Chemical Reactions
When bonds break and substances that were not there appear
How this deck works. Every slide is colour-coded: Read for the explanation, Look for diagrams and models, Do for experiments and simulations, and Check for questions. Move with the buttons at the bottom, or the ← → keys on a keyboard.
Now the bonds break.
Where you left off · Topic 2
You built molecules out of atoms and turned the energy up. The atoms jiggled harder, but the bonds held and the formula never changed. Water stayed water. Methane stayed methane.
This topic is what happens when the bonds do not hold. Atoms let go of their old partners, grab new ones, and what comes out the other side is a substance that was not there before.
It is happening inside you right now, in the pan on your stove, in the engine of every car on I-66, and slowly on the underside of every truck that drove through road salt last February.
Old bonds break, new bonds form
A chemical reaction is a process in which substances are changed into different substances. The atoms are not destroyed and no new atoms appear — they simply let go of the partners they were bonded to and bond to different ones.
The substances you start with are the reactants. The substances you end up with are the products. Everything in chemistry is written in that order: reactants first, then an arrow, then products.
Take the flame on a gas stove:
- Reactants: methane (CH₄) from the gas line, and oxygen (O₂) from the air in your kitchen.
- Products: carbon dioxide (CO₂) and water vapor (H₂O).
Look at what happened to the atoms. Every carbon, hydrogen and oxygen atom that went in is still there afterward. They are just bonded in a completely different arrangement, and that new arrangement is a different substance with different properties. Methane burns. Carbon dioxide puts fires out.
Teacher note · common misconception
Students often picture the reactants as being "used up" and gone, replaced by brand-new material. Nothing is used up at the atom level — the same atoms are simply rearranged. Saying "rearranged, not replaced" every time you use the word reaction pays off enormously in Topic 4.
The same atoms, rebuilt
Burning natural gas. Count the atoms on the left, then count them on the right.
Test yourself
In the reaction methane + oxygen → carbon dioxide + water, which substances are the reactants?
Five signals that a reaction happened
You cannot see atoms rearrange. You can see the evidence they leave.
A new smell counts too — that is a new substance reaching your nose. And when a solid suddenly forms out of two clear liquids, that solid has a name: a precipitate.
Teacher note · common misconception
"Bubbles mean a chemical reaction" is the single most common error here. Boiling water bubbles furiously and produces no new substance at all — those bubbles are water vapor, still H₂O. The signs are clues that a reaction is worth investigating, never a guarantee. The only real test is whether a new substance with new properties exists afterward.
Make the reaction run
A₂ and B₂ molecules are bouncing around. Give them enough energy and their collisions will break bonds.
Run it twice, deliberately:
- Leave the energy near 15 and watch for a while. The molecules collide constantly and nothing happens. Gentle bumps are not enough to break a chemical bond.
- Push the energy past 40. Now collisions start converting A₂ and B₂ into AB. This minimum energy a collision needs is called activation energy, and it is the reason a gas stove needs a spark to light even though gas and oxygen are already mixed.
- Watch the atom count at the bottom the entire time. Molecules change; the number of A atoms and B atoms never does. That is the whole idea of Topic 4.
Test yourself
A student sees a pot of water bubbling hard on the stove and says "bubbles — that's a chemical reaction." What is the best response?
Stop an apple from browning
What is actually turning the apple brown?
Materials
- 1 apple (or a banana or potato)
- Lemon juice
- Water
- Plastic wrap
- 4 small plates
- A knife (ask an adult)
Steps
- Cut four similar slices and put one on each plate. Label them 1 to 4.
- Slice 1: leave it uncovered. This is your control.
- Slice 2: brush it all over with lemon juice.
- Slice 3: brush it all over with plain water, so you know the lemon juice is doing something the water is not.
- Slice 4: press plastic wrap tightly onto the cut surface with no air pockets.
- Check all four at 15 minutes, 1 hour and 3 hours. Photograph them side by side each time.
Slice 1 and slice 3 go brown. Slices 2 and 4 stay close to their original color.
Cutting the apple exposes chemicals inside the fruit to oxygen in the air. An enzyme in the apple speeds up a reaction between those chemicals and the oxygen, and the products are brown substances. That is a chemical reaction: the reactants are gone, and what is on the plate now is genuinely new material — which is exactly why you cannot scrub the brown off.
The two protected slices tell you which reactant matters. Plastic wrap works by removing the oxygen. Lemon juice works differently — its vitamin C reacts with the oxygen first, sacrificing itself before the apple can. Plain water does neither, which is why slice 3 browns like the control.
Teacher note
Slice 3 is the slice that makes this an experiment rather than a demonstration. Without it, students conclude "wet things don't brown." Ask them explicitly what slice 3 rules out.
Physical change or chemical change?
Physical change
The substance changes form, but it is still the same substance. No bonds inside the molecules break.
Melting, freezing, boiling, dissolving, cutting, crushing, bending.
Chemical change
Bonds break and re-form. A different substance exists afterward, with its own properties.
Burning, rusting, digesting, baking, rotting, tarnishing.
The test that always works: ask what substance you have at the end. Melt an ice cube and you have water — same substance, physical. Burn a piece of paper and you have ash, smoke and gases — different substances, chemical.
Dissolving trips up almost everyone. Stir sugar into tea and the sugar seems to vanish, which feels dramatic enough to be chemical. But the sugar molecules are intact and still sugar; that is why the tea tastes sweet, and why evaporating the water leaves sugar behind. Physical.
Teacher note · common misconception
Be careful with "chemical changes are irreversible." It is a useful rule of thumb for grade 7 and it is true for burning and toasting, but plenty of reactions run backward — a rechargeable battery does it every night on a charger. Say "usually very hard to reverse" rather than "impossible," so the rule does not have to be unlearned later.
Rust in twenty minutes
Can you feel a chemical reaction happening?
Materials
- Plain steel wool, no soap (0000 grade)
- White vinegar
- A thermometer
- A jar or tall glass
- A small bowl
- Paper towel
Steps
- Put the thermometer in the empty jar, wait 3 minutes, and write down the starting temperature.
- Soak a golf-ball-sized piece of steel wool in vinegar in the bowl for 1 minute. The vinegar strips off the protective coating.
- Squeeze the steel wool out and pat it lightly with paper towel so it is damp, not dripping.
- Wrap the damp steel wool around the thermometer bulb, put both in the jar, and leave the jar open to the air.
- Read the temperature every 2 minutes for 20 minutes. Write every reading down and sketch a graph.
- Leave the steel wool out overnight and look at its color the next morning.
The temperature climbs several degrees, often by 5–10 °F, and by morning the steel wool has turned orange-brown.
Iron atoms in the steel are bonding with oxygen atoms from the air. The product is iron oxide — rust — a reddish, crumbly substance nothing like the shiny grey metal you started with. New substance, new properties: that is a chemical change.
The vinegar is not a reactant here. It is a preparation step that removes the thin protective layer on the steel so the oxygen can reach the iron underneath. The reaction also gives off energy, which is why the thermometer rises with no flame and no heater anywhere. Reactions that release energy have a name, and you meet it in Topic 5.
Teacher note
Soap-filled scouring pads will not work — the label must say plain steel wool. Squeeze out the excess vinegar properly; a soaking wet pad warms far less because the liquid absorbs the released energy. This experiment sets up both Topic 4 (the mass goes up, because oxygen joined) and Topic 5 (energy released).
Physical change or chemical change?
Ask yourself one question each time: is the same substance still there at the end?
Test yourself
Which of these is a chemical change?
How to read a chemical equation
The subscript sits below and after a symbol and counts atoms inside one molecule. The coefficient sits in front and counts whole molecules.
So 2 H₂O means two water molecules: 4 hydrogen atoms and 2 oxygen atoms, 6 atoms in total. Change the subscript instead and you change the substance itself — H₂O is water, but H₂O₂ is hydrogen peroxide, which you would never drink.
Teacher note · common misconception
When students later try to balance an equation, the tempting move is to edit a subscript. Establish the rule here, before it is needed: coefficients can be changed freely, subscripts never can, because changing a subscript changes what the substance is.
Test yourself
How many atoms in total are represented by 3 CO₂?
Eight words you now own
Tap any card to flip it. Tap again to flip it back.
Where you already use this
Baking soda in a batch of cookies
Heat makes baking soda break down and release carbon dioxide gas. Those bubbles get trapped in the dough, and that is why the cookie rises instead of staying a flat disc.
Rust on a truck after a Virginia winter
Road salt speeds up the reaction between iron, oxygen and water. That is what the car wash bays with the undercarriage spray are really for in February.
An airbag deploying
A crash sensor triggers a reaction that produces a large volume of nitrogen gas in about 30 milliseconds — faster than you can blink, which is the entire point.
Tarnished silverware at Thanksgiving
Silver reacts with tiny amounts of sulfur compounds in the air to form black silver sulfide. Polishing does not clean off dirt; it removes a layer of a different substance.
A glow stick at a Fourth of July show
Snapping it breaks an inner glass tube and lets two liquids mix. The reaction releases energy as light, with no flame and no heat — proof that reactions do not need fire.
The lawn outside your window
Photosynthesis: carbon dioxide and water become sugar and oxygen, powered by sunlight. Every meal you eat traces back to that reaction.
Test yourself
You wrap a cut apple slice tightly in plastic wrap and it stays pale, while an uncovered slice turns brown. What does the plastic wrap do?
Topic 3 quiz — 8 questions
One try per question. You get the reasoning right after you answer.
Up next · Topic 4
Conservation of Mass. In the collision lab, the atom count never moved no matter how the molecules rearranged. Next topic you put that on a scale and prove it — and work out why a burning log seems to lose mass while rusting steel wool gains it.