The pure substances are those that have a constant chemical composition and itself. That is, they are homogeneous, so they are only made up of the atoms or molecules that appear in their respective symbols or chemical formulas.
For example, gold (Au), copper (Cu), zinc (Zn), etc. are pure substances. If we find other atoms or elements in them, then they will cease to be pure: gold mixed with copper, or copper mixed with zinc, and so on. Therefore, mixtures are not pure substances.
Water, even if it has minerals, we usually consider it as a pure substance, even when in reality it is not. Juices, on the other hand, are not pure substances, as they contain water and the nutrients of fruits.
Pure substances can be classified into simple (elements) or compounds. They are simple when they only contain a single type of atom, as in the case of the metals above. And they are compounds, on the other hand, if they have two or more atoms forming heterogeneous molecules, such as water (H 2 O), or a set of ions, such as table salt (NaCl).
Water, for example, will cease to be pure if we add sugar to it: a mixture of sugar and water is a mixture, not a pure substance. The same would apply if we dissolved salts in it. That is why sea water is not considered a pure substance.
Propieties of the pure substances
Pure substances have a number of physical and chemical properties:
Pure substances are homogeneous, so it can be said that they have a constant composition throughout their mass or extension. This means that if a pure substance, for example an iron bar, is divided into many pieces, each of them will have the same physical and chemical properties.
Constant magnitudes of its properties
As there is only one type of atoms or molecules in pure substances, their physical or chemical properties are very well defined and do not show great variations. That is, its properties have constant magnitudes or values at the same temperature or pressure.
For example: pure water will always be crystal clear and will boil at the same temperature. However, when mixed with other substances, it can become cloudy or change color, and its properties begin to be different: it changes density , viscosity, smell, the temperature at which it freezes or boils, etc.
Inseparable by simple methods
A pure substance, like a chemical element, has a type of atom that cannot be separated into other components, neither by physical nor by chemical means. In other words, only iron atoms can be obtained from a pure iron block, since these cannot continue to degrade into smaller parts (at least not nuclear).
Another example would be to want to separate gaseous oxygen (O 2 ): no matter how much it is subjected to separation methods, we will only obtain O 2 molecules (or individual O atoms in extreme conditions).
Similarly, with pure water no separation can be achieved: only water molecules will be obtained, no matter how much is distilled, filtered, sieved, etc.
However, water, being a pure compound substance, can break down into its elemental components: hydrogen and oxygen, which is what happens when it is subjected to electrolysis (decomposition by the action of an electric current).
Different physical states
Pure substances can present different states or physical phases, maintaining their composition and purity.
Distilled water is a pure substance made up of only H 2 O molecules . When it freezes, it turns into ice, but its composition does not change. The same happens when it evaporates: the H 2 O molecules remain the same. Water maintains its purity in all three physical states, unless it is mixed with other elements or compounds.
Description by chemical formula
Pure substances can be perfectly described with a chemical formula.
For example, the formula H 2 O describes water and its molecule, so a tank of pure water can be described as H 2 O nothing else. But if we add sodium hypochlorite salts to this tank of pure water, we will have a mixture (pool type) that can no longer be described with the formula H 2 O.
Why? Because there is no longer only water, but we also have dissolved sodium hypochlorite (NaClO), which is not specified anywhere in the H 2 O formula (which is that of water and nothing else).
When a substance (liquid, solid or gaseous) cannot be easily described by a single chemical formula, then it is not pure.
Types of pure substances
There are basically two types of pure substances: the simple ones (the chemical elements) and the chemical compounds.
Simple substances (chemical elements)
A substance consisting of only one type of atom corresponding to a chemical element can be considered as a pure substance. This is the case as long as there is no contamination due to the presence of atoms of another chemical element, or of atoms of several chemical elements.
A piece of pure silver is made up of only atoms of the metal silver, the element (Ag). Likewise, pieces of nickel or copper metals can be considered as pure substances, as long as they are not present in their pieces contaminations of other chemical elements.
Here comes an important point to clarify: alloys are not pure substances, as they result from the mixture of two or more metals. That is why bronze or brass, for example, are not pure substances.
Among other examples of simple pure substances we have:
A pure substance is a chemical compound if it is made up of two or more chemical elements, which form heterogeneous molecules, or a set of ionic networks (salts, oxides, sulfates, etc.).
Water produced in the laboratory by various procedures can be considered pure; But nature’s water, on the other hand, is polluted by various chemical elements and salts, so it cannot be considered pure.
Ammonia is a chemical compound that is in a gaseous state under normal pressure and temperature conditions . Ammonia molecules are made up of three hydrogen atoms and one nitrogen atom (NH 3 ).
A substance called ammonia will be pure if all its molecules are of the same type and have the formula NH 3 . That is, if it is mixed with gaseous oxygen or water vapor, then it will no longer be pure (it will be a gaseous mixture).
Sucrose (sugar) has the chemical formula C 11 H 22 O 11 and regardless of how complex and extensive the formula is, sugar can constitute a pure substance if there are only molecules of the given formula in it.
Similarly, it can occur with sodium bicarbonate (NaHCO 3 ), and with any other compound with chemical formulas.
Examples of pure substances
The examples of pure substances are inexhaustible. Several of them will be listed below:
- Magnesium (simple).
- Calcium (simple).
- Iron (simple).
- Tin (simple).
- Gaseous oxygen (simple, because the O 2 molecule has only O atoms).
- Ozone (simple, because the O 3 molecule has only O atoms).
- Chlorine (simple, because the Cl 2 molecule has only Cl atoms).
- Helium (simple).
- Nitrogen (simple, because the N 2 molecule has only N atoms).
- Sulfur (simple, even though it can present molecules like S 8 ).
- Mercury (simple).
- Carbon dioxide (compound).
- Carbon monoxide (compound).
- Glucose (compound).
- Citric acid (compound).
- Sodium sulfate (compound).
- Sodium hydroxide (compound).
- Distilled water (compound).
- Sodium carbonate (compound).
- Sulfuric acid (compound).
- Pyruvic acid (compound).
- Sodium chloride (compound).
However, in real life it is almost impossible to find substances that are 100% pure: they will always have a small and insignificant amount of impurities, which we often ignore.
Tip for identifying a pure substance
If you can write a formula for a substance or it is a pure element, it is a pure substance.