 Welcome to Changes in Our World, Chemical and Physical. The world around us changes continuously. Because everything has physical and chemical properties, all substances can go through physical and chemical changes. Chemical change occurs when new substances are produced with new chemical properties. When the chemical bonds of substances are broken and released, or new chemicals are formed, we have different chemical properties. Rusting is an example of this. Iron reacts with oxygen in a chemical reaction that creates a new substance, iron oxide, or what we call rust. Can you think of other examples of chemical change? One example is fuel combustion. Hydrocarbons are burned, giving off acids and water vapor. A physical change does not alter the chemical properties of a substance. Liquid water and solid water look different, but they are chemically identical. A physical change takes place when substances change phase or form. A form change occurs when the size or shape of a substance is altered. Cutting, grinding, and crushing are good examples. Can you think of other examples of form change? Here are a few examples. Gravel crushed from rocks, wheat ground into flour, and coffee beans being ground. A phase change occurs when a substance shifts between solid, liquid, or gas. Water and the water cycle are a good example. The water cycle illustrates physical phase changes. Notice that the lake is water, the clouds are water vapor, the snow is water, and the river is water. They are all water, but are in a different phase. Can you think of other examples of phase change? A couple of examples are water vapor or steam rising from a lake. The water changes to vapor in the air. Wax melting, heat melts wax, changing it from a solid to a liquid. Chemical and physical changes work together. Frequently, substances change chemically and physically in the same environment. Let's look at each item below to see both chemical and physical changes. The rock cycle. There are three main types of rock. Igneous rocks are called fire rocks. They are formed when melted rock cools and becomes solid. These new rocks can be formed above or below the ground. Granite is an example of igneous rock. Sedimentary rocks are formed when wind or water break down other rocks into smaller pieces. These pieces form layers and after a long time they are cemented together to form a new rock. Sandstone is an example of a sedimentary rock. Metamorphic rock is formed or morphed into another kind of rock. Igneous and sedimentary rocks are broken apart. Extreme pressure produces heat and a new rock is formed. Decomposition. Composting and digestion. In a forest, dead leaves fall from deciduous trees and form a thick layer on the forest floor. Decomposition reduces these leaves to compost and then to nutrients through digestion. These nutrients are returned to the soil and enable new plant growth to take place. Candle burning. Physical change. The heat melts the wax, changing it from a solid to a liquid. Chemical change. The melted wax combines with the oxygen in the air during combustion. This combination produces carbon dioxide and water vapor. Digestion. A physical change occurs as food is chewed. A chemical change occurs as enzymes, acids and other chemicals in the mouth, stomach and intestines react with the food particles to form substances that the body can use or store. You have completed this learning activity, changes in our world, chemical and physical.