Chemical and Physical Changes — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. Which observation is the strongest evidence that a chemical change has happened?
A) A liquid becomes solid when cooled
B) A color changes and a new gas is given off
C) A piece of metal is bent into a curve
D) A sugar cube dissolves in hot tea
Color change combined with gas release strongly suggests a new substance has formed, which is a chemical change. The other choices are physical changes.
2. A student burns a piece of paper and weighs the leftover ash. The ash weighs less than the paper. What happened to the missing mass?
A) Mass was destroyed in the chemical reaction
B) It floated away as smoke and gases that were not weighed
C) The paper was turned into pure energy with no mass
D) The scale was broken before the reaction
When paper burns, atoms leave as gas and smoke. If those were captured, the total mass would still equal the original paper plus oxygen used.
3. Which everyday example shows BOTH a physical change and a chemical change?
A) Boiling water in a pot until it becomes vapor
B) Cutting a tomato into slices for a salad
C) Lighting a candle so the wax melts and the wick burns
D) Folding a paper airplane and throwing it
When a candle is lit, the wax melts (physical change) while the wick burns (chemical change), so both kinds of changes occur.
4. Two clear liquids are mixed in a beaker. Which result best shows that a chemical change took place?
A) The liquids combine and stay clear
B) The mixture turns yellow and a solid forms at the bottom
C) The temperature stays exactly the same
D) The mixture can be separated by simple filtration
A new color and a precipitate forming are clear signs that new substances have been created — a chemical change.
Part B: Fill in the Blank
Write the correct answer on each line.
1. When a bike chain rusts in the rain, it has gone through a chemical change because iron and oxygen made a new substance.
Forming a new substance with new properties is the definition of a chemical change.
2. When a popsicle melts in the sun, it has gone through a physical change because the liquid is still the same substance.
Melting changes a solid to a liquid without forming a new substance, which makes it a physical change.
3. Sour milk smells bad because tiny living things have caused a chemical change in the milk.
Microorganisms create acids and other compounds that change milk into a new substance, which is a chemical change.
4. Crushing a soda can changes its shape but not the metal it is made of, so it is a physical change.
Changing only shape or size without making a new substance is a physical change.
5. By the law of conservation of matter, atoms are rearranged but never destroyed during a chemical change.
Atoms are only regrouped in chemical changes, so the total amount of matter stays the same in a closed system.