water drop

Physical & Chemical Changes in Chemistry

In this post, we will focus most of our attention on physical changes in chemistry with a brief look at chemical changes.


Physical change is a change to a substance that does not alter the chemical composition. For example, boiling water is a physical change. Generally, physical changes are easy to reverse, such as when steam is cooled to become liquid water.


Chemical change is a change that alters the chemical composition of a substance. An example would be various forms of cooking, such as frying potatoes to make french fries. Unlike physical changes, chemical changes are much harder to reverse. Just as it is impossible to turn french fries back into raw potatoes.

A specific type of physical change is called phase change. There are several different types of phase changes, as listed below.

  • melting
  • vaporizing
  • freezing
  • condensing
  • sublimation

Many of these are obvious, but they will be explained for clarity. Melting involves a substance moving from a solid to a liquid. Vaporizing takes place as a substance moves from liquid to gas. A substance that moves from a gas to a liquid is called condensing. Freezing is the process of a liquid becoming a solid. Sublimation is a solid moving straight to a gas.

The first four-phase changes are commonly seen in water. Ice melts to become liquid water, water boils/evaporates (vaporizes) to become steam. Water freezes to become ice; in the early morning, it is common in many places to see water on plants due to condensation. Sublimation is tricker to see on a day-to-day basis. The most common example involves carbon dioxide, aka dry ice, which is a favorite tool for Halloween. Other substances that sublimate include arsenic, iodine, and naphthalene (used for mothballs).

Phase changes are related to the kinetic theory of matter, which we will now turn our attention to.

Kinetic Theory of Matter

The kinetic theory of matter states that Molceults have space between them and are in constant random motion. We can say that the more heat, the faster the motion because more energy is present. For solid, the molecules can vibrate, but that is essentially it. All solids are vibrating, such as tables, chairs, desks, etc. However, the vibration is random, and thus the vibrations cancel each other.

Liquids can clearly move about, and this is why they cannot keep a single shape but is formed by their circumstances. This also applies to gasses. The real difference between the various phases is the space around molecules and the speed at which they are moving. When energy is added, molecules move apart and move faster. This explains a solid becoming a liquid and a liquid a gas.

Water breaks many rules in relation to the Kinetic theory of matter. When water freezes, instead of the molecules getting closer together, they actually push out and are thus less dense than water. This is one reason why ice floats and why you would find frozen ice on the top of a lake. The ice floats to the top, and by being on top, it insulates the animals inside the lake from the cold above.


Physical changes play a major role in all of our lives. The phase changes of water are used for various purposes in everyday life. It is beneficial to understand these concepts as they are so commonly encountered.

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