Chemistry for the gifted and talented 5 Candle investigation Student worksheet: CDROM index 03SW This worksheet is also provided on the CDROM as both a PowerPoint presentation 03PP and a teacher-editable Word document 03WD please refer to the Use section below. Discussion of answers: CDROM index 03DA Topics Combustion and reliability of data. Level Able students aged 11 13. Prior knowledge Combustion requires oxygen. Rationale Experiments to show can be frustrating for very able students if they already know what the experiment will show. The approach used here is to change the experiment into one for which they will not know what happens. They have to use critical thinking to evaluate the alternative suggestions in the concept cartoon. They can evaluate some real experimental data... this might stimulate motivation to carry out the experiment to obtain better evidence. The students are then asked to briefly think about the difficulties of showing slight effects, as in medical research. They need to use creative thinking to produce their own concept cartoons. Good examples of concept cartoons to show the students can be found in Concept cartoons in science education 1.
6 Chemistry for the gifted and talented Use This could be used to follow on from a class investigation into the effect of beaker size on the length of time the candle burnt. The PowerPoint version (CDROM index 03PP) could be used as a focus for class discussion, using a projector as an alternative to the worksheets. The Word version (CDROM index 03WD) could be used as a differentiation activity for more able students in the group who can accurately predict the outcome of the size of beaker experiment. When this activity was trialled, the students enjoyed making their own concept cartoons. These could be tried out on other students and a prize could be offered for the best one. If the students plan and carry out their own investigations then it will be an activity for a whole lesson, otherwise it could simply be an episode at the start or end of a lesson. 1 S. Naylor and B. Keogh, Concept cartoons in science education, Sandbach: Millgate House, 2000.
Candles in beakers investigation
What do you think? The candles will burn for the same time because the beakers are the same size. The shortest candle will run out of wax quickest. 12 9 3 6 The shortest candle will generate the most convection because the flame is at the bottom of the beaker. It will therefore burn the longest. The shortest candle will go out first because carbon dioxide is more dense than air and will settle to the bottom and put out the flame.
Plan your own investigation There are two key points here: How can you ensure that you carry out a fair test? How can you demonstrate to others that the data you will obtain is reliable?
The experiment
The experiment cont d A new long candle was burnt for a short while so the wick was not new. It was secured to the bench and a one litre beaker was put over it upside down. The time taken for the candle to go out was measured and the experiment was repeated. The candle was cut down in length and the experiment repeated (the cut off pieces were included in the beaker so that the volume of air remained a constant).
Data were obtained Time candle burnt/s Candle length/cm Exp 1 Exp 2 Exp 3 Average 11.5 27 20 18 21.7 6 22 24 18 21.3 2.5 19 24 19 20.7
Evaluation What do these data show? Describe the relationship between the length of the candle and the time that it burns in the beaker. Scientists publish their findings in scientific journals are these data good enough to publish? If not why not?
Evaluation cont d The impact of candle length on the time that the candle burns is, at most, small. Scientists in the field of medicine do experiments which show slight changes in rates of certain diseases when analysing particular dietary components. For example, an investigation they might do would address the question - do people who eat a lot of oily fish have slightly less chance of heart disease? How would they design their investigation to show meaningful results even though they are expecting only a small effect?
Your turn Can you devise your own concept cartoon about candles burning? Your cartoon could: have alternative questions someone might ask have different shaped containers rather than all beakers alter the experiment so water is drawn up into the beaker as the candle burns have more than one candle in each beaker.
Chemistry for the gifted and talented Student worksheet Candle investigation Concept cartoon What do you think? The candles will burn for The shortest candle will run the same time because the out of wax quickest. beakers are the same size. The shortest candle The shortest candle will generate the most will go out first because convection because the flame carbon dioxide is more dense is at the bottom of the beaker. than air and will settle at the It will therefore burn the bottom and put out longest. the flame. Discuss each suggestion, decide what you think about each one and what you think will happen in the experiment. continued on page 2 Student worksheet 03SW Candle investigation Page 1 of 3
Chemistry for the gifted and talented Plan your own investigation to decide which prediction was true There are two key points. How can you ensure that you carry out a fair test? How can you demonstrate to others that the data you will obtain is reliable? Your turn to be creative Can you devise your own concept cartoon about candles burning? You need a drawing to set the scene in the middle and alternative suggestions from people in speech bubbles. These could be alternative predictions, explanations or suggestions for investigations. Your cartoon could: have different shaped containers rather than all beakers; alter the experiment so water is drawn up into the beaker as the candle burns; compare one candle in 500 cm 3 with two candles in 1000 cm 3 etc; and compare different length wicks. continued on page 3 Student worksheet 03SW Candle investigation Page 2 of 3
Chemistry for the gifted and talented The experiment A new long candle was burnt for a short while so the wick was not new. It was secured to the bench and a 1litre beaker was put over it upside down. The time taken for the candle to go out was measured and the experiment was repeated. The candle was cut down in length and the experiment repeated (the cut off pieces were included in the beaker so that the volume of air remained a constant) Time candle burnt/s Candle length/cm Exp 1 Exp 2 Exp 3 Average 11.5 27 20 18 21.7 6 22 24 18 21.3 2.5 19 24 19 20.7 Evaluation 1. What does this data show? Describe the relationship between the length of candle and the time that it burns in the beaker. 2. Scientists publish their findings in scientific journals are these data good enough to publish? If not why not? 3. The impact of candle length on the time that the candle burns is, at most, small. Scientists in the field of medicine do experiments which show slight changes in rates of certain diseases when analysing particular dietary components. For example, an investigation they might do would address the question do people who eat a lot of oily fish have slightly less chance of heart disease? How would they design their investigation to show meaningful results even though they are expecting only a small effect? Student worksheet 03SW Candle investigation Page 3 of 3
Chemistry for the gifted and talented Discussion of answers Candle investigation Concept cartoon The only unreasonable contribution was the suggestion that the wax would run out. If you think the experimental results are reliable then the carbon dioxide argument gains credibility. Planning your own investigation Some key points here are to make it a fair test and make the results reliable. Fair test The wick needs to be in a similar state each time (new candles need to burn for a little while). The volume inside the beaker must be the same. If you are going to cut down a candle then the cut pieces must be put inside the beaker. If you are going to support the candle at different heights then the supports must be inside the beaker all the time. The air needs to be fresh each time. Carbon dioxide can dissolve in water so the beaker should be dry each time. Reliability Any difference in the time is likely to be small so the experiment needs to be repeated many times to get a reliable result. Evaluation These data show that further work is necessary to be sure of a conclusion. They also shows that there is quite a lot of variation in the repeated results. These data suggest that a more extensive investigation might show that the shorter candle went out quickest. A scientist would want much clearer evidence before publishing. The experiment needs to be repeated many more times. In medical research large numbers of people (a large sample group) are included to reduce the effect of randomness in disease. Discussion of answers 03DA Candle investigation Page 1 of 1