Chemical Detection C3 Topic 3 – Chemistry Extension GCSE

This article will go into detail Topic 3, Chemical Detection which includes analysing substances, test for positive an negative ions, acids and alkalis, calculating moles, Avogadro’s law and titrationHowever, you are more than welcome to skip to the parts most relevant to you.

Analysing Substances
There are two ways you can analyse a substance: qualitative which looks for quality or different colours such as the test for ions and quantitative which is a numerical answer.
This topic involves a lot of identifying ions both positive and negative. The tests for each ion must be unique otherwise you can’t interpret the data. 

Test for Positive and Negative Ions
As you will now see, there is a lot of test you have to know and understand about ions. I’ll start off by going through the positive ion tests and then the negative ion tests. 
But first, what is an ion? An ion is atom of molecular with a net electric charge due to a loss or gain of one or more molecules. All the ions tests have precipitation reactions which is a reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. That insoluble compound is called a precipitate.

Positive Ion Tests

  • H+  use acid/base indicator and look for colour change or react it with carbonate and if there is fizzing, positive hydrogen ions are present.
  • Na+ (yellow/orange), K+ (lilac), Cu2+ (green), Li+ (bright red) and Ca2+ (brick red) all use the flame test. Soak a splint in water, stick the ‘stuff’ (being ions) on the splint then place in flame and look for different colours for different ions.

We can also test for positive metal ions through adding the ions to sodium hydroxide solution:

  • If there is a white precipitate, add more NaOH (sodium hydroxide) solution in. If the precipitate dissolves, the ion is Al3+. If the precipitate doesn’t dissolve and is insoluble, the ion is either Ca2+ or Mg2+.
  • If there is no precipitate, warm the solution and if there is a smell of ammonia coming off or the solutions turns litmus paper blue, the ion is NH4+. If there is no present ammonia, there are no ions detected.
  • If there is a coloured precipitate forming a reddish/brown colour, the ion is Fe3+.
  • If there is a coloured precipitate forming a green (turning slowly brown) colour, the ion is Fe2+.
  • If there is a coloured precipitate forming a light blue colour, the ion is Cu2+.

Negative Ions
Negative ion test doesn’t use the flame test or sodium hydroxide test but uses dilute hydrochloric acid, nitric acid and acid/base indicator.

  • OH- is present using acid/base indicators and reaction when heated up with an ammonium salt.

By adding dilute hydrochloric acid…

  • If the gas turns a bichromate paper green, the ion is SO3-2.
  • If there is no fizzing, add BaCl2 (barium chloride) and if a white precipitate forms, the ion is SO4-2.
  • If there is fizzing, conduct the lime water text and if the lime water goes cloudy/milky, the ion is CO3-2.

By adding dilute AgNO3 (silver nitrate) and HNO3 (nitric acid)…

  • If there is a white precipitate, the ion is Cl-.
  • If there is a cream precipitate, the ion is Br-.
  • If there is a yellow precipitate, the ion is I-.

Calculating Moles
Moles are a type of measurement in chemistry for the number of particles. One mole =6.023x10 to power of 23
The number of atoms in one mole is also known as Avogadro’s constant. Now to calculate the moles of a solid mass you need to know this:

Moles = mass / rmm (relative molecular mass)

To work out the moles in a solution it is:

Moles = concentration x volume

To work out the moles in any gas it is:

Moles = volume / 24 

As for each one of the above they can obviously be rearranged and always remember, if you know the moles to anything, you will be able to work out how many atoms there are by multiplying the moles to 6.023×10 to power of 23.

One Response

  1. Anonymous May 19, 2012

Leave a Reply