[C8] Chemical analysis

What is a pure substance?

A single element or compound that is not mixed with any other substance

What type of substances melt and boil at specific temperatures?

Pure substances

What type of substances melt and boil over a range of temperatures?

Impure substances

If a substance boils between 55 and 58 ᵒC, will it be pure or impure?

Impure

Is carbon dioxide pure or impure?

Pure

Is oxygen pure or impure?

Pure

Is water from a tap pure or impure?

Impure as contains dissolved salts

What is a formulation?

A mixture that has been designed as a useful product

Are formulations pure or impure?

Impure

What is chromatography used to separate?

Mixtures of soluble susbtances

Name the two phases in chromatography

Stationary phase - where molecules can't move Mobile phase - where molecuels can move

What phase is the paper in chromatography?

Stationary phase

What phase is the water in chromatography?

Mobile phase

Why do substances separate in paper chromatography?

Substances move up the paper when they are dissolved in the solvent. The different substances have different solubilities in the solvent. Therefore they travel different distances up the paper

How could you tell if a substance was impure by looking at a chromatogram?

There would be more than one spot in a vertical column above the original spot

How could you tell if a substance was pure by looking at a chromatogram?

There would be only one spot in a vertical column above the original spot

Why should the baseline be drawn in pencil?

Pencil is insolube. If it was drawn in pen the ink would dissolve and so the line would run

Why should the level of water be below the baseline?

If it was above the baseline the spots would wash off the paper

What is the solvent front?

The distance travelled by the solvent

When should you remove the chromatography paper?

When the solvent has nearly reached the top

Why is the chromatography paper often attached to a glass rod when carrying out chromatography?

So that it stands up in the water

Why should a lid be placed on the beaker during chromatography?

To saturated the atmosphere in the beaker with solvent vapour

The higher the Rf value the .......... the distance tavelled by the substance

Greater

What equation is used to calculate Rf values?

Rf value = distance travelled by spot ÷ distance travelled by solvent

How many spots will a pure susbtances produce on a chromatogram?

One

Describe how you would set up a chromatogram

- Draw a baseline in pencil near the bottom of a piece of filter paper - Add a spot of ink to this line and allow it to dry - Place the paper in a beaker of solvent (usually water), make sure the level of solvent is below the baseline - Add a lid to the beaker to stop the solvent evaporating - Allow the solvent to run up the paper - Remove the paper from the beaker just before the solvent reaches the top of the paper

What is the test for hydrogen?

- Use a burning splint - If hydrogen is present it will burn with a squeaky pop

What is the test for oxygen?

- Use a glowing splint - If oxygen gas is present the glowing splint will relight

What is the test for carbon dioxide?

- Bubble gas through limewater (calcium hydroxide solution) - If the gas is carbon dioxide the limewater will turn from colourless to milky

What is the test for chlorine?

- Add damp litmus paper - If chlorine gas is present the litmus paper is bleached and turns white

What colour of flame do lithium ions produce?

Crimson

What colour of flame do sodium ions produce?

Yellow

What colour of flame do potassium ions produce?

Lilac

What colour of flame do calcium ions produce?

Orange-red

What colour of flame do copper compounds produce?

Green

How do you clean the wire before a flame test?

Dip in concentrated hydrochloric acid, heat in bunsen burner, dip in concentrated hydrochloric acid again

What colour of bunsen burner flame do you use for flame tests?

Blue

What colour of precipitate do Al³⁺ ions produce with sodium hydroxide solution?

White precpitate (dissolves in excess sodium hydroxide solution)

What colour of precipitate do Mg²⁺ ions produce with sodium hydroxide solution?

White precipitate (insoluble in excess sodium hydroxide solution)

What colour of precipitate do Ca²⁺ ions produce with sodium hydroxide solution?

White precipitate (insoluble in excess sodium hydroxide solution)

How can you identify the dfference between Mg²⁺ and Ca²⁺ ions?

Carry out a flame test, Ca²⁺ ions produce a orange-red flame, Mg²⁺ ions do not

What colour of precipitate do Cu²⁺ ions produce with sodium hydroxide solution?

Light blue precipitate

What colour of precipitate do Fe²⁺ ions produce with sodium hydroxide solution?

Green precipitate

What colour of precipitate do Fe³⁺ ions produce with sodium hydroxide solution?

Brown precipitate

What is the test for carbonate ions? (CO₃ ²⁻)

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

What is the test for sulfate ions? (SO₄²⁻)

1. Add dilute hydrochloric acid, followed by barium chloride solution 2. A white precipitate will form

Name the white precipitate formed when sulfate ions barium chloride.

Barium sulfate (BaSO₄)

What is the test for halide ions?

"Add dilute nitric acid and then silver nitrate solution. If a precipitate forms, there are halide ions present

"

What colour of precipitate do iodide ions form with dilute nitric acid and silver nitrate solution?

Yellow

What colour of precipitate do chloride ions form with dilute nitric acid and silver nitrate solution?

White

What colour of precipitate do bromide ions form with dilute nitric acid and silver nitrate solution?

Cream

Why do metal ions release light when heated?

The electrons gain energy and are excited to higher energy levels. Electrons then emit radiation when they drop back down

How is the light released by metal ions analysed in flame emission spectroscopy?

It passes through a spectroscope and a line spectrum is produced

How can a metal be identified from a line spectrum?

Each metal has a unique line spectrum. The line spectrum can be compared to a database

What are advantages of instrumental techniques compared to traditional methods of analysis?

Faster, more accurate, more sensitive

What are disadvantages of instrumental techniques compared to traditional methods of analysis?

More expensive, require specialist training, results have to be compared with other results

Name the intrsumental technique used to identify metal ions in samples?

Flame emission spectroscopy