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. Chemical changes, on the other hand, are not reversible: A log burned in a fire turns to ashes, but the ashes cannot be changed back into a log.

What Is a Physical Change?

In a physical change, the material involved is structurally the same before and after the change. Examples of some physical changes are texture, shape, temperature, and a change in the state of matter. A change in the texture of a substance is a change in the way it feels. For instance, a block of wood may feel rough when you run your finger across it. If you rub that block of wood with sandpaper to smooth the surface, it will no longer feel rough. The wood itself has not changed during sanding to become a new material, though—only the texture of the surface changed. A piece of metal may be heated in a fire until it glows, but the metal is the same material before heating and after cooling. Similarly, when a material changes phase, it only changes physically; the substance is still the same. Think about ice melting into water, and then water being heated up and turning into steam. The chemical structure of water is the same whether it is a solid (ice), liquid, or gas (steam).

What Is a Chemical Change?

A chemical change occurs when the composition of a substance is changed. Chemical changes require the breaking and forming of chemical bonds during a chemical reaction. This results in the rearranging of atoms in substances to form the products of a chemical reaction, which become brand new molecules. These new molecules cannot be easily reverted back to their original state.

Sometimes it is difficult to tell if a chemical reaction has taken place. To help determine whether there has been a reaction, chemists consider a few basic indicators. These include a change in temperature, a change in color, or the development of an odor. The formation of a precipitate or the formation of a gas may also indicate a chemical reaction.

In a chemical alteration, a temperature change occurs as a result of the breaking or formation of chemical bonds. When the chemical bonds of the reactants are broken, sometimes excess energy is released. This can cause heat to be discharged, leading to an increase in temperature. Burning wood is an example of a reaction that releases excess energy as heat. A chemical cold pack in a first aid kit is an example of a chemical reaction that absorbs heat energy, rather than releasing it. This results in cooling.

An example of a color change signaling a chemical reaction can be observed when iron reacts with oxygen to produce iron oxide. A common example is when an iron nail is left outside. Over time, it develops a reddish-brown rust.

Rotting food illustrates odor development as a result of a chemical change. When food spoils it may produce a foul odor. This is because of chemical reactions that take place as the food begins to break down and go bad. These changes lead to the formation of new substances that have unique smells.

Another common sign of a chemical reaction is the formation of a precipitate. This happens when chemicals dissolved in a solution are mixed together and an insoluble solid forms in the liquid mixture. This solid is a precipitate. The creation of a new, solid substance from two liquid substances indicates that a reaction has taken place that has altered the original substances.

A chemical reaction may also release a gas. For example, a mix 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 these two household substances.

Chemical Change or Physical Change?

Sometimes it is difficult to determine whether a change is physical or chemical. For example, think about dissolving table salt (sodium chloride) into liquid water. The solid table salt is added to the water and disappears. This dissolution is easily identified as a physical change, because if the water is allowed to evaporate, the salt remains after all the water has evaporated. In other words, the salt has not been permanently altered. However, table salt is an ionic compound. Ions are negatively or positively charged atoms or molecules (because of the loss or gain of an electron). When ionic compounds are added to water, they dissociate or break apart into ions. In the case of table salt, salt added to water dissociates into sodium ions (positive) and chloride ions (negative). This would seem to be a chemical change. However, scientists do not consider this to be one, because the solid table salt remains after the water is evaporated. The atoms recombine into their original arrangement once the water is removed.

The formation of alloys is another example of a change that is difficult to identify as either physical or chemical. An alloy is a metal that has different properties from the metals that are mixed together to make it. Brass is a common example, which can be found on musical instruments such as trumpets, trombones, and saxophones. Brass is made up of about 60 percent copper and 40 percent zinc. However, brass has different properties than either copper or zinc alone. Brass, though, is not made using a chemical reaction. The copper and zinc atoms are both in brass, but they do not chemically bond together. As a result, brass represents a physical change instead of a chemical change.

Remember that a physical change is a change in properties such as texture, shape, or state. In contrast, a chemical change represents the formation of a new substance after the atoms are rearranged by means of a chemical reaction. Unlike many physical reactions, a chemical reaction cannot be easily reversed and sometimes cannot be reversed at all.