Chemical and Physical Changes — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. If you add a few drops of food coloring to a glass of water, what type of change has occurred and why?
A) Physical change because the dye spreads but stays the same substance
B) Chemical change because the water turns a new color
C) Physical change because new molecules are formed in the water
D) Chemical change because oxygen bubbles appear from the dye
Adding food coloring is a physical change because the dye molecules and water molecules stay the same; they just mix together. No new substance forms during this process.
2. A student lights a birthday candle and the wax slowly drips down the side. Which statement best describes the changes?
A) Only a chemical change occurs as the wax drips
B) Only a physical change occurs from the burning wick
C) Both: melting wax is physical, burning wick is chemical
D) Neither change occurs because the candle stays a candle
Melting wax is a physical change because the wax molecules stay the same. Burning the wick is a chemical change called combustion that produces new substances like carbon dioxide and water vapor.
3. Predict what happens when you mix milk and lemon juice and the milk forms small white clumps.
A) A physical change because the clumps are still liquid milk
B) A chemical change because new substances called curds form
C) A physical change because lemon juice only dissolves in milk
D) A chemical change but no new substance is created
Acid in lemon juice reacts with milk proteins, forming new solid substances called curds. New substances with new properties signal a chemical change.
4. A scientist mixes two clear liquids and a yellow solid suddenly forms at the bottom. What does this tell her?
A) A physical change happened because liquids changed colors
B) A physical change happened because solids formed naturally
C) A chemical change happened because a new substance appeared
D) No change occurred since the liquids are still liquids
When two clear liquids form a colored solid, a new substance has been created. The formation of a precipitate is one of the strongest signs of a chemical change.
Part B: Fill in the Blank
Write the correct answer on each line.
1. An apple slice turning brown after sitting in air is caused by oxidation, a chemical reaction with oxygen.
Oxidation is a chemical change where substances react with oxygen, producing new compounds. The brown color shows new substances have formed in the apple.
2. Burning a candle wick is a type of chemical change called combustion, which needs oxygen to occur.
Combustion is a chemical change that combines fuel with oxygen, producing carbon dioxide, water vapor, heat, and light as new products.
3. Bending a paperclip changes its shape but not its substance, making this a physical change.
Bending only changes the shape of the metal; the atoms still form the same substance with the same chemical properties, so it is a physical change.
4. When iron reacts with oxygen and water, the new orange substance that forms is called iron oxide.
Iron oxide, also called rust, is the new substance formed when iron undergoes oxidation. Its different color and crumbly texture confirm a chemical change.
5. The starting substances in a chemical reaction are called reactants, while the new substances are products.
Reactants are the original substances that undergo a chemical change. They rearrange their atoms to form products, the new substances after the reaction.