Elements, Compounds and Mixtures

– Elements are the basic ‘building blocks” of all matter, which cannot be broken down into simpler substances by any physical and chemical methods.

– Atoms are the smallest possible unit of an element.

– Compounds are substances which consist of at least two or more elements chemically combined together.

– Molecules are the smallest possible unit of a compound and consist of at least two or more atoms chemically combined. Molecules may be comprised of atoms from the same element (eg. $Cl_{2}$) or atoms of different elements (ie. compound eg. $H_{2}O$).

– Mixtures are a combination of at least two or more atoms or molecules not chemically combined together.

Elements, Compounds and Mixtures– Some differences between compounds and mixtures are shown in the table below:


 Pure substances, with two or more elements chemically joined together (a chemical reaction takes place when a compound is formed) Not pure substances; where two or more elements and/or compounds are mixed together (not chemically joined together. A chemical reaction does not take place)
 Have a fixed composition eg. water is always made up of 2 hydrogen and 1 oxygen atoms Do not have a fixed composition
 Cannot be separated into their components using physical methods Can be separated into their components with physical methods like filtration, distillation
 Have fixed melting and boiling points Do not have fixed melting and boiling points
 Have different chemical properties from their components Have the properties of their components
 Energy (may be in the form of heat or light) may be given out during their formation Energy is usually not given out or taken in during mixing
 Some examples include: water, carbon dioxide Some examples include: air, salt water, soil

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