Examples of chemical changes in everyday life
A chemical change, or chemical reaction, turns one or more substances into new substances with different properties. Signs that one has happened include a color change, gas bubbles, a new smell, light, a temperature change or a solid forming in a liquid. Chemical changes are usually hard to reverse, unlike physical changes.
- Burning wood — wood reacts with oxygen to produce carbon dioxide, water vapor, ash, heat and light.
- Rusting iron — iron reacts with oxygen and water to form flaky, reddish-brown hydrated iron oxide.
- Frying an egg — heat permanently changes the proteins in the egg white, turning it from clear to solid white.
- Baking a cake — baking powder releases carbon dioxide that makes the batter rise, and the crust browns.
- Toasting bread — sugars and proteins on the surface react (the Maillard reaction) to form brown, flavorful compounds.
- Digesting food — enzymes break starch, proteins and fats into smaller molecules your body can absorb.
- Milk going sour — bacteria turn lactose sugar into lactic acid, which makes the milk thicken and taste sour.
- Making wine — yeast ferments the sugar in grape juice into ethanol and carbon dioxide.
- Photosynthesis — plants use light energy to turn carbon dioxide and water into glucose and oxygen.
- Cellular respiration — your cells use oxygen to break down glucose, releasing energy, carbon dioxide and water.
- Baking soda and vinegar — they react to produce carbon dioxide gas, sodium acetate and water.
- Silver tarnishing — silver reacts with sulfur compounds in the air to form a black layer of silver sulfide.
- Copper turning green — over years, copper reacts with air, water, carbon dioxide and sulfur compounds to form a green patina.
- Lighting a gas stove — methane in natural gas burns with oxygen to form carbon dioxide and water.
- A cut apple turning brown — an enzyme in the fruit uses oxygen from the air to make brown pigments.
- Striking a match — friction starts a reaction between chemicals in the match head and the striking strip, producing a flame.
- Taking an antacid — calcium carbonate neutralizes stomach acid, forming calcium chloride, water and carbon dioxide.
- Using a battery — redox reactions inside the battery push electrons through the circuit as an electric current.
- Cracking a glow stick — hydrogen peroxide reacts with another chemical, and the energy released makes a dye glow.
- Food rotting — bacteria and fungi break the food down into new substances, often with a bad smell.
How to tell a chemical change from a physical change
Ask whether a new substance has formed. In a physical change, such as melting ice or dissolving sugar, the substance stays the same and only its form or state changes, so you can usually get it back by simple means. In a chemical change, atoms are rearranged into new substances with new properties, such as the ash and smoke left when paper burns.
The signs listed above are clues, not proof. Boiling water also makes bubbles, and a metal heated in a flame glows, yet both are physical changes. The real test is whether the material at the end is chemically different from the material at the start.