Grade 8

The overarching Task is to develop a system of strategies to analyse salts to identify them.

They have been dealing with separation of mixtures. Their focus has been on physical properties of substances in order to separate them by simple physical means like filteration, magnetic separation etc. In this exercise, they are given a task to work out a strategy to identify one salt.

The salt they have to identify is common salt or sodium chloride. The aim of the task is to introduce them to the ENV model of thinking. They will look at some physical properties (parameter 1) and see how far they can go i.e. how many values of this parameter can they change.

Step one:

Task: Three salts are given. Two are coloured. One is white. Which is common salt?

Students response: The white one. (the value of the parameter identified is the colour)

Task: Two salts are given. Both are white.

Student response: The colour as a difference no longer valid. Therefore they have to look for another value.

They looked for 2 more values of the parameter “physical properties”, smell, and taste. Both these are invalidated by the safety rules of scientific investigation.

They then look for other ways to change the value of the parameter.

(from their own experience they know salt dissolves in water, it disappears when added to food etc. )

So they check for the solubility I.e. they found one salt dissolves and the other doesn’t. the one that dissolves is identified as common salt.

Task: Two salts are given. Both are white and both dissolve in cold water.

Student response: When they go through the strategies, that is applying differing values 1. colour 2. solubility in water, do not work.

so they have to look for other values. At this point the word variable is introduced.

The students have reached till here. They have to look for more variables. Adding to the ENV. Two more variables that they are going to investigate are a) melting point b) shape of the crystals under the microscope.

After this they would have built up a series of variables. All the variables are different values of the same parameter (physical properties)

After this I will ask them through a discussion to now switch to a new parameter which is chemical properties. Go through a similar exercise of tasks to build up a set of variables for parameter 2 (chemical properties).


# Alexander Sokol 2011-03-01 09:30
Krishna, thanks for the post. A few questions.
1. Did you have a chance to introduce the ENV as a model that, can help the students develop a strategy / algorithm? If yes, can you give details on how it happened and whether the students found it useful? If not, it'd be interesting to know why you decided it was not necessary?
2. Did you have a chance to explicitly deal with the question of building a strategy / algorithm for difficult tasks? °
3. (This is related to previous questions) Did the students find the tasks challenging enough? What I am trying to ask if they felt 'stuck' at any point of working on the questions or was it basicallyt enough to do brainstorming to come up with an idea for an answer?
4. You write that at some point some of the ideas of students were rejected as they didn't meet the safery requirement? Can you give details on how it happened? Did the students reject the ideas by themselves? Did you do it? Was it a guided process? If so, how did the guiding look like?
Looking forward to your comments. In general, it'd be nice to have more details about students' responses to what's going on and how the teacher (you) dealt with them.
# Krishnakali Gupta 2011-03-13 16:52
1. Yes I did mention the ENV model. However, students ate more comfortable with "strategies" and changing "variable" rather than parameter and values. I would like to take them through detaled tour of ENV and algorithm a little later, at the end of part 2. this is because I feel they are entrenched into "aim" and "hypothesis" model of working with science experiments
2.Yes once they got stuck I did get them to discuss the need for a strategy.
4. When they moved from colour, smell, taste was an obvious choice(several students suggested it) Yes, I did have to point out in this way " So would you taste it? yes, we all know what NaCl tastes like, but could you risk it?" Student response: no (unanimous)

Hope this clarifies some areas. I have just posted the part 2 of this activity. It is still not complete, but ongoing.
Thanks Alexander for taking time and interest.
# Alexander Sokol 2011-03-13 22:06
Comfortable doesn't necessarily mean working better, does it? What I am curious about is whether you see any difference for the students when explicitly applying the ENV model in comparison to what they already know.
Another interesting point is how students can build a strategy if they've got no tools for it. Wouldn't we be encuraging them to make trials and errors insrtead of thinking in an organised way?
# Krishnakali Gupta 2011-03-19 10:15
Comfortable means being more familiar with certain vocabulary. Yes, even though I am having to constantly urge them to GO BACK and see what and how they did in the previous task, some of them have already started turning the pages of their notebook to revisit the earlier exercises, before responding.
They do not have the tools to start with. Agreed. However, the first step is to do trial and error. The idea is that with each subsequent task, the trial and error becomes less and they start looking for LOGICAL answers. The organising of thoughts does not happen at one go. A minimum (I sincerely hope not endless)guesswork is the starting point if we are to judge the whole overarching task.
# Alexander Sokol 2011-03-20 09:02
I agree that learning to think takes time. I am still curious though why we can't aim at eliminating guess work. I understand that it may be necessary to help them feel that it's not useful but can't we aim at getting rid of guessing at all in the long run? Why should we always start with trial and error?
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