![]() ![]() To form water, however, hydrogen and oxygen atoms must undergo chemical changes. There are the same number of water molecules present before and after the change, and water’s chemical properties remain constant. When water freezes, it becomes hard and less dense, but it is still chemically the same. ![]() To change between these states, water must undergo physical changes. Water is the only known substance on Earth that exists naturally in three states: solid, liquid, and gas. Water, for example, is made up of two hydrogen atoms and one oxygen atom. In a physical change, a substance’s physical properties may change, but its chemical makeup does not. This concept is called the Law of Conservation of Mass. ![]() The same amount of matter exists before and after the change-none is created or destroyed. Matter can change form through physical and chemical changes, but through any of these changes matter is conserved. It includes molecules, atoms, fundamental particles, and any substance that these particles make up. Matter is anything that has mass and takes up space. Atoms that were in a dinosaur millions of years ago-and in a star billions of years before that-may be inside you today. And because matter is never created or destroyed, it cycles through our world. Matter makes up everything visible in the known universe, from porta-potties to supernovas. ![]()
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