A pure substance is matter that contains only one type of element or one type of compound, with a fixed, definite set of properties. Every sample of a pure substance you examine will have exactly the same chemical composition. This distinguishes it from a mixture, where the ingredients can vary in proportion. Gold, oxygen, distilled water, and table sugar are all pure substances.
What Makes a Substance “Pure”
In chemistry, “pure” has a specific meaning that differs from everyday usage. A pure substance contains only one kind of matter. That means every molecule or atom in the sample is identical, and the composition never changes no matter where the sample comes from or how it was made.
This definition splits pure substances into two categories: elements and compounds. Elements are the simplest form, made of only one type of atom. Compounds are made of two or more elements locked together in a fixed ratio. Both qualify as pure because their composition is constant and predictable.
The key distinction from mixtures is that fixed ratio. In a compound like water, hydrogen and oxygen are always present in the same 1:8 proportion by mass. Saltwater, on the other hand, can have varying amounts of salt dissolved in it. You can make it saltier or more dilute, which makes it a mixture. You cannot make water “more hydrogen” without creating an entirely different substance.
Elements: The Simplest Pure Substances
An element is pure matter composed of only one type of atom. Hydrogen, oxygen, carbon, nitrogen, gold, silver, iron, sodium, and chlorine are all elements. You cannot break an element down into simpler substances by any chemical reaction. A bar of pure gold contains nothing but gold atoms, and its properties (color, density, melting point) are the same whether the gold was mined in South Africa or Australia.
Elements can exist as single atoms, like helium, or as molecules made of the same element, like oxygen gas (O₂). Both forms count as pure substances because only one type of element is present.
Compounds: Fixed Combinations of Elements
A compound forms when two or more elements combine chemically in a constant ratio. Water is always two hydrogen atoms bonded to one oxygen atom. Table sugar (sucrose) is always 12 carbon, 22 hydrogen, and 11 oxygen atoms arranged the same way. This principle was first formally stated in 1799 by Joseph Proust and is known as the Law of Definite Proportions: a chemical compound always contains its elements in the same fixed ratio by mass, regardless of its source or how it was prepared.
A compound’s chemical formula tells you exactly what’s inside and in what proportions. That formula never varies. If it did, you’d have a different compound entirely. This is what makes compounds pure substances rather than mixtures.
Unlike elements, compounds can be broken down into simpler substances, but only through chemical reactions. Running an electric current through water separates it into hydrogen and oxygen gases. You cannot separate water by filtering, boiling, or any other physical method, because the hydrogen and oxygen atoms are chemically bonded together.
How Pure Substances Behave Differently From Mixtures
One of the most practical ways to tell a pure substance from a mixture is by watching how it changes temperature. When you heat a pure crystalline solid, its temperature rises steadily until it reaches the melting point. Then the temperature holds constant, staying flat until every bit of the solid has melted. Only after the phase change is complete does the temperature start climbing again. The same pattern happens in reverse during freezing and at the boiling point.
Mixtures behave differently. They tend to melt and boil across a range of temperatures rather than at one sharp point. If you heat saltwater, for instance, the boiling temperature shifts as water evaporates and the remaining solution becomes more concentrated. A pure substance’s melting point and boiling point are reproducible constants (as long as pressure stays the same), which is why chemists use these values to identify and verify the purity of a sample.
Everyday Examples of Pure Substances
Pure substances show up in daily life more often than you might expect:
- Distilled water is a pure substance. It contains nothing but H₂O molecules.
- Table sugar (sucrose) is a pure compound with a fixed formula.
- Baking soda (sodium bicarbonate) is another pure compound found in most kitchens.
- Diamond is pure carbon arranged in a crystal structure.
- Oxygen gas in a medical tank is a pure element.
Notice that distilled water is on this list, but tap water is not. Tap water contains dissolved minerals, residual chlorine from treatment plants, and other trace substances. Its composition varies depending on where you live and how your water is treated, which makes it a mixture. Similarly, the air you breathe is a mixture of nitrogen, oxygen, water vapor, carbon dioxide, and suspended dust particles, with proportions that change from ground level to the upper atmosphere and even from day to day.
What “Pure” Means in a Lab
In practice, absolute purity is nearly impossible to achieve. Even laboratory-grade chemicals contain trace impurities. Chemists use standardized purity grades to communicate how close a substance is to truly pure. The most common benchmark is ACS grade, set by the American Chemical Society, which requires a purity of at least 95% and tightly controls what impurities are allowed. Analytical Reagent (AR) grade chemicals meet similar thresholds, typically exceeding 95% purity with specific limits on each contaminant.
For most classroom and everyday purposes, though, you don’t need to worry about these lab standards. When your chemistry textbook calls water a pure substance, it means the idealized version: nothing but H₂O. The concept is about composition being fixed and uniform, not about whether a real-world sample has been perfectly purified.
Pure Substances vs. Mixtures at a Glance
- Composition: Pure substances have a fixed, constant composition. Mixtures have variable proportions.
- Separation: Mixtures can be separated by physical methods like filtering or evaporation. Compounds require chemical reactions to break apart, and elements cannot be broken down at all.
- Properties: Pure substances have sharp, consistent melting and boiling points. Mixtures melt and boil over a range.
- Examples: Pure substances include gold, distilled water, and sugar. Mixtures include saltwater, air, and trail mix.