Required practical - Decay, Environmental change, biodiversity, deforestation Flashcards
Describe how to investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
Start by labelling a test tube ‘lipase’
Use a pipette to place 5cm^3 of lipase solution into the test tube
Now label another test tube ‘milk’ and add five drops of the indicator Cresol red
Now add 5cm^3 of milk and 7cm^3 of sodium carbonate solution to the milk test tube
At this point, the solution should be purple. That is because sodium carbonate solution is alkaline and cresol red is purple in alkaline conditions
At this point, place a thermometer into the test tube containing the milk,
Now we place both the test tubes into a beaker of water at our first chosen temperature.
Start at 20 degrees celcius which is around room temperature
At this point, we need to wait until the temperature of our solutions is the same as the water in the beaker
Now use a (a calibrated dropping - a dropping pipette with a scale) pipette to transfer 1cm^3 of lipase solution to the test tube containing milk and we stir the solution
At the same time we start a timer
At this point, the enzyme lipase will start to break down (decompose) the fat molecules in the milk. This releases fatty acids and this causes the milk solution to become acidic
In acidic conditions, the indicator cresol red changes to yellow
So once the solution turns yellow, we stop timing and record the results (the timing)
Now we need to repeat the experiment at a range of different temperatures (e.g. 10,30,40 degrees celcius)
Make sure to carry out the experiment three times at each temperature, then calculate the mean time taken for the colour change to occur at each temperature
The independent variable in this experiment is the temperature
The dependent variable is the time taken for the milk solution to turn yellow
The control variables are the volumes of the different solutions
We need to make sure that the volumes of lipase, milk, sodium carbonate solution and cresol red do not change.
Important points
It is very important that we use a clean test tube for the milk solution for each experiment
This is because any traces of lipase from the previous experiment will trigger the reaction before we are ready.
Because we are looking for a change of colour, it can be difficult to decide the exact point to stop the timer. We can reduce the effect of this, by sharing data with other groups and calculating a mean
Explain why it is very important that we use a clean test tube for the milk solution for each experiment
investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
It is very important that we use a clean test tube for the milk solution for each experiment
This is because any traces of lipase from the previous experiment will trigger the reaction before we are ready.
Explain why it may be difficult to decide the exact point to stop the timer in this experiment
And suggest a solution to this.
investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
Because we are looking for a change of colour, it can be difficult to decide the exact point to stop the timer. We can reduce the effect of this, by sharing data with other groups and calculating a mean
Independent variable of experiment
investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
The independent variable in this experiment is the temperature
Dependent variable of experiment
investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
The dependent variable is the time taken for the milk solution to turn yellow
What are the control variables of the experiment
investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
The control variables are the volumes of the different solutions
We need to make sure that the volumes of lipase, milk, sodium carbonate solution and cresol red do not change.
state the problem with this experiment
and the possible solution
Describe how to investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
Decay is a very slow process so it is very hard to observe that in a lesson.
Solution: To get around that problem, we are going to model decay by using the enzyme lipase. So what we will be looking at is not strictly decay but it should give results that approximate to what we would see with decay
if milk is left at room temperature for a few days what happens to it. And why?
if milk is left at room temperature, then after a couple of days it goes sour
That is because bacteria in the milk start the decay process
why does sour milk taste acidic
milk becomes sour because bacteria in the milk start the decay process
These bacteria use enzymes to produce acidic molecules and that is why sour milk tastes acidic
draw a graph for this experiment
investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRl-4DVCMxx7pAujXyWnM2fT64xeJASp7CHuxUTKfQRnb3Rm2FJ
Explain results of this graph
investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change
https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRl-4DVCMxx7pAujXyWnM2fT64xeJASp7CHuxUTKfQRnb3Rm2FJ
https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRl-4DVCMxx7pAujXyWnM2fT64xeJASp7CHuxUTKfQRnb3Rm2FJ
https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSJ4gIK_ftj4E7csgxa883FPLRPqZkOJ–hSBc8LU2Fh3eYXDWO
https://www.youtube.com/watch?v=LeH5FUKSPzU&list=PL9IouNCPbCxVuf3dVIq6kHQ0b27Hu-fgW&index=11
time: 4:01
At low temperatures, the reaction is slow
That is because enzymes work slowly at lower temperatures
At a certain temperature the reaction is taking place at its fastest rate
This is the optimum temperature
In conditions which are warmer than the optimum temperature, the reaction slows down and it might even stop completely
That is because the enzyme denatures at higher temperatures
In terms of decay, decomposing microorganisms work faster in warm conditions but not in hot conditions
This is because their enzymes denature when the temperature is too high
Another method is…
1.Place 20 cm3 of fresh milk into three beakers
2.Decide the three temperatures you will investigate. Write these onto the sides of the beakers. They may be 5, 20 and 35°C.
3.Use universal indicator paper or solution to determine the pH of the milk in the three beakers
4.Cover each beaker in cling film and incubate at the appropriate temperature
5.Use universal indicator paper or solution to determine the pH of the milk in the three beakers after 24, 48 and 72 hours
0 hours 24hr 48hr 72hr 5°C 6.5 6.4 6.4 6.0 20°C 6.5 6.1 5.5 4.8 35°C 6.5 5.1 4.8 4.8
pH
What conclusions can you draw from this data
At 5°C the pH of the milk reduced the least [1 mark].
This suggests that the least decay has occurred at this temperature [1 mark].
The milk incubated at 20°C and 35°C both reduced to pH 4.8 [1 mark].
This suggests that the decay after 72 hours is similar at these temperatures [1 mark].
However, the reduction in pH occurs much quicker at 35°C [1 mark]
. The pH reaches its lowest after 48 hours not 72 hours as is the case with the milk incubated at 20°C.
These results show the decay of milk occurs quickest at 35°C [1 mark] and then stops.
Another method is…
1.Place 20 cm3 of fresh milk into three beakers
2.Decide the three temperatures you will investigate. Write these onto the sides of the beakers. They may be 5, 20 and 35°C.
3.Use universal indicator paper or solution to determine the pH of the milk in the three beakers
4.Cover each beaker in cling film and incubate at the appropriate temperature
5.Use universal indicator paper or solution to determine the pH of the milk in the three beakers after 24, 48 and 72 hours
0 hours 24hr 48hr 72hr 5°C 6.5 6.4 6.4 6.0 20°C 6.5 6.1 5.5 4.8 35°C 6.5 5.1 4.8 4.8
pH
What limitations might there be in drawing these conclusions
The students did not take repeat readings to look for concordant results. They could be more confident in their conclusions if they took repeat readings.
Another method is…
1.Place 20 cm3 of fresh milk into three beakers
2.Decide the three temperatures you will investigate. Write these onto the sides of the beakers. They may be 5, 20 and 35°C.
3.Use universal indicator paper or solution to determine the pH of the milk in the three beakers
4.Cover each beaker in cling film and incubate at the appropriate temperature
5.Use universal indicator paper or solution to determine the pH of the milk in the three beakers after 24, 48 and 72 hours
0 hours 24hr 48hr 72hr 5°C 6.5 6.4 6.4 6.0 20°C 6.5 6.1 5.5 4.8 35°C 6.5 5.1 4.8 4.8
pH
How would you modify the procedure described above to find out if skimmed milk decayed less quickly than fresh milk
Use similar quantities of fresh and skimmed milk but keep both samples at 35°C. The pH of the milk would need to be measured at the start, and after 24 and 48 hours, but not 72 hours.
Evaluate the impact/the effect of environmental change on the distribution of a species
