When animals in and around the stream eat these plants, their bodies use the stored chemical energy to power their cells and move around. If you added up all the carbon, hydrogen, and oxygen atoms on either side of the equation, the sums would be equal matter is conserved in this chemical change. This equation says that six carbon dioxide molecules combine with six water molecules to form one sugar molecule and six molecules of oxygen. Light energy allows these bonds to break and reform to produce sugar and oxygen, as shown in the chemical equation for photosynthesis:ĦCO 2 + 6H 2O + light → C 6H 12O 6 (sugar)+ 6O 2 The atoms come from carbon dioxide in the air and water in the soil. However, the light energy doesn’t produce the atoms that make up those sugars-that would break the Law of Conservation of Mass-it simply provides energy for a chemical change to occur. In a process called photosynthesis, these organisms convert light energy from the sun into chemical energy stored in sugars. Next, consider the plants and algae living in and along the stream. Water gives a very clear example of how matter cycles through our world, frequently changing form but never disappearing. As the liquid water flows through the canyon, it may evaporate (another physical change) into water vapor. The snow must undergo a physical change-melting-to join the stream. But in the context of the canyon stream, it began in the mountains as snow. Of course that’s not where the water began-it’s been cycled all over the world since Earth first had water. For many canyon streams, the water comes from higher elevations and originates as snow. Consider a stream flowing through a canyon-how many chemical and physical changes are happening at any given moment? First, let’s consider the water. 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 H 2O (water). Both hydrogen and oxygen are diatomic-they exist naturally as bonded pairs (H 2 and O 2, 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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