Elements, Compounds and Mixtures: Classification of Matter, with Examples

Pure substances and mixtures: the first decision

According to Chemistry LibreTexts, in the introductory chemistry course at the College of Marin, a pure substance keeps the same composition at every point of the sample, and its properties do not change either. Melting point, color and boiling point form a single set across the whole sample. A material made of two or more substances is a mixture.

That is where classification starts. Composition always identical: pure substance. Composition that varies from sample to sample: mixture.

Practical example: table salt (sodium chloride) is a pure substance, because its composition is uniform and definite. Salt water is not, because the amount of dissolved salt can vary from sample to sample.

In a mixture each component keeps its own identity and properties, and the amounts of the components can vary, whereas a compound has a fixed, definite ratio. The melting point gives the difference away as well. As the Wikipedia entry on melting point reports, a pure substance melts over a narrower range than an impure sample of the same material or a mixture containing it. The more other components there are, the lower the melting temperature falls and the wider the range grows. Eutectics are the exception: mixtures that melt at a sharp, constant temperature, like a pure substance.

	Pure substance	Mixture
Composition	Constant at every point of the sample	Can vary from sample to sample
Properties	One single set	Each component keeps its own
Melting	Narrow range	Wider range (except eutectics)
Example	Table salt	Salt water

Homogeneous and heterogeneous mixtures

In a homogeneous mixture, the South Puget Sound Community College text explains, every sample has the same composition as any other. The most common case in chemistry is the solution, one substance completely dissolved in another, and all solutions are homogeneous mixtures. Alloys are too: the Wikipedia entry on mixtures describes them as solid solutions, homogeneous mixtures of metals. A homogeneous mixture has a single phase, like a pure substance.

In a heterogeneous mixture the composition is not uniform and there are two or more phases. Soil and vegetable soup are examples. Oil and water form two separate layers. Orange juice with pulp holds solid bits and liquid, so it belongs among the heterogeneous mixtures.

Practical example: salt in water is homogeneous, with uniform composition and one phase. Sand in water is heterogeneous, because the components remain clearly distinguishable. Stir in so much sugar that some of it stays undissolved on the bottom, and the sugar-water mixture becomes heterogeneous too.

A homogeneous mixture and a pure substance both look uniform. Chemistry LibreTexts tells them apart by composition, which is always the same in a pure substance and variable in a mixture. The Mixture entry adds that the distinction depends on the scale of sampling: in practice, a mixture is homogeneous if the property you care about is the same whichever sample you take.

Elements and compounds

Elements and compounds are both pure substances. An element resists being broken down into chemically simpler components; Treccani describes it as a simple substance whose atoms all share the same atomic number, and unlike compounds it does not break down further in ordinary chemical reactions. A compound contains more than one element, and for that very reason it can be broken down into simpler components. Water, for example, is a compound of hydrogen and oxygen.

About 118 elements are known, according to Chemistry LibreTexts, which does not give a year for the figure. For elements 113, 115, 117 and 118, IUPAC published two reports in 2016 validating their discovery, and on 28 November 2016 it announced the approved names and symbols. How the elements are ordered and read is the subject of the periodic table. In the eighteenth century, Treccani recalls, people stopped thinking in terms of the four pre-Socratic elements and began identifying elements by experiment: Lavoisier identified thirty-three, Treccani says.

The same element can show up in more than one form, called allotropes. According to LibreTexts, oxygen has two common allotropes, O2 (the one in air) and O3 (ozone), which has three atoms bonded instead of two. Carbon has many: diamond is transparent and the hardest natural material known, graphite is black, opaque and soft enough to leave a mark on paper. Diamond barely conducts electricity, and graphite is an excellent conductor.

The chemical and physical properties of a compound may look nothing like those of its elements. Combine sodium, a soft metal with a shine, and chlorine, a green gas that smells sharp, and the result is sodium chloride, a white crystalline solid. Water forms when hydrogen and oxygen join chemically in a fixed ratio, 2 atoms of hydrogen to 1 of oxygen.

The law of definite proportions, formulated by Joseph Proust in 1797, says that a compound contains its elements in a fixed mass ratio, whatever its origin or method of preparation. In a sample of pure water, oxygen makes up about 8/9 of the mass and hydrogen 1/9. At the end of the eighteenth century the concept of a compound was not yet well defined and the law was new: Claude Louis Berthollet argued that elements could combine in any proportion, and the law was disputed.

Material	Type	Reason
Oxygen (O2), ozone (O3)	Element	A single element, in two forms
Diamond, graphite	Element	Two forms of carbon
Water	Compound	Hydrogen and oxygen, 2 to 1
Sodium chloride	Compound	Sodium and chlorine combined
Sodium bicarbonate (NaHCO3)	Compound	A pure substance, not a mixture
Salt water	Homogeneous mixture	The salt can vary
Brass	Homogeneous mixture	Alloy of copper and zinc
Vegetable soup	Heterogeneous mixture	Non-uniform composition

Sea water, hydrogen peroxide, air and baking soda

Sea water is a solution of salts in water: water is a compound, but what comes out of the sea also contains salt in varying amounts, so it is not a pure substance.

According to PubChem, hydrogen peroxide as sold is an aqueous solution of the compound, a strong oxidizer used as a bleach and a local antiseptic. It comes in different concentrations, such as the 3% one, and it is a homogeneous mixture of a compound in water. Pure hydrogen peroxide is unstable and breaks down quickly into oxygen and water.

Baking soda, by contrast, is a compound of sodium and bicarbonate ions with the formula NaHCO3: a pure substance, a white crystalline powder. According to PubChem, heated above 50 °C it begins to give off carbon dioxide, water and sodium carbonate.

Air is a gaseous mixture. According to NOAA (page updated July 2024), dry air is 78.084% nitrogen, 20.946% oxygen, 0.934% argon and 0.042% carbon dioxide by volume. Water vapor is almost always present, up to about 4% of the total volume.

How mixtures are separated

Separating a mixture takes only techniques that rely on physical changes, and the pure substances it is made of always come back. According to the Mixture entry, the mechanical methods are distillation, chromatography, filtration, gravity separation and centrifugation.

• Filtration: holds back particles large enough to be caught by a porous material. Soil suspended in water can be filtered with a coffee filter, and tea is separated from the leaves the same way.
• Evaporation: separates a dissolved solid from a liquid. This is how table salt is obtained from sea water, and in salt pans the heat comes from the sun.
• Simple distillation: once the mixture is heated, the more volatile component becomes vapor first, at the lower temperature. The vapor, cooled in a condenser, turns back into liquid and is collected as the distillate.
• Chromatography: the components of the mixture cross a medium and advance at different speeds. Thin-layer chromatography separates the colored compounds of a spinach leaf.
• Decantation: the top layer is poured away and the denser liquid or the solid stays at the bottom; the property exploited is density. It works for immiscible liquids or for a solid in suspension.
• Centrifugation: it uses centrifugal force: the denser components move outward, and the separation depends on the size, shape and density of the particles and on the viscosity of the medium.

A compound, by contrast, is broken down by a chemical reaction, as baking soda is when heated above 50 °C.