Chemical and Physical Changes — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
When iron rusts, it is just a physical change because the iron looks different on the outside.
Corrected: When iron rusts, it is a chemical change because iron and oxygen combine to form a new substance called rust.
Rusting creates iron oxide, which has different properties from iron, so a new substance has formed — a chemical change.
2. Fix the sentence:
Burning wood is a physical change because the wood just gets smaller as it burns away.
Corrected: Burning wood is a chemical change because the wood combines with oxygen to make ash, smoke, and gases.
Burning produces new substances such as carbon dioxide, water vapor, and ash that cannot turn back into wood.
3. Fix the sentence:
Baking cookie dough in the oven only changes the shape, so it is a physical change.
Corrected: Baking cookie dough is a chemical change because heat causes new substances to form, giving cookies their brown color and crisp texture.
Baking causes irreversible reactions that create new compounds, so it is a chemical change.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. When leaves change color in fall, chemicals inside the leaf break down, which is a chemical change.
Color change caused by new substances forming is a sign of a chemical change.
2. A nail left outside in the rain forms a reddish-brown coating called rust, which is a sign of a chemical change.
Rust is iron oxide, a new substance, which proves a chemical reaction has occurred.
3. When wood burns, the new substance smoke floats away as a dark cloud, showing a chemical change.
Smoke is made of new substances released during burning, indicating a chemical change has happened.
4. Chemical changes are usually irreversible because we cannot easily turn the new substances back into the original ones.
Most chemical reactions form new substances that cannot be changed back into the original materials.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Explain why baking cookies is considered a chemical change in Grade 5 science.
Sample answer: Baking cookies is a chemical change because the heat causes the ingredients to react. Sugars brown, dough firms up, and gases form, creating new substances. The baked cookies cannot be turned back into raw dough, which proves a chemical change happened.
Chemical changes form new substances with new properties and are usually irreversible.
2. Describe two examples of chemical changes outdoors and explain how you know they are chemical.
Sample answer: A bike chain rusting outside is a chemical change because iron combines with oxygen and water to make rust, a new orange substance. A campfire burning logs is a chemical change because the wood becomes ash, smoke, and gases that cannot become wood again.
Both examples form new substances that have different properties than the starting materials.