Asia-Pacific Forum on Science Learning and Teaching, Volume 4, Issue 2, Article 2 (Dec., 2003)
Pamela MULHALL, Amanda BERRY and John LOUGHRAN
Frameworks for representing science teachers' pedagogical content knowledge
Previous Contents


A PaP-eR on Chemical Reactions NB Shaded 'call out' boxes have been added to highlight researchers' interpretive frames but do not form part of the PaP-eR.

Understanding What Substances Are

This PaP-eR discusses the importance of students developing an understanding of the idea of substance and how substances differ, as a precursor to recognising and understanding chemical reactions.

A succinct description of the science pedagogical content knowledge that will be elaborated in this narrative
Chemical reactions tend to be presented to students as processes in which new substances are formed. I used to consider this idea to be unproblematic for students and tended to focus instead on developing student understanding of scientific explanations for the behaviour of a reaction at the atomic level. After I had been teaching awhile, I became aware that students often aren't sure what a 'substance' is and find it hard to decide if a chemical reaction has occurred: when they 'see' a chemical reaction taking place, they do not automatically 'see' that new substances are formed because they do not think this way about matter.  Narrative voice is that of a thoughtful teacher reflecting on past experience.

Teacher recognizes this is a complex concept. Teacher's knowledge develops in action.

Over the years I have often heard this kind of conversation between students in prac groups as they are doing an experiment: Teacher questions what is usually seen to be 'unproblematic', i.e. students' understanding of what is a substance.

Teacher knowledge derived from experience of practice; noticing a pattern in the way students respond.

Pat (recording the group's notes about the prac): What happened?
Kim: It went fizzy.
Pat: Did you see any new substances?
Kim: Nope.

Sam: What shall I write down was formed?
Chris: A blue colour

PaP-eRs are located in a particular classroom context. Each class is different and the teacher's responses to that class will be based on specific knowledge of the particular group as well as general knowledge about teaching this content at this year level.
While students such as these 'see' bubbling, they don't make the connection that a new substance - gas - is formed. They may 'see' a colour change but not that a different substance - powder - is now floating around in the test tube. So I spend a lot of time trying to develop student awareness of substances. I do this by trying to develop student awareness of the ways in which 'stuff' differs. In chemistry terms that means considering the physical and chemical properties of a piece of 'stuff' but early on I don't worry about this distinction.  
Teacher makes decisions (based on awareness of students, content, curriculum, pedagogy).
Teacher's knowledge about properties of substances (content knowledge) and knowledge about how students learn (pedagogical knowledge) together inform the decision that is made (PCK).
Teacher recognizes that student learning develops slowly and through a variety of approaches.
We do a lot of activities where basically the task is for students to try to describe as accurately as possible a number of different substances. I pick out a few substances that have a similar property (for example colour) and ask how we know they are different. Sometimes it's hard to be sure (unless we perform a chemical analysis - but that is a long way down the track for these students) but the important thing is to start students thinking about the differences between things and what makes a substance different from the rest. It's really important to have some pure powders among the examples students have to consider and some whole pieces of the substances that these powders come from, such as a piece of iron and fine iron filings: this helps them to realise that powders are the same 'stuff' or 'substance' as the thing they came from, and that size is not a good way of distinguishing substances.  
Teacher recognises the importance of providing a range of opportunities for students to grapple with one of the big ideas of science.
When I think they have the idea, I push them further to start considering situations where a new substance may have been produced. Of course this is getting into chemical reactions, but I don't use that term yet. I am still focussing on the idea of substance and whether students can distinguish between different substances. I give students this handout to discuss in small groups. Teacher is constantly monitoring student learning. This informs what will be done next.


The event

Group decision (with reason)

The foul smell of food gone bad


The rusting of a nail


Cheese being grated


Baking a cake


A tree growing from a seed



Teacher's understanding of the content informs his/her choice about which everyday examples are useful to include for students' learning.
As the discussions progress, I wander around and listen. Conversations like the following tell me they are getting the idea.

Gina: Well grated cheese looks the same as the block of cheese it came from.
Teresa: Tastes the same too - yum!
Tom: Yes, it's just smaller bits of the big cheese but still the same stuff.

Hugh: Of course a cake is a new substance!
Con: Yeah, looks and feels totally different to the eggs, butter, flour and sugar.
Tim: In fact you can't even see them anymore

Teacher's understanding of content and pedagogical approach (listening, not intervening) helps develop teacher's awareness of students' understanding. Teacher is listening for and at the same time, listening to.
If I hear an observation like Tim's, I store it in the back of my mind for the next lesson when I want to develop the idea that when new substances are formed, the original ones disappear. For the moment though I am pleased that the students can recognise when new substances are formed. (The tree growing from a seed is a tricky one - the leaves look new but what about the bark? There are often arguments between the students about this - what's important here is not the actual decision that students make but their reasoning and I like to emphasise that it's situations like this, where scientists are not sure of the answer, that often lead scientists to doing further experiments.)

Teacher is sensitive to what is occurring 'within the moment' and considers its influence on future learning decisions.
Sense of teacher's reasoning why learning experience is constructed in this way.
Ultimately I want students to see things they have seen before, both in the world around them and in the lab, in a new way so that the conversations in the lab I mentioned at the start run like this:

Pat (recording the group's notes about the prac): What happened?
Kim: It went fizzy.
Pat: So a gas was produced?
Kim: Yes.

Sam: What shall I write down was formed?
Chris: A blue substance.

Teacher's purpose for his/her teaching linked to big science idea which he/she sees as problematic in terms of student learning.

Teacher awareness of his/her role in providing opportunities for learning.

Exchanges like this between students tell me that they are starting look at the world through the lens of 'substance' rather than just properties (eg colour) and behaviour (eg bubbling). Soon they will be ready for the concept 'chemical reaction'.    


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