What do these two changes have in common?
saliva breaking down a piece of bread
silver jewelry tarnishing
A: Both are chemical changes.
B: Both are only physical changes.
C: Both are caused by cooling.
D: Both are caused by heating.
Step 1: Think about each change.
Saliva breaking down a piece of bread is a chemical change. Bread is made up mostly of a chemical called starch. Saliva breaks the bonds between atoms in the starch molecules.
The atoms then link together to form smaller, simpler molecules of sugar. The sugar is a different type of matter than the starch.
Metal turning less shiny over time is called tarnishing. Silver jewelry tarnishing is a chemical change. The silver reacts with sulfur in the air to form black tarnish. The tarnish is a different type of matter that was not there before the change.
Step 2: Look at each answer choice.
Both are only physical changes.
Both changes are chemical changes. They are not physical changes.
Both are chemical changes.
Both changes are chemical changes. The type of matter before and after each change is different.
Both are caused by heating.
Neither change is caused by heating.
Both are caused by cooling.
Neither change is caused by cooling.