A pure compound is a substance made of two or more different elements chemically bonded together in a fixed ratio. Every sample of that compound, no matter where it comes from or how it was made, contains exactly the same proportion of elements. Water is a classic example: any pure sample of water is always 11.19% hydrogen and 88.81% oxygen by mass, whether it came from a glacier, a lab, or a rain cloud.
What Makes a Compound “Pure”
In chemistry, the word “pure” has a specific meaning. A pure substance has a constant composition and a single set of physical properties throughout the entire sample. There are only two kinds of pure substances: elements (made of one type of atom) and compounds (made of two or more types of atoms bonded together). So when chemists say “pure compound,” they mean a substance where every molecule or unit has the same chemical formula, with no other substances mixed in.
This consistency is described by the law of definite proportions: a given compound always contains the same elements in the exact same proportions by mass. Carbon dioxide, for instance, always consists of one carbon atom bonded to two oxygen atoms. You can’t have a version of carbon dioxide with extra oxygen floating around loosely. If extra oxygen were present, you’d have a mixture, not a pure compound.
Compounds vs. Elements
Both compounds and elements qualify as pure substances, but they differ in one key way. An element is made of only one type of atom. Gold is gold all the way through. A compound, by contrast, contains atoms of different elements locked together by chemical bonds. Table salt (sodium chloride) combines sodium and chlorine in a 1:1 ratio. Aluminum oxide combines aluminum and oxygen in a 2:3 ratio. These ratios are always whole numbers and never vary.
The important distinction is that a compound can be broken down into simpler substances through chemical reactions, while an element cannot. You can split water into hydrogen gas and oxygen gas using electricity, but you can’t break hydrogen or oxygen down any further by chemical means.
Compounds vs. Mixtures
This is where most confusion happens. A mixture combines two or more substances, but those substances are not chemically bonded. Each component keeps its own properties. Tea, for example, is a solution of various compounds dissolved in water. It looks uniform, but it’s not a pure compound because you can separate its components through physical methods like filtration or evaporation.
A pure compound requires chemical methods to break apart. The bonds holding its atoms together are strong, and simple filtering or boiling won’t separate the elements. This is fundamentally different from a mixture, where the ingredients are just physically blended. Salt water is a mixture because the salt and water molecules aren’t bonded to each other. Table salt itself is a compound because its sodium and chlorine atoms are bonded in a crystal structure.
Two Main Types of Compounds
Pure compounds fall into two broad categories based on how their atoms are bonded.
Ionic compounds form when metals combine with nonmetals. One atom transfers electrons to another, creating positively and negatively charged particles that attract each other. Table salt is ionic: sodium gives up an electron to chlorine, and the resulting charged particles pack into a rigid crystal. Other common ionic compounds include calcium chloride, potassium bromide, and barium nitrate.
Molecular compounds form when nonmetals share electrons with each other. Water, ammonia, and carbon dioxide are all molecular compounds. These tend to have lower melting points than ionic compounds and often exist as liquids or gases at room temperature.
How to Tell If a Substance Is Pure
Pure compounds have sharp, consistent physical properties, and this is one of the most practical ways to check purity. A pure crystalline solid melts at a single, precise temperature. The transition is so sharp that melting points can be measured to within 0.1°C. Chemists use this to identify unknown compounds: glucose melts at 150°C, fructose at 103 to 105°C, and sucrose at 185 to 186°C. If you have an unknown white powder, its melting point can help you figure out what it is.
Mixtures, by contrast, melt over a broad temperature range and typically at lower temperatures than any of their pure components. If a substance that should melt at exactly 150°C instead softens gradually between 140°C and 148°C, that’s a strong sign it contains impurities.
In laboratories, more advanced techniques confirm purity. Chromatography separates a sample into its individual components, revealing whether anything besides the target compound is present. Nuclear magnetic resonance (NMR) spectroscopy can directly measure what fraction of a sample is the intended compound. These tools let scientists verify purity levels with high precision.
Purity in Practice
No manufactured chemical is ever 100% pure in the real world. Industry uses grading systems to communicate how close a substance comes. Chemicals labeled “ACS grade” or “reagent grade” meet standards set by the American Chemical Society and typically exceed 95% purity, with the specific types and amounts of impurities measured and controlled. For most laboratory work, this level of purity is sufficient. Specialized applications like pharmaceutical manufacturing or analytical chemistry may demand even higher purity levels.
Common Examples
Pure compounds are everywhere in daily life. Water (H₂O) is the most familiar, made of hydrogen and oxygen. Table salt (NaCl) is sodium and chlorine. Baking soda (NaHCO₃) contains sodium, hydrogen, carbon, and oxygen. Sugar, or sucrose (C₁₂H₂₂O₁₁), is built from carbon, hydrogen, and oxygen atoms bonded in a precise arrangement. Carbon dioxide (CO₂) in the air you exhale is a pure compound of carbon and oxygen.
In each case, the compound has properties completely different from its individual elements. Sodium is a reactive metal that explodes in water. Chlorine is a poisonous gas. Combine them chemically, and you get table salt, something you sprinkle on food. That transformation, where bonded elements produce a substance with entirely new characteristics, is what defines a compound.