C8 : Chemistry Analysis

What does it mean if a substance is pure?

It is a single element or compound, not mixed with any other substance

What properties do pure substances have that can differentiate them from impure substances?

Pure substances have specific melting and boiling points whereas impure substances don’t.

What is a formulation?

A mixture that has been designed for a specific use

How are formulations made?

Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties

Give examples of formulations (7)

• Fuels

• Cleaning agents

• Paints

• Medicines

• Alloys

• Fertilisers

• Foods

What is paper chromatography used to separate + identify?

Used to separate mixtures and can give information to help identify substances

What are the 2 types of phases in chromatography?

• Stationary phase

• Mobile phase

What is the stationary phase + give an example? (in general terms)

Where the molecules can’t move → a solid or a really thick liquid

What is the mobile phase + give an example (in general terms)?

Where the molecules can move → a liquid or a gas

What is the stationary phase in paper chromatography?

The chromatography paper

What is the mobile phase in paper chromatography?

The solvent

List the equipment needed for paper chromatography (6)

What 2 things could be used as a solvent?

• Water

• Ethanol

What phase do components normally separate in?

Stationary phase

What 2 things determine the amount of time substances spend in each phase?

• How soluble the solvent is

• How attracted they are to the paper

Which phase does a more soluble substance spend more time in + what attraction will it have to the paper + how far will it be carried up the paper?

• Spends more time in the mobile phase

• Less attracted to the paper

• Will be carried up very high on the paper

Which phase does a less soluble substance spend more time in + what attraction will it have to the paper + how far will it be carried up the paper?

• Spends less time in the mobile phase

• More attracted to the paper

• Won’t be carried up very high on the paper

How would you differentiate a pure substance from a impure substance using chromatography?

A pure substance would only have one dot in any solvent whereas an impure substance would have more than one

What is the result of a chromatography analysis called?

A chromatogram

What is the equation for the R_f value?

If you change the solvent what would happed to the R_f value?

The R_f value would change

Explain how paper chromatography can be used to separate and tell the difference between coloured substances (7)

1. Draw a horizontal pencil line 2 cm from the short edge of the chromatography paper + Mark pencil spots at equal intervals across the line but not too close to the end of the paper

2. Use a glass capillary tube to put a small spot of each colouring on the pencil spots. A small spot ensures that the colouring separates clearly. Label each spot in pencil

3. Pour about 1 cm3 of water into the beaker

4. Suspend the paper in the beaker so that bottom edge of the paper dips into the water + Make sure the pencil line is above the water surface

5. Wait for the water solvent to travel at least three quarters of the way up the paper. Remove the paper and draw another pencil line at on the dry part of the paper right next to the wet edge. This is the solvent front, the distance travelled by the solvent

6. Hang the paper up to dry thoroughly

7. Calculate the Rf values for each spot. Rf value is calculated by distance travelled by substance/ solvent front.

What is the test for chlorine?

The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white

What is the test for oxygen?

The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen

What is limewater known as?

Calcium hydroxide

What is the test for carbon dioxide?

The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy)

What is the test for hydrogen?

The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound

What can flame test be used to identify?

Metal ions (cations)

Explain how to do a flame test (3)

• Dip a clean platinum wire loop into dilute HCL and then hold it in a blue flame (from a bunsen burner) until it burns without any colour

• Then dip the loop into the sample you want to test and put it back in the flame

• Record the colour produced

What colour flame does the compound lithium emit?

Crimson flame

What colour flame does the compound sodium emit?

Yellow flame

What colour flame does the compound potassium emit?

Lilac flame

What colour flame does the compound calcium emit?

Orange-red flame

What colour flame does the compound copper emit?

Green flame

What happens if a sample containing a mixture of ions is used for the flame test?

some flame colours can be masked

What is the test for carbonates?

Carbonates react with dilute acids to form carbon dioxide gas. Carbon dioxide can be identified with limewater

What is the test for sulfates?

Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid

What is the test for halides?

Halide ions in solution produce precipitates with silver nitrate solution in the presence of dilute nitric acid

What does chloride give in the test for halides?

A white precipitate of silver chloride

What does bromide give in the test for halides?

A cream precipitate of silver bromide

What does iodide give in the test for halides?

A yellow precipitate of silver iodide

What is the test for metal hydroxides?

Adding a few drops of sodium hydroxide solution can be used to identify some metal hydroxides

What colour is the precipitate for calcium?

White

What is the ionic equation for the precipitate formation of calcium?

What colour is the precipitate for copper (II)?

Blue

What ionic equation for the precipitate formation of copper (II)?

What colour is the precipitate for iron (II)?

Green

What ionic equation for the precipitate formation of iron (II)?

What colour is the precipitate for iron (III)?

Brown

What ionic equation for the precipitate formation of iron (III)?

What colour is the precipitate for magnesium?

White

What ionic equation for the precipitate formation of magnesium?

What colour is the precipitate for aluminium?

White at first, but then redissolves in excess NaOH to form a colourless solution

What ionic equation for the precipitate formation of aluminium?

What happens during a flame emission spectroscopy? (4)

1. The ion is heated up

2. When the electrons in the ions cool down they transfer energy as light

3. The light passes through a spectroscope which detects different wavelengths of light to produce a line spectrum

4. Each ion produces a different pattern of wavelengths and has a different line spectrum

What does the intensity of the spectrum indicate?

The concentration of that ion in solution

What is the advantages of using machines such as instrumental methods such as flame emission spectroscopy? (3)

• Very sensitive

• Very fast

• Very accurate