Physical changes do not produce a new substance. Chemical changes result in the production of a new substance and cannot be reversed.
The process of rusting, or oxidization, exemplifies a chemical reaction. Here is an oxidized copper lion statute in front of the Chicago Art Institute and the Aon Center.
Photograph by Paul Damien
Matter is anything that has mass and takes up space. The building blocks of matter are tiny particles called atoms. Atoms can be combined with other atoms. Grouped together, they form molecules .
Matter can go through changes. Some changes to matter are physical. Other changes are chemical. The two kinds of changes are very different from each other.
Physical changes can often be reversed. For example, think of an ice cube in a hot room. The ice will melt and become water. That water can then be refrozen. It can become ice again.
Chemical changes cannot be reversed this way. Think of a log in a fire. As the wood burns, it turns to ashes. The burning is a chemical change. Those ashes cannot be changed back into a log.
What Is a Physical Change?
In a physical change, the structure of a material does not change. The shape, texture, and temperature of the material can change though. The material can also change from one state of matter to another. The three most common states of matter are solid, liquid, and gas.
Texture is how something feels. A block of wood may feel rough on your fingers. Sandpaper can be used to make the wood smooth. After sandpapering the wood, the feeling of it changes. The molecules of the wood do not. This is an example of a physical change.
A physical change can also be a change of phase. In other words, matter may change between being a solid, liquid, or gas. Think about ice melting into water. That water can then be heated up. It will turn into steam. The molecules of the water do not change. They are the same whether the water is liquid, solid ice, or steam, which is a gas.
What Is a Chemical Change?
Chemical changes are different from physical changes. During a chemical change, the molecules of a material do change. The material becomes something new and different. That is because its chemical bonds are broken and the atoms get rearranged. Chemical bonds hold molecules together.
Scientists have ways to check if a chemical change has happened. They look for changes in temperature or color. A change in odor can also be a sign.
A chemical change can lead to a change in temperature. Burning wood is one example. This chemical change releases energy as heat.
A color change can be part of a chemical reaction . For example, if you leave an iron nail outside, it will change color. It turns reddish brown. That is because the iron reacts with oxygen in the air. The react ion makes a new substance called iron oxide, or rust.
The smell of rotting food can also be a sign of a chemical change. When food spoils, it starts breaking down. These changes lead to the formation of new substances. They produce new smells, which are often unpleasant.
Chemical Change or Physical Change?
Sometimes it is hard to tell if a change is physical or chemical. The formation of alloys is one example. An alloy is a metal made from a mixture of other metals. It does not have the same properties as the metals it is made of.
Brass is an example of an alloy. It's used to make musical instruments like trumpets. Brass is made up of copper and zinc. It has different properties than either copper or zinc alone. Yet it is not made by a chemical reaction.
That may seem confusing. After all, brass is different from both copper and zinc. It is its own new thing. The copper and zinc atoms are both present in brass. However, they do not chemically bond together. No chemical bonds are formed. None are broken, either. That is why brass represents a physical change instead of a chemical one.
