Chemical and Physical Changes — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. In the reaction between baking soda and vinegar, baking soda and vinegar are the reactants that react together.
Reactants are written before the arrow in a reaction and are what combine or change.
2. The new substances formed after a chemical reaction are called products, like the carbon dioxide gas from vinegar and baking soda.
Products are the substances created when reactants chemically combine or rearrange.
3. Before a chemical reaction, 50 grams of reactants are sealed in a flask; after the reaction, the total mass must still be 50 grams.
In a closed system, atoms are only rearranged, so the total mass stays exactly the same.
4. When a candle burns, wax and oxygen are reactants that combine to make carbon dioxide, water vapor, and heat.
Combustion reactions need oxygen as one of the reactants to react with the fuel.
5. If a chemical change releases heat, scientists describe the reaction as exothermic, meaning energy leaves the reaction.
Exothermic reactions release energy, often as heat, when reactants form products.
6. A change that absorbs energy and feels colder is called an endothermic reaction, like an instant cold pack.
Endothermic reactions take in energy from the surroundings, often making things feel cooler.
7. Mixing a clear silver nitrate solution with a clear salt solution forms a white solid called a precipitate, a sign of a chemical change.
Precipitates form when dissolved substances react and create a new solid that does not dissolve.
8. Even though a wooden log seems to disappear when it burns, the matter is not lost — it has changed into ash, gases, and smoke.
All atoms from the wood end up in ash, smoke, and gases — total mass is conserved.
9. Chemical reactions rearrange atoms but never create or destroy them, which is why mass is conserved.
Atoms are simply regrouped in chemical reactions, supporting the law of conservation of matter.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Substances that go into a reaction
→ Reactants
Conservation of matter
Substances formed by a reaction
→ Products
Precipitate formation
Total mass stays the same in a sealed reaction
→ Conservation of matter
Reactants
White solid forms from two clear liquids
→ Precipitate formation
Products
Reactants, products, conservation of matter, and precipitates are core ideas Grade 5 students use to describe chemical reactions.