Elements, Mixtures and Compounds
Chemistry is the study of physical matter, which is classified in many different ways, such as state of matter (gas, liquid or solid), chemical form (element, mixture or compound), chemical structure (atoms or molecules, etc.) and so on.
It is important to understand the words element, mixture and compound, because these convey useful information.
substance, which can be used to refer to either an element or a compound - but not to a mixture because a 'substance' always has a definite composition. The most general word is matter which can be used to refer to any of 'substances', 'elements', 'mixtures' or 'compounds'.
The categories of elements, mixtures and compounds are described in words below:
- consist of only one type of atom - which may, or may not join together to form molecules or large structures, therefore:
- can exist as either atoms (e.g. argon) or molecules (e.g., nitrogen)
- cannot be broken down into a simpler type of matter by either physical or chemical techniques - though some larger elements break-down spontaneously due to being radioactive.
Elements are listed in the periodic table.
Many elements are found in nature and so may be called 'naturally occurring elements'. Other elements have not been found in nature but can be produced in the laboratory. A few more elements are thought to exist but are very rare and even if produced would only exist for a very short time because they are radioactive and would quickly decompose into other elements whose atoms are smaller.
Read more about elements.
- consist of two or more different elements and/or compounds - physically intermingled,
- can be separated into their constituent parts by physical means (e.g. distilation of liquids or seperating magnetic and non-magnetic solids using a magnet), and
- have many of the properties of their constituent parts (e.g. the element 'oxygen' is part of the mixture 'air' and some of the properties of air are due to the oxygen, albeit somewhat reduced compared with pure oxygen due to the presence of the other constituents of the mixture called 'air').
There are many different types of mixtures, some of which have special names. These include:
- Homogeneous Mixtures - in which the two or more substances that form the mixture are evenly distributed throughout the mixture, e.g. vinegar is a homogeneous mixture of ethanoic acid and water.
- Heterogeneous Mixtures - in which the two or more substances that form the mixture are not evenly distributed throughout the mixture, e.g. oil and water.
- Solutions - a special type of homogeneous mixtures in which one substance (called the 'solute') is dissolved in another substance (called the 'solvent'), e.g. salt water is salt dissolved in water - in such a way that the salt no-longer exists as solid particles within the water.
- Suspensions - heterogeneous fluid mixtures containing solid particles large enough for sedimentation, which means that the particles (compare with the 'solute' part of a solution) will eventually settle to the bottom of the container (unlike in the case of colloids, below), e.g. particles of sand in water.
- Colloids - heterogeneous mixtures in which one is substance microscopically dispersed evenly throughout another substance (for comparison, the size of the particles of 'solute' are greater than in the case of a solution, but much smaller than in the case of a suspension). There are many naturally occuring colloids, e.g. milk. Colloids are very important in biology and medicine.
- Alloys - mixtures in which the main elememt (or elements) are metal(s). A more technical definition of an alloy is 'a partial or complete solid solution of one or more elements in a metallic matrix'. Common examples of alloys include bronze, brass and steels.
These different types of mixtures are interesting but may not be essential knowledge for GCSE Chemistry. The descriptions are non-technical (omitting details, e.g. particle sizes), but sufficient for comparison of these different types of mixtures.
Read more about mixtures.
- consist of atoms of two or more different elements bound together chemically,
- can be broken down into a simpler type of matter (elements)
by chemical means; but not by physical means
- always contains the same ratio of component atoms.
- have properties different from their component elements (e.g. the compound water (H2O) is a liquid at room temperature and pressure and has different chemical properties from those of the two elements, hydrogen (H2) and oxygen (O2), from which it is formed).
A compound is a pure substance that consists of two or more elements chemically combined in a fixed proportion, that can be further subdivided into simpler substances by chemical (not physical) means only.
So, if a quantity of a material consists of atoms of two or more elements joined together, always in the same ratio, then the matter forming that material is a compound.
A molecule is the smallest part of a compound whose properties are those of the compound.
A compound can be represented by using a chemical formula.
Read more about compounds.
A common question concerns the smallest identifiable units within elements, mixtures and compounds, i.e. the smallest units whose properties are those of the bulk material. Are they atoms or molecules, or something else ?
See the page about 'Elements Mixtures and Compounds' and 'Atoms and Molecules'.