Measuring equipment - model answers

Activity 1 - Measuring the volume of a Test Tube

1. Describe any variation in your results for volumes of the test tubes

Students should say how much variation there is, the range of values, or the maximum and minimum values

2. Suggest why there are differences in the volumes measured.

There are a few common causes of uncertainty in measurements:

  • Problems related to the skill of the user - reading the lowest point of the meniscus, etc.
  • Errors in the functioning of the measuring instrument - unlikely with a measuring cylinder, but a graduated pipette filler may leak.
  • Properties of the substance / object being measured - the surface tension of water allows a test tube to be filled beyond the rim. It is hard to say exactly when it is full.
  • The act of measuring - something in the way the measurement takes place alters the measurement - e.g. a thermometer may cool a liquid slightly, drops of water may stick to the sides of a beaker.
  • Changes in the environment, e.g. temperature - may cause the liquid to expand, or evaporate

3.  What is the measuring error (the uncertainty) of each piece of equipment?  This is half the smallest graduation. 
Add this to your results table.

This depends on the equipment used in the lab

4. If you used the piece of equipment more than once to take one measurement you must multiply the error by the number of times it was used.

Add this to your results table

If a small measuring cylinder 20 ml is used three times to find the volume of a beaker of water 50 ml and the uncertainty is +/-1ml each time, then the total uncertainty will be +/-3ml

5. Which piece of equipment gives the most accurate answer?

This depends on the equipment used in the lab

Activity 2 - Measuring Mass of Rice Grains

1. Complete the uncertainty of each measurement in the table?
Which method 1 – 4 gives the most precise measurement? Explain Why.
Number of rice grains Uncertainty of one measurement Overall uncertainty
1 +/- 0.01g +/- 0.01g
2 +/- 0.01g +/- 0.005g
10 +/- 0.01g +/- 0.001g
100 +/- 0.01g +/- 0.0001g

When using the balance all the raw measurement have the same uncertainty.
This is the size of one of the smallest units displayed on the balance, 0.01g

2. Complete the Overall uncertainty column.
Note: If you divide a total mass by two you must divide the uncertainty by two.
Which method 1 – 4 gives the most precise measurement? Explain Why.
See the table above for overall uncertainties.
The balance is has an uncertainty of +/- 0.01g
(unless the manufacturer says otherwise, some small jewelry balances claim 
+/- 0.01g but in reality they are not this accurate.)
If you make one measurement of multiple objects then you can divide this measurement to find the size of one object (mean average) You can also divide the uncertainty.
However this assumes that there is not a significant variation in size between the objects.
If the experiment was measuring the sizes of different seeds for different species then this method would not be appropriate - it would be better to measure each seed individually

 

Activity 3 - Making a starch solution and using a colorimeter

1. How many times smaller than 1 litre is 10ml?

1 litre is 1000ml  so 10ml is 100 times smaller than a litre.

2. How many grams of starch would you need to add to make 1 litre of each of  the solutions?

One hundred times more than the sheet, 1g, 3g, and 10g.

3. What is the difference between the units dm3 and dm-3?

dm3 means 'litres' and dm-3 means "per litre".

4. Explain what gdm-3 stands for.

grams per litre

Taking it further

There is a good explanation of calculating uncertainties on this webpage: http://www.wikihow.com/Calculate-Uncertainty

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