A chemical change occurs when a substance reacts and forms one or more new substances. Chemical changes occur when a cake bakes in an oven, leaves on a tree change color, and food is digested in your stomach.
How can you recognize a chemical change? You have to look for clues. For example, when food spoils, it often gives off an unpleasant odor. Three common types of evidence for a chemical change are a change in color, the production of a gas, and the formation of a precipitate.
A change in colorEdit
Over time, a shiny silver (Ag) bracelet that is exposed to air will darken. As a match burns, it shrivels up and turns black. A new copper (Cu) roof has a reddish color while and old copper roof has a greenish color. In each of these examples, a change in color is a clue that a chemical change has produced at least one new substance.
Production of a gasEdit
An example of producing a gas is mixing vinegar (C2H4O2) with baking soda (NaHCO3). Bubbles of carbon dioxide (CO2) form immediately. A similar chemical change happens when you use baking powder as an ingredient in a cake recipe. Bakig powder is a mixture of baking soda and one or more acids that react when wet. As the cake bakes, the bubbles of carbon dioxide expand and cause the cake to rise.
Formation of a precipitateEdit
Another chemical change you can observe in the kitchen is the curdling of milk. If you add lemon juice (H3C6H5O7) or vinegar to milk, small bits of white solid will appear from the liquid. Any solid that forms and separates from a liquid mixture is called a precipitate. When an acid is added to milk, proteins in the milk undergo a chemical change that alters their structure, causing them to stick together in clumps. They form cottage cheese.