Ecosystems have many chemical and physical changes happening all at once, and matter is conserved in each and every one-no exceptions. Hydrogen and oxygen are gases at standard temperature and pressure, whereas water is a colorless, odorless liquid. The difference in this case is that the substances before and after the change have different physical and chemical properties. In chemical changes, just as in physical changes, matter is conserved. Notice that there are the same number of hydrogen atoms and oxygen atoms on either side of the equation. This equation says that it takes two molecules of hydrogen and one molecule of oxygen to form two molecules of water. Chemists write out this chemical reaction as: In the right conditions, and with enough energy, these diatomic bonds will break and the atoms will join to form H2O (water). Both hydrogen and oxygen are diatomic-they exist naturally as bonded pairs (H2 and O2, respectively). The addition or subtraction of atomic bonds changes the chemical properties of the substances involved. For a chemical change to occur, atoms must either break bonds and/or form bonds. 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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