Chemistry: Structured Paper C3 - Topic 3

Chemical Detection. C3 Topic 3 In the Book.

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Analysing Substances

Chemical Analysis is very useful.

- Used for crime investigations and how much alcohol someone carries.

- What substances are present in foods?

- Purity of water.

- Quality Control.

QUALITATIVE - what is present

QUANTATIVE - how much is present

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Tests for Positive Ions


SODIUM (Na) gives orange/yellow flame.

POTASSIUM (K) gives lilac flame.

CALCIUM (Ca) gives brick red flame.

COPPER (Cu) gives blue/green flame.

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Add Sodium Hydroxide

Add Sodium Hydroxide and Look for coloured precipitates.

Calcium (Ca) > White

Copper (Cu) > Blue

Iron (Fe) (II) > Sludgy Green

Iron (Fe) (III) > Reddish Brown

Aluminium (Al) > White > Clear

Ammonium (NH4) > Smell

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Tests for Negative Ions

Hyrdrochloric acid - can help detect Carbonates and Sulphites


Carbonates give off CO2 with HCl !!

You can test for carbon dioxide using lime water.


Sulphites give off SO2 with HCl

You can test for sulphur dioxide using damp potassium dichromate paper. The paper turns from orange to green


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Tests for Negative Ions 2

Test for Sulphates with HCl and Barium Chloride

Sulphate Ions produce a white precipitate.

> The origional compound was a sulphate.


Test for Halides with Nitric Acid and Silver Nitrate
(Chloride, Bromide, Iodide ions)

A chloride gives a white precipitate of silver chloride. 

A bromide gives a cream precipitate of silver bromide.

An iodide gives a yellow precipitate of silver iodide.


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Tests for acids and alkalis

An Indicator - a dye that changes colour.

LITMUS: Blue litmus paper turns red if the solution is an acid.
*lots of H+ ions are present.

Red litmus paper turns blue if the solution is an alkali.
*lots of OH- ions are present. 


To Test for alkalis: - Heat the substance with an Ammonium Salt.

The smell is quite distinctive.

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Measuring Amounts - Moles

A MOLE = 6.023 x 10(23)

One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass of that substance.

Divided by 
Relative atomic mass (OF ELEMENT OR COMPOUND)

Eg. How many moles are there in 66g of carbon dioxide?

Mr of CO2 = 12 + (16 x 2) = 44
No of moles = Mass (g) / Mr = 66/44 = 1.5 moles

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Calculating Volumes

Avogadros Law - "One mole of any gas occupies 24dm(cubed)"

VOLUME OF GAS = MASS OF GAS divided by Mr of Gas

                                 all times by 24.

Example: whats the volume of 4.5 moles of chlorine at RTP?

1 mole = 24dm(cubed), so 4.5 moles = 4.5 x 24 = 108. 


You can calculate volumes in reactions if you know the masses.
2.7g of C gives 9.9g of CO2.
Volume = Mass divided by Mr (all times by 24)

so, (9.9/44) x 24 = 5.40dm(cubed)

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  • Titrations allow you to find out exactly how much acid is needed to neutralise a quantity of alkali.
  • Using a pipette, add some alkali to a conical flask, along with two or three drops of indicator.
  • Fill a burette with acid (DO THIS AT EYE LEVEL)
  • Add the acid to the alkali a bit at a time. - giving the conical flask a regular swirl - this will help the indicator change colour.
  • Record the volume of acid used to neutralise the alkali.
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The Calculation of a Titration - Moles

Step 1; work out how many moles of the known substance you have.
No. of moles = concentration x volume = 0.1 x (25/1000) = 0.0025 moles.

Step 2; write down the equation for the reaction
2NaOH + H2SO4 > Na2SO4 + 2H2O .. and work out how many moles of the unknown stuff you must have had...

Every 2 moles of sodium hydroxide > there was 1 mole of sulphuric acid.

So, if you had 0.0025 moles of sodium hydroxide, you must have had half that amount of sulphuric acid: 0.0025 ÷ 2.

Step 3; Finally, work out the concentration of the 'unknown' stuff:
(Concentration = number of moles  ÷ volume)

= 0.00125 ÷ (30/1000) = 0.0417 moles per dm(cubed)

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Drinking water = needs to be good quality!

  • microorganisms in drinking water can cause cholera and dysentery.
  • filtered to take these out.
  • good water = good life.

Water is a really useful Solvent.

  • water dissolves most ionic compounds.


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