Chemical and Physical Changes — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. Bubbles forming when vinegar is poured on baking soda show that a gas has been produced, a sign of chemical change.
Bubbles indicate carbon dioxide gas is being created, which is a clue that a chemical reaction is happening.
2. When a glow stick lights up after bending, the production of light is a sign of a chemical change.
Some chemical reactions release energy as light, which is evidence that new substances are forming.
3. If a reaction makes the test tube feel warmer, energy is being released as heat, indicating a chemical change.
Heat release is a common sign that a chemical reaction has produced new substances.
4. A sour or rotten smell from food going bad is another clue that a chemical change has occurred.
New gases with different odors are formed when food spoils, signaling a chemical change.
5. When two clear liquids are mixed and a solid forms at the bottom, that solid is called a precipitate and signals a chemical change.
A precipitate is a new substance that forms during a chemical reaction between dissolved chemicals.
6. The law of conservation of matter says that the total mass before and after a chemical reaction stays the same.
Atoms are only rearranged in chemical reactions, so total mass is preserved even if the substances look different.
7. If 10 grams of baking soda reacts with 20 grams of vinegar in a sealed bag, the total mass after the reaction is 30 grams.
By conservation of matter, the mass of the products equals the mass of the reactants — 10 + 20 = 30 grams.
8. The starting substances in a chemical reaction are called reactants, and the substances that form are called products.
Reactants are the materials that combine or change, and products are the new substances created.
9. A surprising color change in a mixture, like clear liquids turning blue, is one of the most obvious signs of a chemical reaction.
A new color usually means a new substance has formed, which is a sign of chemical change.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Bubbles in vinegar and baking soda
→ Gas produced as a sign of chemical change
Light produced as a sign of chemical change
Glow stick lighting up
→ Light produced as a sign of chemical change
Conservation of matter in a reaction
10 g + 20 g sealed reaction equals 30 g
→ Conservation of matter in a reaction
Reactants forming a new product
Iron and oxygen combine to form rust
→ Reactants forming a new product
Gas produced as a sign of chemical change
Each scenario shows a different sign of a chemical change or the law of conservation of matter — gas, light, mass conservation, or new product formation.