A home-made smell remover
| Y10 (09/2007) | |||
| a) |
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Neutralisation | Difficult |
| b) | The smell disappears because a reaction occurs. | Moderate | |
| c) |
i)
ii) iii) iv) |
No (Baking soda and water) It depends (Vinegar and baking soda) It depends (Yoghurt and water) Yes (Yoghurt and vinegar) |
Prediction – moderate
Prediction – very difficult Prediction – very difficult Prediction – difficult |
The smell is a specific property of amines. These are changed chemically into a salt and water when they react with an acid and neither of these new chemicals has the same type of smell.
This item is designed to provide indications of students' ability to think about what actually happens in a chemical reaction, and to tease out a reasoning sequence, not just to repeat symbolic ideas about general reaction patterns (e.g., "acid plus base gives salt plus water"). We also took the opportunity to look for indications of "it depends" thinking, which is so important to understanding the complexity and (often) unpredictability of systems interactions.
Students found many parts of this item very difficult, but there are some indications of beginnings of understandings from which to build. Just over one third of students could name an acid/base reaction as neutralisation. Nearly half of them could explain that after more chemicals are added the smell has been "cancelled out" in some way. These students do understand that a change has taken place, but they seem less aware of why and how that could cause a smell to go away. Only 13 of the 294 trial students explicitly said that this was because the chemical that caused the smell had been changed into something else. Other answers commonly indicated a sense of disguise – the fishy smell was masked by a stronger smell and hence disappeared. A third of the students did not attempt an explanation at all.
Part C of the question required several reasoning steps:
- Recognition of the pH of the substances: a lot of students thought that yoghurt must be a base because they know that milk is basic. There are indications that the pH of baking soda (and perhaps of vinegar) is more commonly known – more students made correct predictions here, even if they could not explain them.
- Working out the likely reaction between the substances mixed for use. This is the first place where there is potential for an acid/base reaction but obviously the thinking will be incorrect if the pH has already been identified incorrectly.
- Factors such as dilution then need to be taken into account, which is where the "it depends" aspect comes into play.
- Finally, working out the likely reaction between this mixture and the amine – the second potential acid/base interaction.
There are opportunities here to explore smell as a property specific to chemicals, and change of smell as an indicator of chemical change. It may be that students know this intuitively when a smell appears but it may not be so obvious when the smell disappears. (If students are using smell as an indicator of chemical change, you could use your safety precautions about how to smell unknown chemicals as a way of illustrating that scientists work in particular ways, making explicit another aspect of nature of science).
Teasing out the reasoning steps, as above, could be modelled to help students understand the process used to arrive at explanations for seemingly simple interactions of everyday chemicals. This type of thinking could then be related to other everyday situations where materials with differing chemical properties could come into contact with each other. The "thinking steps" template could be useful for this purpose.
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Making Thinking Visible To predict if each mix of chemicals could neutralise a base (fish amines). For this to happen, the end product of each mixing would need to be an acid. Reasoning steps:
Now you are ready to choose between yes/no/ it depends! Go back to Step One and do the same for the next pair.
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