Describing Chemical Equations
In the experiment you performed, calcium chloride and sodium bicarbonate were mixed in a solution of phenol red indicator. These starting chemicals are called reactants. When you mixed these chemicals you were able to see many changes occur. These changes represented a chemical reaction; where a substance changes into a new substance. These new substances that you created are called the products of the reaction. To describe this reaction, scientists use a chemical equation-a combination of symbols and chemical formulas that represent the starting and ending substances in a chemical reaction. For our experiment, the equation would be:
2 NaHCO3(s) + CaCl2 (s) CaCO3 + CO2(g) + 2 NaCl + H2O (catalyst)
The arrow, , in a reaction is read as yields, produces, or forms. The letters in parentheses after each chemical formula is its phase of matter: (s) solid, (l) or (l) liquid, or (g) gas. If a substance has been dissolved in water, it will be followed with (aq) for aqueous. If a reaction is possibly reversible, a double arrow will be used to show that the reaction can go in both directions, or .
Earlier in the year, we learned that a physical change is a change to a substance that alters its size, shape or state of matter. These include changes such as crushing sugar, melting ice and dissolving salt. With a chemical change we observe evidence of a new substance being formed. For example, in your experiment you noted that bubbles of carbon dioxide formed. This indicated that a gas was being released. You also noted that the temperature changed which indicates that energy is being absorbed or released. Because we added the phenol red solution, you were able to observe a color change occurring. After allowing the reaction to sit for several minutes, you were able to observe a solid (precipitate) that collected at the bottom of the solution. The formation of a precipitate is another indicator of a chemical reaction. Because the gas released (carbon dioxide) is odorless, you were not able to observe an odor change however, this too is an indicator of a chemical change.