Until now, matter has changed shape and phase but stayed itself. This week the atoms rearrange and something genuinely new appears.
Take a moment and think about your favourite baked good — a cookie, a cake, a loaf of bread. You start by mixing flour, sugar, eggs, and other things. You put it in the oven. What comes out is delicious and completely different from the batter you started with.
So what happened? You cannot pick the eggs back out. You cannot separate the flour. The batter did not just get warm — it became something else entirely.
Melted ice can be frozen right back into ice. Crumpled foil can be flattened again. But a baked cake can never go back to being batter. That difference is the whole point of this week.
A chemical reaction is a process that takes matter and reorganizes it into a new type of matter. The atoms are not created or destroyed — they let go of their old partners and join new ones.
The matter that goes into a chemical reaction. In the baked good, the reactants are each of the ingredients: the flour, the sugar, the eggs.
The matter that results from the reaction. In this example, the cake is the product. Heat from the oven helps power the reaction.
Here is the reaction that happens in a burning candle. Count the atoms on each side.
This is the question you will be asked most often, so learn the test properly.
The matter is still the same substance. Only its shape, size, or phase changed.
A new substance was made. The atoms rearranged.
Bubbles alone are not proof. Boiling water bubbles like crazy and is only a physical change — the gas coming off is still water. Look for two or more signs before you decide.
Ask yourself: is it still the same substance, and could you easily get it back? Tap your answer.
Signs: vigorous bubbling (a gas is produced) and the cup turns noticeably colder — this reaction absorbs energy instead of releasing it.
The reactants were baking soda and vinegar. The products are carbon dioxide gas, water, and a dissolved salt. You cannot get the vinegar back.
The scale is the real lesson: the mass stays the same, because the balloon trapped the gas. Do this in an open cup instead and the mass drops — not because matter vanished, but because the CO₂ floated away. Atoms are never destroyed.
Pick a reaction. The counter tallies every atom on both sides. See if you can find one where the totals do not match.
D rusts most (constant water plus air), then B at the waterline, then A slowly. C barely rusts at all — the oil sealed out the oxygen.
Rusting is a chemical reaction: iron + oxygen + water → iron oxide. The reddish flakes are a genuinely new substance. They are brittle where iron was strong, and you cannot scrape them back into iron.
Why cup C matters: it identifies which reactant is essential. Take away oxygen and the reaction stops. That is why cars are painted and why tools are oiled.
Right now your cells are reacting the sugar from your breakfast with the oxygen you just breathed, releasing the energy that lets you read this. The products are carbon dioxide and water — which you breathe straight back out.
Plants run the reverse reaction. They take carbon dioxide and water, use energy from sunlight, and build sugar and oxygen. You breathe out what they need; they release what you need.
Gasoline reacts with oxygen hundreds of times per second. The reaction releases energy so fast that it pushes the pistons and moves the whole car.
Bacteria react with sugar left on your teeth and produce acid, which reacts with tooth enamel. Brushing removes the reactants before the products can damage anything.
Cover the definition, say it in your own words, then flip to compare.
Food & Energy. You just learned that reactions release energy. Next week we follow that energy into living things: why every animal must eat, and where the energy in your lunch originally came from.
Question for tonight: trace your dinner backward as far as you can. Where did that energy start?