• 12-15

The class progress so far: The students are working on the task of identifying common salt out of many salts. They have explored "physical properties" of the given salts, like colour, solubility etc. They have been dealing with one parameter (physical property) and changing the value of it from colour, smell, solubility and shape (of the crystals under the microscope). They also have become aware of the fact that each of these values is what in an experiment is called "variable".

So their algorithm at the end of this phase looks like this:

A salt may be identified by looking for these variables 1. colour 2. smell 3. whether it is soluble in a) cold water b) hot water and 4. its crystalline shape.

After this there were no more values that could have changed for the given salts.

Exercise: A salt is given to them, (carbonate) They know it is a carbonate salt. They are given instructions to add an acid to it and test for the gas that is coming out. It is not hard to predict this, it is carbon dioxide gas.

Class discussion: To go back to the identification of common salt, what could you do?

Student response: Since the physical properties are already done, we need to look for more variables. Yes but how? If could make the sodium chloride "give out" some gas that we could test for? (this response was from one of the students, very tentative).

Teacher question: Why do you want it to "give out" a gas?

Student response: Then we could test for it

Teacher: So which gas could possibly come out of sodium chloride? (a list is drawn up on the board from the gases they have tested for in the lab. in earlier lessons. Nitrogen dioxide, oxygen, carbon dioxide, chlorine and bromine and iodine)

Student response: chlorine

Teacher: Why?

Student: There is no nitrogen, or oxygen or carbon or bromine or iodine in the formula NaCl. It does contain Chlorine, therefore it could be made to give out chlorine gas.

Teacher: How?

Student: by doing the same thing that was done to the carbonate salt i.e. by adding acid to it.

So now we are at this point. The students would then add acid to all the salts and test for the gas that comes out in each case.

At the end of this they would have explored 2 parameters, physical properties and chemical properties and explored ENV model for physical properties of 4 different values and one value for the parameter chemical property, namely add an acid and test for the gas that comes out.

After the end of this I intend to use another salt that they would be able to use their ENV model to identify.

Then I would introduce a third salt where the value of the chemical property (add gas and identify the gas evolved) would not help. So they would have to look for more parameters or variables.