GCSE Chemistry: Chemical Analysis (T8)

What is a pure substance?

A single element or compound, not mixed with any other substance and no impurities

What does a pure substance have?

A specific melting and boiling point

How Can We Use Melting and Boiling Point Data to Tell if a Substance is Pure?

1. Pure substances melt at a specific fixed temperature→have a specific, sharp melting and boiling point.
   

   • Example: Pure water melts at exactly 0°C and boils at 100°C.

   • If sharp and correct = pure

2. Impure substances:
   

   • Melt/boil over a range of temperatures (not sharply).

   • May have higher or lower melting/boiling points than the pure substance.

   • if off or spread out = impure

What do impure substances have?

A range of melting and boiling points.

What is a formulation?

A complex mixture designed as a useful product with specific amounts of each components

How do formulations have the properties we need?

The quantity of each component is carefully measured.

What are examples of formulations?

Fuels, cleaning products, paints, medicines, alloys, fertilisers and foods.

What does paper chromatography do?

Allows us to separate substances based on their different solubilities

Can give information to help identify substances

What a solvent

A substance that dissolves a solute resulting in a solution

What does solubility mean?

ability to dissolve

What is paper described as in paper chromatography?

The stationary phase- as it doesn't move

What is the solvent described as in paper chromatography?

The mobile phase- as it moves

Describe paper chromatography

Method:

• 1. First draw a pencil baseline across a piece of chromatography paper.

• 2. Then place a dot of the first colour on the pencil line and next to that the dot of the other colours (making sure there’s an appropriate amount of space between the colours)

• 3. Place the chromatography paper is put in a jar with the solvent. Make sure that the solvent level is below the level of the spots otherwise the dye will wash off into the solvent. As the solvent moves up the paper, the dyes in the mixture separate from each other.

• Single spot = pure substance

• Mixture of 2 = impure

How can you tell if the compounds pure during chromatography?

It will produce a single spot in all solvents whereas the compounds in a mixture may separate into different spots depending on the solvent.

What happens in paper chromatography to the substance if it's more soluble?

It travels further up the paper the more soluble it is.

Why do we draw our starting line in pencil?

Because if we drew the line in pen, the pen ink would move up the paper, with the solvent.

How can you identify unknown substances using paper chromatography?

• Carry out chromatography.

• Use formula (distance travelled by substance/distance travelled by solvent) to calculate Rf value -Look this value up on a database in order to identify the substance

What is the Rf value formula?

Distance travelled by substance / distance travelled by solvent

What’s an issue regarding the identification of an unknown substance using paper chromotography?

Several different substances could have the same Rf value so you should repeat the experiment using different solvents in order to narrow it down further.

What's another issue regarding the identification of unknown substances experiment?

If the substance has never been analysed before then there will not be an Rf value on the database so you need to carry out further analysis.

Using paper chromatography how can you tell if certain chemicals are in the mixture.

They line up.

Required practical: chromatography

Describe how paper chromatography can be used to identify substances

• EXACMPLE:

• have a sample of food colouring which is a mixture of chemicals we are going to call this → U = unknown

1. Four known food colouring → labels A-d

2. Method :

3. Use a ruler to draw a horizontal pencil(doesn’t dissolve) line on the chronographs paper → around 2cm from the bottom of paper

4. Mark 5 pencil spots at equal spaces across the line at least 1cm clear at each side

5. Using capillary tube to add a spot of of each of known food colours & unknown colour onto pencil spots → keeping small to prevent spreading into eachoehrr later

6. Pur water in beaker to depth of 1cm→ solvent

7. Attach paper to glass rod using tape and lower into beaker , the bottom dipping into water the pencil line above the water and must touch side of walls add lid on beaker to avoid evaporation

8. Water will move up the paper and the colours will be carried up

9. Remove paper when water has reached around 3 quarters up and use pencil to mark the point where water reached and hang paper to dry

10. Unknown colours ahve separated → mixture of colours

11. To identify the chemicals in the colours use the Rf value

What is the test for hydrogen?

Put a lit splint where the gas is. If hydrogen is present then the gas will burn rapidly with a pop sound.

What is the test for oxygen?

Putting a glowing splint into a test tube containing a gas. If the splint relights (bursts into flames) then oxygen is present.

What is limewater?

An aqueous solution of calcium hydroxide

What is the test for carbon dioxide?

Shake the gas with limewater or bubble it through limewater and if the solution turns cloudy, carbon dioxide is present.

What is the test for chlorine?

Place damp litmus paper in gas and if it bleaches the litmus paper white, chlorine is present.