Topic 8 - Chemical Tests

What is purity in chemistry?

A substance which only contains one compound or element throughout.

How to test for purity?

• Melting Point

• Boiling Point

• Paper Chromatography

The MP and BP of a substance can be compared with the MP and BP of a pure substance, the closer it is, the more pure it is.

A pure substance only creates one spot in paper chromatography whereas a impure substance can produce many.

What is a formulation?

A formulation is a useful mixture with a specific purpose that are made by following a formula. Each component has a exact quantity to contribute to the properties of the formulation so it meets its function.

Examples of formulations?

• Paint

• Pills

• Cleaning Products

• Fertilisers

• Fuels

What is Paper Chromatography?

An analytical method used to separate substances in a mixture and identify them.

What are the two phases in Paper Chromatography?

Mobile Phase - Where the molecules move as a liquid or a gas.

Stationary Phase - Where the molecules can’t move as a solid or thick liquid.

What is an R_f Value?

The ratio between the distance travelled by the dissolved substance and the distance travelled by the solvent.

R_f = Distance travelled by substance / Distance travelled by solvent

What are the 4 Tests for Common Gases?

Chlorine - It bleaches damp litmus paper turning it white (It may start by turning red as a solution of chlorine is acidic).

Oxygen - If you put a glowing splint in a test tube with oxygen, it relights.

Carbon Dioxide - Bubbling carbon dioxide through limewater (Calcium Hydroxide) turns it cloudy.

Hydrogen - If you hold a lit splint at the end of a test tube, there will be a squeaky pop from the hydrogen burning.

What are the 3 tests for Anions?

Carbonates - Adding a few drops of dilute acid (eg HCl) and the using the Carbon Dioxide test to see if the bubbles turn limewater cloudy.

Sulfates - Adding a few drops of Barium Chloride solution to the solution will create a white precipitate.

Halides - Adding a few drops of silver nitrate creates a white (Chloride), cream (Bromide) or yellow (Iodide) precipitate.

What are the flame tests?

Dip a platinum wire loop in HCl and then hold it in a blue flame to clean it. Then dip it in the HCl again and then then the sample you want to test. Clean with HCl and then the Bunsen between samples.

Lithium - Crimson

Sodium - Yellow

Potassium - Lilac

Calcium - Orange-Red

Copper - Green

What are the precipitate tests?

Many metal hydroxides are insoluble and form a precipitate in a solution. Add a few drops of sodium hydroxide to your solution and it could form a precipitate.

Calcium - White

Copper(II) - Blue

Iron(II) - Green

Iron(III) - Brown

Aluminium - White and then redissolves to colourless

Magnesium - White

What is Flame Emission Spectroscopy?

A sample is heated and the ions produce colours as light. Each ion produces a unique spectrum. The intensity indicates the concentration. It can be used to test form multiple ions in a mixture.

What are the advantages of instrumental analysis?

• Very Sensitive

• Very Fast

• Very Accurate