Chemical and Physical Changes — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. If 16 grams of one reactant combines with 8 grams of another, the products will weigh 24 grams total.
The law of conservation of mass states that matter is neither created nor destroyed during a chemical change, so 16 plus 8 equals 24 grams.
2. A starting substance in a chemical reaction is called a reactant, while the substance formed is called a product.
Reactants are the starting materials that combine during a chemical change, while products are the new substances that form from them.
3. If 50 grams of vinegar reacts with 5 grams of baking soda in a sealed bag, the bag still weighs 55 grams.
In a closed container, all matter stays inside, so the total mass of 50 plus 5 equals 55 grams remains constant during the reaction.
4. When wood burns in an open fire, the ash weighs less because gases like carbon dioxide escape into the air.
Mass seems lost when wood burns because gas products escape into the air. If we collected all gases, the total mass would still equal the starting wood plus oxygen.
5. The new substance formed during a chemical reaction is called a product of that reaction.
A product is the substance that results from a chemical change, formed when reactants combine and rearrange into something new.
6. If 10 grams of hydrogen reacts with 80 grams of oxygen to form water, the water weighs 90 grams.
By the law of conservation of mass, the total mass of products equals the total mass of reactants, so 10 plus 80 equals 90 grams of water.
7. Mass is neither created nor destroyed; it is conserved during any chemical reaction.
Conservation of mass is a fundamental law of science: the total mass before a reaction always equals the total mass after, even if substances change form.
8. When iron rusts, it gains mass because oxygen from the air joins with the iron to form iron oxide.
Rust weighs more than the original iron because oxygen atoms from the air bond with iron atoms, adding to the total mass of the new compound.
9. If 12 grams of carbon burns completely with 32 grams of oxygen, the product weighs exactly 44 grams.
Carbon plus oxygen forms carbon dioxide, and 12 plus 32 equals 44 grams, demonstrating the law of conservation of mass.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Total mass of 5g + 7g reactants
→ 12 grams
Reactant
Word for starting materials
→ Reactant
12 grams
Word for new substances formed
→ Product
Conservation
Mass law during reactions
→ Conservation
Product
Five plus seven equals 12 grams. Reactants start a reaction, products form from it, and conservation describes how mass stays equal throughout.