Pure Substance and Mixtures

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

Something can be said to be pure if it has had nothing added to it

If a substance has impurities it will melt/boil over a temperature range - The boiling point will be higher and the melting point will be lower

Formulations are useful mixtures with a precise purpose. Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties Formulations can include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods

Chromatography

Chromatography can be used to separate mixtures and can give information to help identify substances. Chromatography involves a stationary phase and a mobile phase. Separation depends on the distribution of substances between the phases.

Rf Value = distance moved by substance/ distance moved by solvent

Different compounds may have different Rf values in different solvents which can be used to help identify the compounds he compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

REQUIRED PRACTICAL:

Method:

1. Use a ruler to draw a horizontal pencil line 2 cm from a short edge of the chromatography paper. Mark five pencil spots at equal intervals across the line. Keep at least 1 cm away from each end.

 2. Use a glass capillary tube to put a small spot of each of the known colourings on four of the pencil spots. Then use the glass capillary tube to put a small spot of the unknown mixture on the 5th pencil spot. 

Try to make sure each spot is no more than 5 mm in diameter. 

Label each spot in pencil.

 3. Pour water into the beaker to a depth of no more than 1 cm.

 4. Tape the edge of the chromatography paper to the glass rod.  The paper needs to be taped at the end furthest from the spots. 

Rest the rod on the top edge of the beaker.  The bottom edge of the paper should dip into the water.

Ensure that the:

·       pencil line is above the water surface

·       sides of the paper do not touch the beaker wall.

5. Wait for the water solvent to travel at least three quarters of the way up the paper.  Do not disturb the beaker during this time.

Carefully remove the paper.  Draw another pencil line on the dry part of the paper as close to the wet edge as possible.

 6. Hang the paper up to dry thoroughly.

7. Measure the distance in mm between the two pencil lines.  This is the distance travelled by the water solvent. 

Measure and record the same distance for each food colouring in the table below.

8. For each of the four known colours, measure the distance in mm from the bottom line to the centre of each spot.  Write each measurement in the table.

9. Use the following equation to calculate the R_f value for each of the known colours.

Testing for gases

Hydrogen - Hold a lit splint at the open end of a test tube containing gas, if hydrogen is present a squeaky pop noise will be heard

Oxygen - Insert a glowing splint into a test tube of the gas, if oxygen is present the splint will relight

Carbon Dioxide - Bubble gas released through limewater, if carbon dioxide is present then the limewater will turn cloudy

Chlorine - Hold damp litmus paper above the gas released. if chlorine is present then it will be bleached white