemical change has happened.