Physical and Chemical Changes

The Law of matter conservation states that matter cannot be created or destroyed, only changes its form. It can go through either a physical or chemical change. In this article, we will learn what these changes, what they look like, and how they help us identify substances

• This article is about physical and chemical changes
• Firstly, we will define physical and chemical properties
• We will look at examples of these properties and how we can utilize them to identify unknown substances
• Then we will dive into the definition of physical and chemical changes
• We'll identify examples of each of these changes
• Lastly, we'll learn the key differences between these two

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.

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.

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

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

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.