What is a physical change?

We explain what a physical change is, how it occurs and how it differs from a chemical change. In addition, examples and explanations.

1. What is a physical change?

Physical changes are the type of transformations of matter that alter its state, but never its composition or nature . That is, these are transits between one form or another of the matter, without a significant chemical alteration, that is, without one type of matter becoming another by some type of chemical reaction.

As the name implies, physical changes involve alterations in some of the physical properties of matter , such as its state of aggregation , its hardness, its shape, size, color, volume or density , among others.

Only rarely does this type of change imply a substantial rearrangement of atoms (as occurs in the formation of crystals). Usually, physical changes are usually reversible .

Physical changes are also the result of a physical method , which usually consists of the modification of energy , pressure or other variables in which matter is found. Many of these methods are also useful for separating mixtures of matter, although they are not useful for separating the components of a chemical compound (the result of a chemical reaction).

2. Examples of physical change

Some examples of physical change are as follows:

• Evaporation of liquids , increasing their amount of energy ( heat ) to convert them into gases, although without modifying their nature. Water vapor, for example, is still chemically water (H ₂ O), even if it is in a gaseous state .
• Gas condensation , the inverse process to the above, which takes place by extracting energy (heat) from a gas, allowing it to change to the liquid state . This is what happens when we bathe with hot water and steam condenses in the mirror, fogging it with minimal droplets.
• Solidification of liquids , as is the case with the removal of energy (heat) or sometimes simply by resting them long enough. The simplest example is the freezing of water on solid ice, without changing its chemical composition at all.
• Solids solutions in liquids , such as when we dissolve salt in water or sugar in coffee: we can stop observing the added solids, but not feeling their effect on the mixture . It will be enough to evaporate the liquid to find the new solid at the bottom of the container, in its unchanged chemical form.
• Magnetization of metals , such as iron and the like, when coming into contact with a source of electrical or magnetic energy . We can do this with clips and a magnet : if we subject the former to contact with the magnet, we will notice how they partially acquire their magnetic charge and attract each other, without altering their shape or chemical composition.

3. Physical change and chemical change

Unlike physical changes, chemical changes do imply a reaction in matter and therefore the alteration of its nature. These processes are usually irreversible and consume or release energy , since in the process one or more chemical substances become others, recombining their atoms in an always specific way.

Chemical changes do not respond to physical methods, and that is why we cannot separate the components of a chemical compound by physical changes : if we boil water, the resulting vapor will continue to consist of oxygen and hydrogen; while if we react water with sulfur trioxide (SO ₃ ) we will obtain sulfuric acid (H ₂ SO ₄ ), a totally different compound.