Matter is anything that has mass and takes up space. The tiny particles called atoms are the basic building blocks of all matter. Atoms can be combined with other atoms to form molecules.

Matter is capable of undergoing changes, which are classified as either physical or chemical. Physical changes in matter are often reversible. For example, an ice cube can melt into liquid water, and then the liquid water can be refrozen into an ice cube. A chemical change is very different. A burning log demonstrates this. The log turns to ashes. However, its ashes cannot be changed back into a log. The burning was a chemical change. Unlike many physical changes, it cannot be easily reversed.

What Is a Physical Change?

In a physical change, the structure of the changed material remains the same before and after the change. Physical changes may affect the texture, shape, and temperature of a substance. They can also affect the material's state of matter.

A change in the texture of a substance is a change in the way it feels. For example, a block of wood may feel rough when you run your finger across it. Rubbing the wood with sandpaper smooths the surface, so it no longer feels rough. This is an example of a physical change: The texture of the wood has changed, but the wood's molecular structure has not.

A physical change may also involve a change of phase between solid, liquid, and gas. When a material changes phase, it only changes physically; the substance is still the same, even though its form is different. Think about ice melting into water. It can then be heated up and turned into steam. The phase changes, but the structure of water does not. The water molecules are still water molecules, whether the water is in liquid form, in solid form as ice, or in gas form as steam.

What Is a Chemical Change?

Chemical changes are very different from physical changes. A chemical change takes place when the composition of a substance is altered. This requires the breaking and forming of chemical bonds during a reaction. Chemical bonds hold molecules together.

A chemical change results in the rearranging of atoms in substances. This rearrangement leads to the creation of brand new molecules. The new molecules are the products of the chemical reaction. They cannot be easily reverted back to their original state.

Sometimes it is not easy to tell whether a chemical reaction has taken place. To figure it out, scientists look for a few basic signs. Indicators of a chemical change can include a change in temperature, color, or odor.

In a chemical change, temperature may rise or fall. This release of heat is the result of the breaking or formation of chemical bonds. Burning wood is an example of a reaction that releases energy as heat. The opposite may also indicate a chemical reaction. A chemical change can cause heat to be absorbed. The result is a decrease in temperature. A chemical cold pack in a first aid kit demonstrates this kind of chemical reaction. It absorbs heat energy, which results in cooling.

A color change can also signal a chemical reaction. A common example can be observed when you leave an iron nail outside. The iron reacts with oxygen in the air. The reaction produces iron oxide. As a result, the nail develops a reddish-brown rust.

The odor of rotting food also indicates a chemical change. The food undergoes chemical reactions as it begins to break down. These changes lead to the formation of new substances. These substances produce unique, often, unpleasant smells.

Another sign of a chemical reaction may be the formation of a precipitate. When certain chemicals are mixed together in a solution, a new solid can form in the liquid mixture. This solid is a precipitate. It shows that a chemical reaction has taken place. The change has altered the original substances.

A chemical reaction may also release a gas. For example, a mixture of vinegar and baking soda will immediately start bubbling and foaming. The bubbles are a release of carbon dioxide gas, a product of the chemical reaction between the two common substances.

Chemical Change or Physical Change?

Sometimes it is difficult to tell whether 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. The alloy's properties are different from the properties of the metals used to make it.

Brass is a common example of an alloy. It's used to make musical instruments including trumpets and trombones. Brass is made up of about 60 percent copper and 40 percent zinc. It has different properties than either copper or zinc alone. Yet it is not the result of chemical reaction.

That may seem confusing, because brass is different from both copper and zinc. However, though the copper and zinc atoms are both present in brass, they do not chemically bond together. No chemical bonds are formed or broken. As a result, the creation of brass represents a physical change, not a chemical one.