Yes, water is scientifically considered a pure substance.
That being said, the water that we find out in the world is rarely a pure substance.
Confused? In this article, we’ll explain what a pure substance is, and why it is that water does and does not qualify.
Is Water a Pure Substance? (Explained)
What Is A Pure Substance?
A pure substance is a material that is composed of only one of something. This could be one kind of element, or one kind of molecule.
For example, gold is a example of a pure substance, because it is composed only of the element gold.
Gold is often combined with other metals in jewelry, and when that happens, it is no longer considered a pure substance.
Another example of a pure substance that is made up of more than one type of substance is carbon dioxide.
Carbon dioxide is a molecule formed when carbon and oxygen chemically bond to each other.
Carbon dioxide is a gas that is made up only of these bonded molecules. There are no other molecules or elements. Just the carbons and oxygens joined up to each other, repeated over and over again throughout the gas.
Why Is Water a Pure Substance?
As a pure substance, water is more like carbon dioxide than it is like gold.
Water is made up of two substances that are chemically bonded together. Two hydrogens bond to one oxygen to form a water molecule. In water, this molecule exists on repeat, millions of times over.
There are no other kinds of molecules necessary to make up water.
This is why water is considered a pure substance.
But If Water Is A Pure Substance, Why Did You Say That It is Also NOT A Pure Substance?
Scientifically (and theoretically), water on its own is a pure substance.
But in our world, water rarely (if ever) exists in this form.
In most cases, water only exists as a pure substance when humans make that happen by removing all of the other components you’d find in water.
Take your tap water, as an example.
Most likely, even though you can’t see it, your tap water contains tens or even hundreds of different kinds of substances, such as chlorine in small amounts to kill bacteria or other harmful organisms, those bacteria or harmful organisms, minerals such as calcium, iron, zinc, or potassium, iron oxide, fluoride for your teeth, and much more.
Since tap water contains more than just the simple H2O compound, it is not a pure substance.
The same is true with water samples you could collect around the world, such as seawater (contains salt), lake water (contains tons of organic materials and bacteria), and even drinking water from plastic bottles (minerals).
How Is Water As A Pure Substance Made?
Since the impurities that plague water are often too small to filter out (though you can filter some), the primary way to make pure water is through distillation.
To make pure water (aka distilled water), you’d evaporate the water molecules into a gas, and then re-condense those gaseous molecules in another container. The impurities and extras (minerals, bacteria, etc) are left behind a when the water molecule changes phase from liquid to a solid.
The impurities don’t make the jump to a gas along with water.
If Water Is Made Of Two Substances, Why Isn’t Water A Mixture?
This is a great question, and one that we answered in a separate article: Is Water A Mixture (Or A Compound)?
Is Water a Pure Element?
No, water is not a pure element. First, I’m not even sure that there is such a thing as a pure element.
An element is a substance that can’t be broken down into a simpler substance.
If that element is chemically bonded to another substance, it is not an element.
If the element is mixed up with another substance, but those two substances are not bonded to each other, the element is still an element. And then resulting substance is called a mixture.
Water is not an element. Water can be broken down into simpler substances (hydrogen and water).
So it doesn’t make much sense to try and figure out whether water is a pure or non-pure element.