Short answer
- In chemistry, a substance is matter with a definite, constant composition and one set of properties throughout the sample. A pure substance is a substance made of only one kind of particle, either one element or one compound.
- An element is a pure substance made of only one type of atom. It cannot be broken down into simpler substances by an ordinary chemical change.
- A compound is a pure substance made of two or more elements chemically bonded in a fixed ratio. It can be broken down only by a chemical change, not by filtering or another physical step.
- A mixture is a physical blend of two or more substances. Its composition can vary, its components keep their own identities, and it can usually be separated by physical methods.
- A homogeneous mixture, also called a solution, looks uniform throughout. A heterogeneous mixture does not look uniform, and different parts of the sample have different compositions.
- The most common mistake is to treat "uniform" as the same as "pure." Salt water looks uniform, but it is a homogeneous mixture, not a pure substance, because the amount of salt in it can vary.
- The most useful purity test is comparison: a pure substance has characteristic melting and boiling behavior that stays consistent when pressure is stated, while a mixture usually melts or boils over a range. No single test proves absolute purity on its own.
Why These Words Cause Confusion
Everyday English uses “substance” loosely. A news report may call any material a substance, and a food label may call a product “pure.” Chemistry uses the same words in a narrower, testable sense, and that gap explains most homework errors.
In chemistry, the question is never whether a material looks clean, clear, natural, or simple. The question is whether every part of the sample has the same composition, and whether that composition is fixed. If both answers are yes, you are dealing with a pure substance. If the composition can change from sample to sample, or from one part of a sample to another, you are dealing with a mixture, however uniform it looks.
“Purity” in chemistry also does not mean “good,” “safe,” or “natural.” Pure sodium chloride is table salt in its chemically pure form, but pure carbon monoxide is poisonous and pure sulfuric acid is corrosive. Purity describes composition only.
Finally, “substance” itself is used in two ways. Some general writing uses it for any kind of matter, including mixtures. This article follows the stricter teaching usage: a substance has a uniform and definite composition, so a mixture, whose composition can vary, is not a substance in that strict sense. When another source seems to disagree, check which definition it is using.
The Classification Map
All matter in this system falls into two large groups, pure substances and mixtures. Pure substances split into elements and compounds. Mixtures split into homogeneous mixtures and heterogeneous mixtures. Most mistakes come from placing an example one level too high or one level too low.
| Group | Definition in one line | Can composition vary? | Separated by physical methods? |
|---|---|---|---|
| Pure substance: element | One type of atom only | No | No |
| Pure substance: compound | Two or more elements bonded in a fixed ratio | No | No, only by a chemical change |
| Homogeneous mixture | Uniform throughout, but made of two or more substances | Yes | Yes, in principle |
| Heterogeneous mixture | Not uniform, with visibly or measurably different parts | Yes | Yes, often easily |
What Counts as a Substance
A substance, in the strict sense used here, is a form of matter with a constant composition and properties that are constant throughout the sample. Chemistry LibreTexts states the point directly: a pure substance has a constant composition, meaning that it is the same everywhere, and properties that are constant throughout the sample, so there is only one set of properties, such as melting point, color, and boiling point, throughout the matter.
That definition has two built in tests. First, composition: does every portion contain the same substances in the same proportions? Second, properties: does every portion behave the same way when you measure a characteristic property? Distilled water passes both tests. Every portion is water, H2O, and every portion shows the same characteristic behavior under the same conditions. Salt water fails the first test, because one sample may be barely salty and another may be very salty, even though both look like clear water.
A substance is not defined by its state. Ice, liquid water, and water vapor are all the same pure substance, water, in different states. State is a condition, not an identity.
Pure Substances: One Kind of Particle
A pure substance is made of only one kind of particle. For an element, that particle is one type of atom. For a compound, that particle is one type of molecule or formula unit, built from more than one type of atom in a fixed arrangement. In both cases, the key word is “one.” A sample that contains two kinds of particles mixed together, even in tiny amounts, is not chemically pure, even if it looks like a single material.
In practice, chemists speak of purity as a matter of degree while keeping the definition strict in theory. Almost every real sample contains traces of something else. A laboratory sample described as pure has been purified and tested to a stated standard for its intended use. It does not mean that scientists have proved that not a single foreign particle is present, which would be very hard to prove. It means that the sample behaves as one substance within the limits of the tests used, and that any remaining impurity is below the level that matters for the purpose at hand.
That point matters for reading labels. A bottle labeled “pure water” in a store may mean distilled water, or it may mean water with nothing added for flavor. Neither claim is the same as a laboratory statement that a sample is a pure substance to a tested standard. Always ask: pure compared with what, tested how, and pure enough for what purpose?
Elements: The Simplest Pure Substances
An element is the simplest kind of pure substance. It is made of only one type of atom, and it cannot be broken down into chemically simpler substances by an ordinary chemical change. Chemistry LibreTexts gives aluminum, used in soda cans, as an example of an element, and notes that there are about 118 elements in the known universe. The International Union of Pure and Applied Chemistry periodic table likewise covers 118 elements.
Each element is defined by its atomic number, the number of protons in its atoms. All atoms of carbon have the same atomic number, and all atoms of gold have a different atomic number. That is what makes carbon and gold different elements.
Two refinements keep this definition honest. First, an element can appear in more than one form. Diamond and graphite are both pure carbon, but their atoms are arranged differently, so one is a hard, clear gemstone and the other is a soft, dark material used in pencils. These different forms of the same element are called allotropes. They do not change the element, but they do change its properties, which is why form and identity must be kept separate.
Second, atoms of the same element can have different numbers of neutrons. These variants are called isotopes. A natural sample of an element may contain more than one isotope, and chemists still call it that element, because the atomic number, not the neutron count, defines the element. Isotopes do not turn an element into a mixture of elements.
Common elements include oxygen, hydrogen, carbon, nitrogen, iron, copper, aluminum, gold, and silver.
Compounds: Fixed Ratios, New Properties
A compound is a pure substance made of two or more elements chemically bonded together in a fixed ratio. Water is the classic example: every water molecule has the same ratio of hydrogen atoms to oxygen atoms, expressed by the formula H2O. Sodium chloride, common salt in its pure form, has sodium and chlorine in a one to one ratio, expressed as NaCl. Carbon dioxide has one carbon atom bonded to two oxygen atoms, expressed as CO2.
The fixed ratio is the heart of the definition. It is sometimes called the law of constant composition: a given compound always contains its component elements in the same ratio, no matter where the sample came from or how it was made. Pure water from a laboratory, from a melted ice cube, or from a chemical reaction is still H2O in the same ratio. If the ratio changes, you no longer have the same compound. You have a different substance, or a mixture.
The second heart of the definition is that a compound has properties different from its component elements. Sodium is a reactive metal, and chlorine is a reactive gas, yet sodium chloride is a stable white solid used as salt. Hydrogen and oxygen are both gases at room temperature, yet water is a liquid at room temperature. That change in properties is the visible sign that a chemical bond has formed and a new substance exists, not just a blend of the old ones.
That is also why a compound cannot be separated by physical methods. Filtering will not split water into hydrogen and oxygen. To break a compound, you must break chemical bonds, which is a chemical change. For the fuller account of chemical changes, see the companion article on chemical and chemical changes, mentioned in the notes at the end of this article. This article does not repeat that coverage.
Mixtures: Blends That Keep Their Identities
A mixture is a physical combination of two or more substances. The components are not chemically bonded to each other, they keep their own identities and properties, and the proportions can vary. Chemistry LibreTexts describes a mixture as a physical blend of two or more components, each of which retains its own identity and properties in the mixture.
Salt water is the standard example. Dissolving salt in water changes only the form of the salt. You can recover both by physical means, for example by evaporating the water and leaving the salt behind. No new substance has been formed, and no chemical bond between salt and water needs to be broken.
Because the proportions can vary, mixtures do not have one fixed set of properties. A weak salt solution and a strong salt solution are both salt water, but they taste different, they have different densities, and they behave differently when heated. That variability is the diagnostic feature. If you can change the proportions and still have the same kind of material, you are looking at a mixture, not a pure substance.
Mixtures are everywhere, far more common in daily life than pure substances. Air, seawater, soil, milk, blood, concrete, salad dressing, and most foods are mixtures. Blending lets us tune properties: steel is stronger and more useful for many jobs than pure iron.
Homogeneous Mixtures: Uniform, but Not Pure
A homogeneous mixture is uniform throughout. Its composition is the same in every part of a given sample, so it looks like a single material. All solutions are homogeneous mixtures. Salt water, sugar dissolved in tea, vinegar, and air are standard examples.
Air deserves a closer look, because it is often misclassified as a pure substance. Air is a mixture of gases, mainly nitrogen and oxygen, with smaller amounts of argon, carbon dioxide, water vapor, and other gases. Its composition is fairly stable in broad terms, but it is not fixed: humidity changes the water vapor content, and local pollution changes the trace gases. Air also has no single melting point or boiling point as a substance. It is a homogeneous mixture, not a pure substance, and not a compound.
Brass is another useful case. Brass looks like a single golden metal, but it is an alloy, a homogeneous mixture of copper and zinc. Different brasses contain different proportions of copper and zinc, which is why brass for musical instruments, brass for plumbing fittings, and brass for decorative work can behave differently. If brass were a compound, its composition would be fixed. It is not, so it is a mixture, even though it is uniform to the eye.
The central lesson is this: uniformity is not purity. Salt water is uniform, but the salt content can vary. Pure water is uniform, and the composition cannot vary without ceasing to be pure water.
Heterogeneous Mixtures: Different Parts, Different Compositions
A heterogeneous mixture is not uniform throughout. Different parts of the sample have different compositions, and you can often see, feel, or measure the difference. Vegetable soup is the textbook example: one spoonful may contain more carrot, another more potato, and the liquid part differs from the solid parts. Oil and water shaken together form another example: they separate into layers, each layer a phase with its own composition.
Other everyday heterogeneous mixtures include soil, granite, muddy water, sand mixed with gravel, a salad, and a pizza. In each case, the sample as a whole has no single composition. You must say which part you mean, because the parts differ.
Two borderline cases deserve care, because they break the simple “looks uniform, so it is homogeneous” rule. The first is a colloid, such as milk. Milk looks uniform to the naked eye, but it contains tiny fat droplets and proteins dispersed in water. It is not a true solution, and in many textbook classifications it counts as a heterogeneous mixture, because its components are present as separate tiny phases, even though they are too small to see without magnification. The safe statement for a learner is this: milk is not a pure substance, and it is not a true solution. Whether a particular textbook files it under heterogeneous mixtures or under colloids, it is a mixture, and that is the classification that matters most.
The second borderline case is a suspension, such as muddy water or orange juice with pulp. A suspension looks cloudy or lumpy, and its solid particles will settle on standing or can be removed by filtering. Freshly squeezed orange juice, with pulp and liquid together, is a heterogeneous mixture. Filtered juice, with the pulp removed, is closer to a homogeneous mixture, but it is still a mixture of water, sugars, acids, and flavor compounds, not a pure substance. Classification always applies to a stated sample, not to a word in the abstract.
How Purity Is Tested and Defined
Purity is not judged by appearance. A beaker of pure water looks the same as a beaker of impure water. Purity is judged by comparing a sample with the known behavior of the pure substance, under stated conditions, using tests that are sensitive to extra components.
The classic school test uses melting and boiling behavior. A pure substance has a sharp, characteristic melting point and boiling point at a stated pressure. An impure substance, a mixture, usually melts or boils over a range of temperatures, and its melting point is often lowered while its boiling point may be raised or lowered depending on the impurity. The standard teaching test is the same: a way of verifying whether a substance is pure is by freezing or heating it, and if the melting and boiling points remain constant, the substance is pure, while variation is observed for mixtures.
Pure water is the familiar reference case. At standard atmospheric pressure, it melts at 0 degrees Celsius and boils at 100 degrees Celsius. Those figures are pressure dependent. At high altitude, where atmospheric pressure is lower, water boils below 100 degrees Celsius, which is why cooking times change in the mountains. A boiling point quoted without a pressure is incomplete.
Melting and boiling tests are useful, but they are not conclusive on their own. Some mixtures have sharp looking transitions, some pure substances decompose before they melt or boil, and a small amount of impurity may shift a melting point by less than a thermometer can reliably show. Chemists therefore use more than one kind of test for serious work. In paper chromatography, for example, a sample is placed on a strip and a solvent carries its components along at different rates. A pure substance should give one spot or band. A mixture gives more than one, revealing that more than one substance is present. Other tests compare density, solubility, crystal shape, or spectral patterns with reference data.
None of these tests proves absolute purity, in the sense of zero foreign particles. Together, they build a case: if every test that should reveal an impurity comes back clean, and the sample behaves exactly like the reference substance, it is reasonable to treat it as pure for the purpose at hand. That is the honest definition of purity in practice: a sample is treated as a pure substance when its composition is constant and its characteristic properties match the reference substance within the limits of the tests applied. The stricter the tests, the stronger the claim.
How Mixtures Are Separated
Mixtures can be separated by physical methods because their components are not chemically bonded. The method chosen depends on the property that differs between the components: particle size, density, solubility, magnetism, or boiling point.
| Method | Property used | Typical example |
|---|---|---|
| Filtration | Particle size, solid does not pass through the filter | Sand from water, pulp from juice |
| Evaporation | Volatility, solvent leaves and solid remains | Salt from salt water |
| Distillation | Boiling point difference, vapor is condensed and collected | Pure water from salt water |
| Decantation | Density and settling, liquid poured off a solid | Clear water poured off settled mud |
| Magnetism | Magnetic attraction | Iron filings from a mixture with sulfur powder |
| Chromatography | Different movement through a stationary phase | Plant pigments separated on paper |
| Centrifugation | Density under rapid spinning | Cream from milk in a dairy process |
Each method recovers a component unchanged. Filtered sand is still sand. Salt recovered by evaporation is still salt. Water recovered by distillation is still water. That is the contrast with a compound: no physical method recovers hydrogen from water or sodium from salt, because those elements are not present as separate components waiting to be picked out. They are bonded into a new substance, and only a chemical change can release them.
Worked Examples: Classifying Everyday Materials
Salt dissolved in water. Start with pure water and pure sodium chloride. Stir them together. The result looks like water. Is it a pure substance? No. The salt content can vary, the components keep their identities, and evaporation recovers both. Classification: homogeneous mixture, a solution. The salt in the mixture is still sodium chloride, a compound, and the water is still water, a compound. The mixture is not a compound, because no fixed ratio joins them.
Oil and vinegar dressing. Shake oil and vinegar together. The mixture may look cloudy at first, but it soon separates into layers. One spoonful from the top is mostly oil, one from the bottom is mostly vinegar. Composition varies by position in the sample. Classification: heterogeneous mixture.
Table salt from the kitchen. A salt shaker often contains more than sodium chloride. Many commercial salts contain anti caking agents and, in iodized salt, a small amount of an iodine compound. The pure compound sodium chloride is a pure substance. The contents of a typical salt shaker are a mixture, mostly sodium chloride with other substances present. For cooking, the mixture is fine. For a purity test, it is not a pure sample.
Common Mistakes, and How to Avoid Them
Clear means pure. Clear liquids can be mixtures. Salt water, sugar water, vinegar, and tap water are all clear, and all are mixtures. Clarity tells you about particles large enough to scatter light, not about composition.
One ingredient on a label means one substance. A label may list one main ingredient while the product contains processing aids or trace additives. “Pure honey” is a marketing and food standards term, not a chemistry claim that honey is a single compound. Honey is a mixture of sugars, water, and other substances.
Natural means pure. Natural materials are usually mixtures. Wood, soil, milk, blood, and seawater are all natural and all mixtures. Laboratory purification, not natural origin, makes a pure substance.
A single word means a single classification. Gold jewelry is often called gold, but most jewelry gold is an alloy, a homogeneous mixture of gold with copper, silver, or other metals for strength. Only gold refined to a stated standard counts as the pure element in a chemistry sense. The same caution applies to “salt,” “water,” “sugar,” and “oxygen.”
Homogeneous means pure. This is the most persistent error: a homogeneous mixture is uniform, but its composition can vary. Uniformity is about distribution within one sample. Purity is about fixed composition across samples. Salt water passes the first test and fails the second.
Why the Distinction Matters
In a laboratory, using a mixture when a pure substance is required, or the reverse, changes results. A reaction that should give a predictable product may give a different yield if the starting material contains an extra substance. A measurement that should be reproducible may drift from day to day if the sample composition drifts. That is why laboratories buy reagents to a stated purity, store them to prevent contamination, and record the sample, not just the substance name, in their notes.
A Five Question Test You Can Use Anywhere
Use these questions in order on any stated sample. Stop at the first question that gives a clear answer, and you will usually reach the right classification.
- Does every part of this sample have the same composition? If no, it is a heterogeneous mixture.
- If it looks uniform, could another sample of the same kind have a different composition? If yes, it is a homogeneous mixture.
- If the composition is fixed, is it made of one type of atom only? If yes, it is an element.
- If the composition is fixed and it contains more than one type of atom bonded together, can it be separated by a physical method? If no, and only a chemical change breaks it down, it is a compound.
- If you are still unsure, what does a property test show? Compare melting behavior, boiling behavior, or a chromatography result with reference data for the suspected pure substance, at a stated pressure. Agreement supports purity. A range, or extra components, points to a mixture.
Frequently Asked Questions
Is water a pure substance or a mixture? It depends on the sample. Distilled water, water on its own, is a pure substance, a compound, H2O. Tap water is a homogeneous mixture, because it contains dissolved minerals and gases in varying amounts. Seawater is a mixture. Always name the sample before you answer.
Are elements and compounds both pure substances? Yes. Elements and compounds are the two kinds of pure substances. Elements contain one type of atom. Compounds contain two or more elements bonded in a fixed ratio. Both have constant composition and one set of characteristic properties.
Is salt a pure substance? Pure sodium chloride, NaCl, is a pure substance, a compound. Kitchen salt as sold is often a mixture, because it may contain anti caking agents and, if iodized, an iodine compound. The compound and the kitchen product are different samples, and they deserve different classifications.
Sources & references
- Chemistry LibreTexts, Classification of Matter (Pure Substances and Mixtures), South Puget Sound Community College course material. Institution teaching page opened 27 October 2026. Covers pure substance and mixture definitions, element and compound definitions, homogeneous and heterogeneous mixtures, phases, salt water and vegetable soup examples, and the statement that there are about 118 elements.
- Standard chemistry teaching material on substance and element definitions, consulted 27 October 2026. Covers the distinction between elements and compounds, allotropes such as graphite and diamond, and the melting and boiling point constancy test for purity.
- International Union of Pure and Applied Chemistry, Periodic Table of the Elements and Isotopes, via the U.S. Geological Survey Publications Warehouse summary. Institution summary consulted 27 October 2026. Covers the periodic table structure and the categorization of the 118 elements.
