Mixtures and compounds

Water - as you might know - is made of hydrogen and oxygen. Does that mean that water is a mixture of those two substances? Or is water a single substance? Let's zoom in and have a look at the atomic level. Here is a mixture of oxygen gas and hydrogen gas.

The mixture is also - a gas. When hydrogen and oxygen react... ... water molecules are formed. In water molecules, there are chemical bonds between the atoms. When atoms of different elements attach to each other with chemical bonds, the resulting substance is called a chemical compound.

What about salt? Ordinary table salt is made up of sodium and chlorine. Is that a mixture, or a compound? At the atomic scale, sodium and chlorine form a crystal lattice. They are attached to each other, which means that table salt - or sodium chloride - is also a compound.

Salt and water - and other chemical compounds - are pure substances, not mixtures. What if you were to dissolve salt in water? Is salt water a mixture or a compound? Let's look at the atomic level. Are there any chemical bonds?

The salt crystals have dissolved, but the atoms haven't formed chemical bonds to the water. No new compound has been formed - salt water is a mixture. There are more differences between mixtures and compounds. Let's look at the properties of the substances involved. Hydrogen and oxygen act very differently on their own.

Both are gases at room temperature. Hydrogen is highly reactive. And pure oxygen is highly flammable. But water is not very reactive. And we actually use it to put out fires.

What about sodium and chlorine? Sodium is a corrosive metal that reacts violently with water. Chlorine is a poisonous, foul-smelling gas. Luckily, sodium chloride - salt - doesn't have any of those properties. So compounds have different properties from the substances that they are composed of.

How about the mixture, salt water? Both the water and the salt retain most of their properties. A third difference between mixtures and compounds is the ratio of the substances involved. In order to form water, we need exactly twice as many hydrogen atoms as oxygen atoms. If we add or remove an atom, it's not water anymore.

The ratio of atoms in a certain compound is always the same. In a mixture, like salt water the ratio can vary. Water with a pinch of salt,... ... or lots of salt... ... is still salt water.

Here's a list of the main differences between compounds and mixtures. What about ordinary table sugar? Is that a mixture or a chemical compound? Let's check the list. Sugar is made up of carbon, hydrogen and oxygen.

But it's not black like coal nor gaseous like hydrogen or oxygen. The atoms are bound together with chemical bonds, forming molecules. The molecules always have twelve carbon atoms, 22 hydrogen atoms and eleven oxygen atoms, so it has a specific chemical formula. Sugar is a chemical compound, not a mixture.