Chemistry: Topic 8: Chemical Analysis

Pure substance

A single element or compound, not mixed with any other substance

What are the melting and boiling points of pure elements and compounds like?

Melt and boil at specific temperatures

Formulation

A formulation is a mixture of compounds in measured quantities that has been designed as a useful product

measured quantities ensures the product has the required properties
many products are complex mixtures in which each chemical has a particular purpose

Examples of formulations

• Fuels

• Cleaning agents

• Paints, medicines

• Alloys

• Fertilisers

• Foods

Alloys

Alloys are mixtures of metals; they are formulations

They are harder than pure metals, so have a particular purpose

What is chromatography used for?

To separate mixtures and can give information to help identify substances

What does chromatography involve?

A stationary phase and a mobile phase
The separation of the substance depends on the distribution of substances between the phases

Rf value

The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent:

Rf = Distance moved by substance / distance moved by solvent

How can you identify a compound using chromatography?

Different compounds have different Rf values in different solvents
You can do tests for what you are looking for beside the unknown compound - if they end up in the same place they are probably the same

Mixtures vs pure compounds in chromatography

Compounds in a mixture may separate into different spots depending on the solvent
A pure compound will produce a single spot in all solvents

What does an Rf value of 0.85 suggest?

That the compound has a higher affinity for/ attraction to the solvent than for the paper

Hydrogen test

A burning splint is held at the open end of a test tube of the gas
Hydrogen burns rapidly with a pop sound

Oxygen test

A glowing splint is inserted into a test tube of the gas
The splint relights in oxygen

Carbon dioxide test

An aqueous solution of calcium hydroxide (lime water) is used
When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky/ cloudy

Chlorine test

When damp blue litmus paper is put into chlorine gas the litmus paper is bleached and turns white
it may turn red for a second as it is slightly acidic

Name the tests for ions

Cations:

• metal ions - flame tests

• metal hydroxides - sodium hydroxide solution

Anions:

• sulfates

• halides

Metal ions - Flame tests

Used to identify some metal ions (cations)

1. dip a platinum wire loop in HCL to clean it

2. hold it in a blue flame from a Bunsen burner until it burns without any colour

3. dip the loop into the same and put it into the flame

4. record the colour

works with the element of compound

If a sample containing a mixture of ions is used some flame colours can be masked/ hidden or mixed by the other ions

Flame test results for Lithium

Crimson flame

Flame test results for Sodium

Yellow flame

Flame test results for Potassium

Lilac flame

Flame test results for Calcium

Orange-red flame

Flame test results for Copper

Green flame

Metal hydroxides

Sodium hydroxide solution can be used to identify some metal ions (cations):

Many metal hydroxides are insoluble and precipitate out of solution when formed
Some of the hydroxides have a characteristic colour

1. Add a few drops of sodium hydroxide solution to a solution of your compound

Sodium hydroxide test for Copper(II)

Forms a blue precipitate

Sodium hydroxide test for Iron(II)

Forms a green precipitate

Sodium hydroxide test for Iron(III)

Forms a brown precipitate

Sodium hydroxide test for Aluminium ions (Al3+)

Forms white precipitates at first
The aluminium hydroxide precipitate dissolves in excess NaOH to form a colourless solution

Sodium hydroxide test for Ca2+

Forms white precipitate

Sodium hydroxide test for Mg2+

Forms white precipitate

Sodium hydroxide tests ionic and balanced equations

Ionic equations:
M?+(aq) + ?OH- (aq) > M(OH)? (s)

M = metal ? = charge
e.g. Cu2+ (aq) + 2OH- (aq) > Cu(OH)2 (s)

Balanced equations:
Na from NaOH and whatever the metal ion was bonded to will react to form a compound together

MX + ?NaOH > M(OH)? + ?NaCl

M = metal X = other metal ? = charge
e.g. CuCL2 + 2NaOH > M(OH)2 + 2NaCl

Test for carbonate anions

React with dilute acids e.g. HCl
Form carbon dioxide gas - fizzing observed - which can be identified with limewater

Test for halide anions

Add a solution of silver nitrate in the presence of dilute nitric acid
; Chlorides form a white precipitate, Bromides form a cream precipitate, Iodides form a yellow precipitate.

Silver chloride halide test

Forms white precipitate

Silver bromide halide test

Forms cream precipitate

Silver iodide halide test

Forms yellow precipitate

Test for sulfate anions

Add a solution containing (barium) Ba2+ cations
e.g. a solution of BaCl2 in the presence of dilute hydrochloric acid
Forms white precipitate

Instrumental methods

Used to detect and identify elements and compounds
They are accurate, sensitive and rapid methods which are useful when the amount of sample is very small.

Flame emission spectroscopy

An instrumental method used for identifying metal ions in solution or measuring their concentration
The sample is put into a flame and the light given out is passed through a spectroscope
The output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations

Cations and anions

Cations are the positive ions; anions are the negative ions.