Required Practicals Paper 2

Question 1

Method for disappearing cross experiment

Answer 1

• Use a measuring cylinder to add 10cm3 of sodium thiosulfatein to a conical flask
• Place flask onto a printed black cross
• Add 10cm3 of HCl
• Swirl the solution and immdiately start the timer and look down through the top of the flask
• Stop the timer once it has gone cloudy (due to sulfur precipitate) and you can no longer see the cross
• repeat with different concentrations of sodium thiosulfate
• repeat to find a mean

Question 2

Problem with disappearing cross experiment

Answer 2

• People have different qualities of eyesight
• This may lead to people getting different results so the practical is not reproducible

Question 3

Method for volume of gas produced by a reaction experiment

Answer 3

• Use a measuring cylinder to add 50cm3 HCl to a conical flask
• Attach a bung and delivery tube into a trough of water and and upturned measuring cylinder filled with water
• Add 3cm Mg ribbon
• Swirl + immediately start the timer once the bung is on
• Every 10s, measure the volume of gas collected until it is full
• Repeat to find a mean
• Repeat with different conc. HCl
• Control temperature, length of Mg and volume of HCl
• To find the rate, you do 1/time

Question 4

Method to carry out paper chromatography ro find the dyes in food colouring

Answer 4

• Use a ruler to draw a pencil start line on the paper about 2cm from the bottom of the paper
• Mark some spots using pencil on the line and leave 1cm clear between each one
• Use a capillary tube to place a spot of the known food colurs and the unknown food colours on the pencil spots
• Pour a solvent e.g. water into a beaker
• Attach the paper to a glass rod using tape and lower it into the beaker so that the paper is in the solvent but the solvent is below the start line
• Attach a lid
• Wait for the solvent to move up the paper and carry the dyes with it and take it out of the water once it is near the top
• Use the pencil to mark the solvent front
• Hang the paper up to dry it
• Line up the spots of the unknown colour with the known colours to see what it is a mixture of

Question 5

Method for experiment to check if water is pure

Answer 5

• Check the pH by adding a drop of the water onto universal indicator paper - if the water is pure the paper should turn green indicating a pH of 7
• Get mass of empty evaporating basin using a mass balance
• Fill with water sample and place on a beaker of water on a tripod and gauze
• Heat until all water has evaporated
• Reweigh evaporating basin and if no change in mass, water was most likely pure as it doesn’t have any dissolved solids

Question 5

How to use paper chromatography to identify an unknown substance

Answer 5

• Set up chromatography in standard way with the unknown chemical on the pencil start line
• Draw a line where the solvent got to (solvent front)
• Measure the distance moved by the unknown chemical
• Measure the distance moved by the solvent
• Use these values in the Rf value equation
• Look this Rf value up in a database but it may not be on there if the substance has never been analysed
• This may need to be repeated with different solvents

Question 6

Method for experiment to purify water

Answer 6

• Set up water in a conical flask on a tripod and gauze. attach a bung an a delivery tube directed into a test tube surrounded with a beaker of ice and water
• Heat water using a bunsen burner
• It will evaporate and travel along the delivery tube
• When it gets to the cold beaker, it condenses and forms distilled water as any dissolved solids are left in the conical flask