How are substances identified? There are two major ways we can describe a substance: physical properties and chemical properties. Learn about how chemists use properties to classify matter as either a mixture or a pure substance.
Look around you. What do you see? What kinds of things surround you? Maybe a notebook, or your keyboard, a coin in your pocket or even a pet? Think about how you may describe those things. What do they all have in common? You may be able to come up with some similarities; but one key similarity is that they are all matter. Matter is everything around you! Your pencil, a book; even you are matter. One of the most important things chemists do is describe matter. Matter is anything that has mass and takes up space. Chemists spend their days working with matter: describing it, making measurements and even changing its form.
Let's take a closer look at a coin in your pocket. It is made from a metal. You can tell it is made from a metal because it is shiny and maybe has a silvery color. Those two descriptions I gave it are called physical properties. A physical property is a characteristic that can be observed or measured without changing the composition. If you toss your coin in a fountain, it will sink. This is because your coin is more dense than the water in the fountain. Density is another example of a physical property. Your coin may be a little wet, but it is still composed of the same material as it was before you tossed it in the fountain. Other examples of physical properties include color, mass, smell, boiling point, volume and temperature.
Rust results when iron reacts with oxygen
Now let's talk about your car. Your car is likely made from a metal as well. How can you tell? It probably has a very similar density to the coin in your pocket. What other properties does a car have? If you have ever seen a 'fender bender' then you would know that metals are quite bendable. When they get hit by something, they dent instead of shattering like glass. This property is known as malleability. It is a property that almost all of the metals share and it is a physical property because your car is still composed the same metal after it is dented. It just may look a little different.
But what happens if we fast forward 10 years. Now what does that same car look like? Chances are that it may be starting to form some rust. Unfortunately, the iron that was used to make your car is not the most stable of the metals. When it is exposed to oxygen it transforms into rust. This ability to react to oxygen to form rust is called a chemical property. A chemical property is a characteristic that can only be determined by changing the chemical identity of a substance. What we started with was iron, but when it reacts with the oxygen in the air, it turns into iron oxide, or rust. What are some of the physical properties of rust? Are they anything like the properties of iron? Are they anything like the properties of oxygen? No. Rust has its own set of properties. First, unlike its metallic counterpart, it is brittle. It's also a dull reddish-brown color. So the ability to react with oxygen is a chemical property of iron. Other examples of chemical properties include reactivity with water, reactivity with acid, pH and my personal favorite: flammability, or the ability to burn.
Mixtures and Pure Substances
The reason properties are so important to a chemist is chemists use them to describe matter and the changes it undergoes. They also use properties to put matter into major categories or classifications. For example, if you have a substance that can be physically separated out into other substances, you have a mixture. A mixture is material made up of two or more substances that are physically mixed but not chemically combined. Take ocean water as an example. The major components of ocean water are water and salt. They can be separated by allowing the water to evaporate. This is a physical separation because we started with salt water, and we ended with salt and water vapor (which is still water). Solutions and alloys are examples of mixtures.
The ocean water mixture separates when the water evaporates into vapor
If you can't separate it physically, then you have a pure substance. There are only two types of pure substances: elements and compounds. An element is a pure substance made up of only one type of atom. Examples of elements include hydrogen, sodium, neon, gold and copper. Let's go back to our rusty car. We started off with two elements: iron and oxygen. They were pure substances that only contained iron atoms or oxygen atoms. When they reacted over time, they chemically combined and transformed into a completely different substance with different properties.
This new substance is called a compound. A compound is a pure substance that is made up of two or more different elements that are chemically combined. Several common compounds include water (H2 O), limestone (CaCO3), salt (NaCl) and carbon dioxide (CO2). It is not easy to separate the elements in a compound because it requires a chemical reaction. For example, if I wanted to separate the oxygen from the hydrogen in water, I would have to do a lot more than just pull them apart. They would have to undergo a chemical reaction to split them.
So let's put this all together. Matter is anything that has mass and volume. It can be described using physical properties, and chemical properties. Physical properties can be determined without changing what it's made of, and chemical properties can only be determined by changing its composition. Matter can be divided into two main types: mixtures and pure substances. Mixtures are made of two or more substances that are physically mixed and can be easily separated without changing what the substance is made of. Pure substances can be further divided into elements and compounds. Elements only consist of one type of atom, and if you chemically combine two different elements you will make a compound.