Physical and Chemical Changes

What are physical and chemical properties?

Let's say you had to pick your friend out of a crowd. How would you find them? You can identify them by things like height, hair color, and how talkative they are. Elements and chemical compounds work the same way. Everyone has their unique properties that help us identify them. A substance can have two main types of properties: chemical and physical. So what are these properties exactly?

Physical Properties

Physical properties can help identify substances without needing to use them in a reaction. There are two types of physical properties: intensive and extensive.

Other examples of physical properties are malleability (the ability of a substance to be molded into thin sheets), hardness, solubility, and electrical conductivity. We can use these properties to determine the identity of unknown compounds or elements.

Let's say you were handed three metal balls. They are all the same size and shape, and all of them are silver-colored. Now, how can we tell them apart? One way is to determine their densities.

All of these metals are similar in color, but as you can see, their densities vary enough that we can tell them apart. After measuring the density of each ball three times for the measure of accuracy, we get these results: ball 1 (2.72 g/cm³), ball 2 (7.87 g/cm³), and ball 3 (10.3 g/cm³). Based on our chart, we see that ball 1 is aluminum, ball 2 is iron, and ball 3 is silver.

Chemical Properties

A key difference between a chemical and physical property is that a physical property describes what a substance currently is while a chemical property describes what a substance can do. This can be a little confusing, so let's break it down. Chemical properties are all about potential: the potential to catch fire, corrode, oxidize, etc. Once a substance actually does any of that, its identity (chemical composition) changes. For example, Iron (Fe) can oxidize, but once it does, it's no longer iron! It is instead iron oxide (Fe(OH)₃).

Now, what about physical properties like a boiling point? Boiling is something liquid can do, right? Not quite, by "can do", we mean "can do a chemical change". The key difference here is that once a liquid boils, it still is the same molecule. Water, no matter what state it's in, has the same boiling point. So by "currently is", we mean "this property can be observed without a chemical change taking place".

Fig. 1. When this iron chain was exposed to moisture, it oxidized and became rusted. The ability to rust is a chemical property of iron. Source: Pixabay.

The capability of iron to rust is a chemical property. So, how can we distinguish substances using chemical properties?

Let's say you are given two silvery lumps of metal and challenged to determine what they are based only on chemical properties. To make it easy, we are told that one is tin and one is potassium. Potassium reacts violently when dropped in water due to the creation of hydrogen gas. Tin, however, doesn't react with water under normal conditions. So we can take a small piece of each of our metals and place them in water (while wearing safety protection!) and whichever one reacts is the potassium.

What are chemical and physical changes?

Now that we've covered chemical and physical properties, let's look at physical and chemical changes.

Physical changes

There are several kinds of physical changes, some common ones are changes in the state of matter (boiling, freezing, etc.), cutting/bending/breaking, separation of a mixture, and creation of a mixture.

When we melt ice, it becomes water. Even though the name of the substance is changed, the atomic structure remains the same.

Mixtures versus chemical reactions

Let's look at another physical change: dissolving salt in water. While the salt is now separated into ions, it can be returned to solid salt if the water is boiled off. Saltwater is a mixture, more specifically, a homogeneous mixture. A homogeneous mixture is a substance made up of two or more compounds or elements that is uniform. If I took a 1 mL sample of 50 mL solution of salt water, that 1 mL would be representative of the whole thing.

In the other type of mixture, a heterogeneous mixture, the composition is not uniform. An example of this is oil and water. If I poured out some of the liquid, I would only get oil (since it is less dense).

The key difference between a mixture and a chemical reaction is that a mixture is reversible. Most importantly, it is reversible using only physical changes such as boiling, sifting, or separating by density.

Chemical Changes

The key component of a chemical change is that the identity of the reactant(s) is changing. The molecule(s) we start with are different from our end product.

Let's look at the reaction of butane and oxygen as an example.

\(2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O\)

In this reaction, butane is converted into carbon dioxide and water. The only way we get butane back is through more chemical reactions. Butane is commonly used as a lighter fluid due to its flammability ( chemical property). The reaction itself is the change.

Fig. 2. The combustion of butane releases heat energy in the form of fire, so it is commonly used in lighter fluid. Source: Pixabay.

Examples of Physical and Chemical Changes

To understand physical and chemical changes, we'll have a look at some examples of both:

Examples of Physical Changes:

1. Melting: When ice melts, it changes from a solid to a liquid state, but its chemical composition remains the same.
2. Cutting: When a piece of paper is cut into smaller pieces, its physical appearance changes, but it is still made up of the same materials.
3. Boiling: When water boils, it changes from a liquid to a gas state, but it is still made up of the same molecules.

Examples of Chemical Changes:

1. Combustion: When wood burns, it undergoes a chemical reaction with oxygen in the air to produce carbon dioxide, water vapour, and ash.
2. Oxidation: When iron rusts, it undergoes a chemical reaction with oxygen in the air to form iron oxide.
3. Digestion: When food is digested in the stomach, it undergoes a series of chemical reactions to break down complex molecules into simpler ones that can be absorbed by the body.

Compare and contrast physical and chemical changes

The differences between these two types of changes can be summarized in the table below: