Elements Compounds and Mixtures

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  • An element is a substance that consists of only one type of atom and cannot be broken down into simpler substances by chemical means

We know if something is an element by heating it. For instance, we know that copper is an element because upon heating copper, it will not be broken down into any other substances. However when we heat limestone, it is broken down into calcium, carbon and oxygen (in the form of carbon dioxide).

Elements can be arranged in the Periodic Table, according to their properties. The easiest way to do that is by saying elements are either metal, or non-metal. Their properties are listed below:

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Compounds and Mixtures

  • A compound is a substance that consists of two or more elements that are chemically joined together

When atoms join together, they make molecules. Elements combine to make compounds. These happen in chemical reactions. A compound is a fixed composition. Each compound has its own formula.  Water is a compound of Hydrogen and Oxygen. Water's formula is H2O.

  • A mixture consists of two or more substances mixed together that are usually easy to separate

A mixture is not a fixed composition, the parts that make up the mixture are not combined. Air is a mixture of nirtogen, oxygen, carbon dioxide, and other substances. Mixtures are usually easy to separate, we can use these methods to separate mixures:


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Differences between Elements and Compounds

There are some differences between elements and compounds


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Differences between Compounds and Mixtures


  • See book for example with iron sulphide.
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There are two ways of evaporation.

  • Crystallisation: can be used to obtain a soluble solid from a solution which contains a number of solutes. Solution heated to evaporation point, then left to cool slowly. The crystals can be removed by filtration
  • Evaporating to Dryness: if a solution is evaporated to dryness, the residue may not be pure


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Filtration and Centrifugation

  • Filtration is used to separate an insoluble solid from a liquid. 
  • The solid is called a residue; the liquid, filtrate
  • You could use filtration to separate salt from salt water.


  • Centrifugation is used to remove very small insoluble solid particles dispersed in a suspension (eg blood cells from blood plasma)
  • Centrifugation is better than filtration is small volumes are involved and if the solid is very fine
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Simple Distillation

  • Simple Distillation is normally used when a solid is dissolved in a solvent and the solvent is required in a pure state (eg. pure water from sea water)
  • The solution is heated and the solvent evaporates.
  • The vapour is cooled , condensed and collected in a separate container from the original solution. This is now called the distillate


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Separating Funnel

  • The Separating funnel is used to separate two liquids which are immiscible and have different densities.
  • The denser liquid is removed first by opening the tap and running it off into a beaker
  • The tap is closed at the junction of the liquids, a new beaker is put in place and the second liquid is collected in that.

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Fractional Distillation

  • Fractional Distillation is used to separate miscible liquids but have different boiling points.
  • All of the liquids will evaporate but those with a lower boiling point will rise up the column first.
  • A fraction is a part collected over a definite temperature range. 
  • This mehtod is used comercially to refine crude oil

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  • Chromatography is used to separate solutes in a sloution.
  • A spot of mixture is placed at he bottom of a ***** of paper, on the base line.
  • This is stood in a small volume of solvent.
  • As the solvent rises up the paper, spots are produced.
  • The base line must be drawn in pencil.

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Mass Spectrometry

Atoms are much to small to be weighed, but by using a machine called a mass spectrometer, chemists can accurately measure the masses of atoms and molcules. 

The masses of all atoms are relative to one twelth the mass of a carbon-12-atom..


This is the mass spectrum of chlorine.

The heights of the two peaks show that for every four chlorine atoms, three are chlorine-35 isotopes and one is a chlorine-37 isotope, ie in the ratio of 3:1. These are isotopes of chlorine

  • Isotopes are atoms of the same element with the same atomic number but different mass numbers because they have different numbers of neutrons.


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Calculating the RAM

As seen in the mass spectrometry card, chlorine has two isotopes: for every four chlorine atoms, three are chlorine-35 isotopes and one is a chlorine-37.

  • Isotopes are atoms of the same element with the same atomic number but different mass numbers because they have different numbers of neutrons.

This then means that chlorine does not have a whole mass number. To calculate the mass number:

(mass of isotope x abundance) + (mass of isotope x abundance) + (so on)

total abundance

So for chlorine, this would be:

(35 x 3) + (37 x 1)


= 35.5

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