What do these two changes have in common?
saliva breaking down a piece of bread
a banana getting ripe on the counter
Both are chemical changes. Both are caused by heating. Both are caused by cooling. Both are only physical changes.
Matter is made of very small particles called atoms. Atoms can be linked together by chemical bonds. When two or more atoms link together, they form a molecule.
In a chemical change, the chemical bonds in the molecules break. The atoms then link together to form different molecules. The types of molecules in matter before and after a chemical change are always different.
Some chemical changes are caused by heating or cooling. For example, burning a piece of paper is a chemical change caused by heating. As paper burns, paper molecules react with oxygen molecules in the air. This reaction breaks the chemical bonds in the molecules. The atoms then link together in a different way to form different molecules. For example, carbon dioxide molecules are formed when paper burns.
In a physical change, chemical bonds do not break. The types of molecules in matter before and after a physical change are always the same.
A change of state is a type of physical change. Changes of state can be caused by heating or cooling. For example, water vaporizing is a physical change that can be caused by heating. Liquid water and water vapor are made of the same type of matter: water.
The law of conservation of mass says that all physical and chemical changes conserve mass. Conserve means to keep the same. So, the total mass before a physical or chemical change is equal to the total mass after the change.
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.
A banana getting ripe on the counter is a chemical change. As a banana ripens, the type of matter in it changes. The peel changes color and the inside becomes softer and sweeter.
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.