Evaporating water
| Y6 (07/2009) | Y8 (07/2009) | ||
| a) | C (Container 3 will have the least amount of water) | moderate | moderate |
| b) | Kevin | easy | very easy |
|
c) i) ii) |
C (Surface area) Links to questions a) and b) and relates to surface area. For example: Because container number 3 was open the most and Kevin's towel was spread out the most. or Gives a logical explanation for their choice (even though it was incorrect) and links to questions a) and b) |
very difficult very difficult difficult |
difficult very difficult difficult |
Predicting is an important part of scientific investigations. Predictions in science are based on a theoretical idea, and so form the basis for planning an investigation that leads to testing personal theories. For example, in this task students are encouraged to develop or use their personal theory about the shape of the container and how this relates to the rate of evaporation. The results of an investigation that do not match their prediction can challenge students (and scientists) to re-examine their personal theories.
By making, testing, and explaining predictions, students can come to the understanding that predicting is not guessing but applying their science understandings to a specific context.
Predicting what might happen encourages students to complete "hands on" investigations with their "minds on" as well. They are likely to be more engaged if they have made some commitment to predicting the result.
Why do students need to know about factors that affect rates of evaporation?
Knowledge about the factors that drive evaporation is fundamental to understanding some of the big issues in science such as water supplies, climate change, and global warming. It also has many practical applications that students are likely to have experienced, such as drying clothes, looking after fish in the aquarium, and cooking. However, although they are likely to have observed the results of evaporation in these contexts, many will have a naïve understanding about the science processes involved.
The majority of students linked the rate of evaporation of water to heat. This is not surprising because at Levels One and Two of the curriculum the achievement objective focuses on what happens to materials when they are mixed, heated or cooled. At Levels Three and Four the relevant achievement objective is very broad, with no reference to other factors that affect evaporation. To avoid the misconception that heat is the only significant factor, students need to be given opportunities to also consider other factors such as surface area, the amount of water, and air movement.
In Question b), most students knew that the spread out towel would dry most quickly, probably because they have experienced this first hand. However, in Question c) ii) few related this to more of the towel's surface being exposed to the air.
- The question is dependant on their getting Question c) i) correct.
- Students have to have some understanding of surface area. In this example the surface area relates to the area of water exposed to the air, i.e., the surface of the water, which is where the water molecules can evaporate from. (There is no expectation that they be able to complete any mathematical calculations.)
- They have to think about two different instances and notice what is common to both, which is more cognitively demanding than identifying the factor for just one instance.
However, about 1/5 of Year 6 students and nearly 1/3 of Year 8 students who answered c) i) incorrectly did make a logical attempt to link their answer to both instances. In most of these cases they had chosen heat as the variable being investigated.
- Because they both had something to do with heat (Y6)
- Because heat is what evaporates water and dries towels (Y8)
These students are developing important skills in justifying their answers.
A few students confused "surface area" with the surface that containers of water or the towel were on, rather than the surface area exposed to the air.
- Because it depends on how hot the surface is
- Because they're all over windows and chairs: hooks and bags
Gaining experience in making and justifying predictions.
A useful strategy to use is Predict, Observe, Explain (POE). The strategy is most suited to Material and Physical World concepts where a quick result is gained. Using contexts where the results are surprising can lead to some interesting classroom discussions. Go to POE to read more about this strategy.
Factors that affect rate of evaporation
If students only consider heat as a factor, investigate a familiar context such as drying washing, getting them to compare different scenarios, and giving reasons for their predictions.
For example:
- Which washing will dry faster, and why?
- Washing hung in the sun or the shade?
- Washing on a cold day or a hot day?
- Washing on a windy or still day?
- Washing pegged on the line by its ends or hung over the line?
For example:
- Will washing dry faster on a cold windy day, or a still, hot day?
Follow up investigations
Students should always be given a chance to plan and carry out investigations to check their predictions and reflect on the results. At Levels 3 and 4, however, they are likely to need considerable support to manage all the variables.
Introducing particle theory
Students who have a good idea of the variables that contribute to the rate of evaporation can be introduced to using particle theory to explain how surface area is a factor in evaporation rate.Some avtivities students can explore at Levels 3 and 4 that will provide a basis for understanding the more complex ideas are:
- Role plays, where the students physically act out being the water molecules, is a useful beginning activity.
- Showing the action of molecules through drawing is another useful activity, especially if students annotate or explain their drawings to others. The disappearing puddle is an example in the context of evaporation.
- Balloons uses the POE strategy in the context of heating and cooling air.
- Where will wood float? uses the POE strategy in the context of wood floating.
- The disappearing puddle is a drawing task about water evaporating from a puddle.
Connected 2, 2002 includes articles on water and the water cycle
Making Puddles, Connected 1, 2000 is about evaporation, although the focus is on the role of heat.
Ministry of Education (2001). Where's the Water? Building Science Concepts Book 15. Wellington: Learning Media. Activity 2, pages 8-9, is about the variables that contribute to a handprint on a paper towel drying.
Ministry of Education (1998). Making Better Sense of the Material World. Wellington: Learning Media. This book includes a chapter on water. Activity 3: Washing Day (page 30) investigates factors that affect washing drying, including wind (but not surface area). The investigation could be extended to include how the washing is hung up. Notes on pages 21 and 22 provide background information for teachers.