Practical skills 2

Question 1

What must you do when planning an experiment?

Answer 1

• RTQ as they may tell you what apparatus to use
• State dependent and independent (think about a suitable range and interval of the independent variable)
• State control of key variables (one that would actually make a difference)

Question 2

What is a hypothesis?

Answer 2

• A prediction of how you think the two variables are related to one another
• It must be stated in a way that you can test by experiment through the collection of quantitative results

Question 3

What could a hypothesis be for an experiment to investigate the relationship between temperature and the rate of respiration of yeast?

Answer 3

1. The rate of respiration of yeast increases with temperature
2. As temperature increases, the rate of respiration of yeast will increase up to a maximum temperature above which it will fall
   - Either of these hypothesis is fine and both of them can be tested by changing the temperature and measuring the rate of respiration
   - You may be asked to sketch graph of you predicted results if your hypothesis is supported

Question 4

Can you prove a hypothesis?

Answer 4

1. You cannot ‘prove’ that a hypothesis is correct by doing one experiment
   - Your results may support your hypothesis, but they cannot prove it
   - You would need to do many more experiments before you can be sure that your hypothesis really is correct in all situations
2. Can disprove hypothesis more easily e.g. if you found that the rate of reparation did not increase as temperature increases, then this suggests that the hypothesis is incorrect
   - Nevertheless, it would be a good idea to do the experiment two or three more times more to make sure that the results can be repeated

Question 5

What is an example of using the right apparatus?

Answer 5

1. As yeast respired, it produced carbon dioxide gas which collects above the liquid in the syringe, increasing the pressure and causing the meniscus to move down the capillary tubing
2. Respirometer may be better but must use what they say

Question 6

How would you change the temperature (independent variable)?

Answer 6

Water bath, measure temperature with thermometer

Question 7

How would you change the pH (independent variable)?

Answer 7

Buffer solutions

Question 8

How would you change the concentration (independent variable)?

Answer 8

Using a stock solution to make up solution with lower concentrations, using the serial dilution technique

Question 9

How would you make up the initial stock solution with a particular concentration?

Answer 9

E.G 1% sugar solution (solution contains 1g of sugar for every 100g of water) (1cm^3 of water has mass 1g)

1. Use a top pan balance to measure out 1g of sugar
2. Place it into 100cm^3 volumetric flask (a kind of flask that enables the volume to be measured very accurately)
3. Add a small amount of distilled water and dissolve the sugar thoroughly
4. Add distilled water to make up to exactly 100cm^3

Question 10

How would you make up a 1moldm-3 solution?

Answer 10

• This means a solution containing 1 mol of the solution in 1dm3 of solution
• A mole is the relative molecular mass of a substance in grams
• Find out mass from mr and use 1 mol
  1. Use a top pan balance to measure out 342g of sugar (sucrose) (add 342g of sucrose to a beaker take the mass of the beaker onto account or tare the beaker to zero)
  2. Put the sugar into a 1dm3 volumetric flask (wash any sugar crystals left behind into the flask)
  3. Add a small amount of distilled water and shake until the sugar has completely dissolved
  4. Add more distilled water until the meniscus of the liquid is exactly on the 1 dm3 mark

Question 11

How would you measure the dependent variable?

Answer 11

• The dependent variable is the rate of respiration of the yeast
• This can be measured by recording the rate of movement of the meniscus
• If you are investigating rate then time must come into your measurements
  1. You must record the meniscus at tine 0 and continue at regular time intervals
  2. You would need to do this at each temperature than you have decided to test

Question 12

What is important to remember with dependent variables?

Answer 12

• Describe exactly how you would measure the dependent variable
  1. Say what measuring instruments you would use
  2. What you would do to are sure that your measurements are made accurately
  3. Say exactly what you would measure and when you would measure it

Question 13

What are the two types of quantitive variable?

Answer 13

continuous or discrete

Question 14

What is continuous data?

Answer 14

• If the variable is continuous, then each measurement count or reading can be any value between two extremes
• Your results will not necessarily be whole numbers
• The results for the yeast reparation rate experiment will be a quantitative and continuous variable

Question 15

What is discrete data?

Answer 15

• If the variable is discrete then each measurement, count or reading can only be one of a set number of discrete values
• E.G you might be asked to count the number of prickles on each lead in a sample of holly leaves and the number of prickles will always be a whole number as you cannot have half a prickle on a leaf

Question 16

What are the two types of qualitative data?

Answer 16

categoric or ordered

Question 17

What are ordered (ordinal) variable?

Answer 17

• Those that although you do not have actual numerical values for them can be organised into an order of sequence
• E.G. a series of Benedict’s test on a set of glucose solutions of unknown concentration and decide on the relative depth of colour of each one
• You can sort these into an order from the one that is least brick-red to the one that is darkest brick-red, but you cannot assign an actual numeric value for the colour of any of them

Question 18

What are categoric variables?

Answer 18

• They are completely discrete and you cannot put them I order
• Each observation first into a particular, clearly defined category
• E.G In a sample of dead leaves taken from a forest floor, you might record the species of tree from which each leaf comes. Each leaf comes from one species - there are no ‘overlaps’ between categories, and there is no way of ordering or ranking them

Question 19

What is the purpose of a control?

Answer 19

• Is to check that it is the factor you are investigating that is affecting the dependent variable, and not some other factor
• E.G in the yeast reparation it would be a good idea to set up at least one syringe with sugar solution but not yeast
• This would be a check that it is something that the yeast is going that is causing the change in position of the meniscus

Question 20

How do you describe the sequence of steps?

Answer 20

1. When you are desiring your planned experiment, make sure that you describe a logical sequence of steps that you would follow
2. It is always worth jotting these down roughly first, to make sure that the steps follow one another in a sensible order, and that everything is fully explained
3. You might like to draw labelled or annotated diagrams to explain some of the steps - it is sometime easier and wicker to fo it that way. Diagrams are very often the best way to describe how you would assemble the apparatus

Question 21

What must you remember when planning an experiment?

Answer 21

• Risk assessment
  1. Do not event significant risks if there are none
  2. But ALWAYS mention risk, even if it is just to say that you do not think there are any significant risks

Question 22

How do you handle risks?

Answer 22

-If you do identify any significant risk, then you should explain how you would minimise them

Question 23

How do you handle risks?

Answer 23

• If you do identify any significant risk, then you should explain how you would minimise them
• E.G in the yeast experiment, you might decide to use temperatures up to 80 degrees Celsius and this is hot enough to burn the skin
• Therefore you would need to take precaution when handling the apparatus in a water bath at that temperature
• You should lift the apparatus in and out using tongs, or use heatproof and waterproof gloves

Question 24

What do you need to include in a conclusion?

Answer 24

• You may be asked how you would use your collected results to reach a conclusion
• Usually, the conclusion will be whether not the results support your hypothesis

Question 25

What is a control experiment?

Answer 25

An experiment in which the factor whose effect is being investigated (the independent variable) is absent; it is used as a standard of comparison