Say hello to Mark and Molly. Each are enjoying a glass of water. Deciding to save some for later Molly puts her glass in the refrigerator but Mark puts his in the freezer. A short time later they go to get their drinks. Molly enjoys her cold liquid water but something has happened to Marks water. His water has physically changed.
Matter can change chemically or physically. An ice cube is frozen water. A closer look into the ice will show that water is H2O or molecules made of hydrogen and oxygen atoms. When the ice cube melts the molecules stay H2O they just change from solid to liquid. Even when water evaporates the molecules might be moving a little faster but they are still water molecules.
Another example of a physical change would be bending iron metal. Iron can be made into I beams or pressed into shapes to make the body of a car. Whether the iron is in a beam or on a car it is still Fe atoms just in a different shape.
A chemical change occurs when an entire new material is created or the atoms of a substance become rearranged. Iron rusting is a chemical change because iron turns into a new material we call rust. The process of rusting can be seen in a chemical formula. Iron + water + oxygen yields iron iii oxide or rust. In this chemical equation we start with iron but end up with something new.
Another chemical change happens inside of your body when you eat sugars. Sugar enters your body as C6H12O6. Then when you breath in oxygen gas a chemical change begins. The sugar is broken down and turned into carbon dioxide and water. The sugar molecule has chemically changed.
Another chemical change happens when you drive a car. Gasoline enters your car as octane. Octane is C8H18. In you engine oxygen is introduced and the octane changes into carbon dioxide and water. This is a chemical change because the octane is no longer a molecule.
You can tell the difference between chemical and physical changes because chemical changes can’t be easily undone but lucky for mark physical changes can be undone and his glass of water has almost changed back to a liquid.