Chemistry C3 Edexcel topic 3

Covers topic 3 - Chemical Detection

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  • Created on: 20-05-12 18:16

Analysing Substances

Qualitative Analysis - A test to find out the ions present in an unknown substance/solution 

*By using the word IONS you are sure to get the maximum marks 

Quantitative Analysis - A test to find out how much of something is present in a substance/solution

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Tests positive Ions - Flame tests

Carryin out a flame test -

1. clean a metal wire loop with hydrochloric acid/distilled water

2. Dip wire loop in a sample of substance/solution

3. Hold wire loop in blue bunsen flame

4. Record colour change

Sodium (Na+) - Gives Orange/yellow flame

Potassium (K+) - Gives lilac flame

Calcium (Ca 2+) - Gives brick red

Copper (Cu-) - Gives blue - green flame


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Tests for positive ions - Add sodium hydroxide

Add a few drops of sodium hydroxide to an unknown solution and look for a coloured precipitate

Precipitate - An insoluable salt formed from a reaction between two solutions 

Ca - White precipitate

Cu - blue precipitate

Fe(II) - Sludgy green precipitate

Fe(III) - Reddish brown precipitate

Al - white ate first then redissolves when excess NaOH is added to form a colourless solution

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Tests for Negative ions - HCL

Hydrochloric acid can be added to detect carbonates and sulphites

With dilute Hydrochloric acid, carbonates give off carbon dioxide - you can test the presences of carbon dioxide by using lime water = It turns cloudy when CO2 is present

With dilute hydrochloric acid, sulphites give off sulphur dioxide - You can test suphur dioxide with damp potassium dichromate(IV) paper = It turns from orange to green

Test for sulphates with HCL and Barium Chloride - Sulphate ions produce a white precipitate of barium sulphate means original compound was sulphate

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Tests for Halides

To test for chlorine, bromine iodine, you add dilute nitric acid followed by silver nitrate

A chloride give a white precipitate of silver chloride

A bromide gives a cream precipitate of silver bromide

An iodine gives a yellow precipitate of silver iodide

*the acid is added to get rid of any carbonate or sulphite ions

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Tests for acids & alkalis

Litmus Paper-     BLUE litmus turns red if acid (H+) are present     RED litmus turns blue if alkali (OH-) are present
 

Universal Indicator - to estimate pH (http://richardanderson.me.uk/keystage3/year7/module5/images/phscale.jpg)

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Moles

One mole of atoms or molecules of any usbstance will have a mass in grams equal to relative formula/atomic mass for that substance

eg. one mole of carbon = 12g

To find the number of moles in a given mass:

Number of moles = Mass in g / Relative atomic mass (R.A.M)

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

RAM of CO2 = 12 + (16 x 2) = 44

No. of moles = Mass (g) / R.A.M = 66/44 = 1.5moles

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Calculating moles

Avagandro's law - one mole of any gas occupies 24dm3 (= 24 000 cm3) at room temperature and pressure (RTP - 25'C and 1 atmosphere)

using this formula you can convert a mass of any gas into volume

Volume of gas (in dm3) = mass of gas / R.A.M of gas x 24


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