Chemical change
Introduction
You can get a number of surprising effects from chemical reactions. In the early and middle school years, only simple reactions are appropriate, such as that between sodium bicarbonate (baking powder) and vinegar (or any mild acid) to produce carbon dioxide. This reaction is the basis for many intriguing activities, with effects that depend on the gas production. Cooking provides some excellent examples of change and is a useful and popular activity in classrooms. In the 'kitchen science' section of this topic you can focus on techniques for observing and understanding the changes that occur in cooking, and use some novel recording strategies.
Key concepts of chemical change
The activities in this topic are designed to explore the following key concepts:
- Changes to materials can be physical (dissolving, melting, evaporating), involving changes in existing substances, and chemical (bicarb reactions, burning), involving the production of new substances.
- Substances can react together to form new substances that are quite different in their properties.
- A gas is a possible product of a chemical reaction.
- Combustion is a chemical reaction.
- A flame needs oxygen to keep burning, as the oxygen reacts with the burning substance.
- Substances can be grouped (e.g. acid/base) according to their chemical properties.
Students' alternative conceptions of chemical change
Research into students' ideas about this topic has identified the following non-scientific conceptions:
- Phenomena such as bubbles are simply effects: they are not seen in terms of the formation of substances. Thus, the categories of 'substance' and 'effect' can be interchangeable. 'Mixing things makes them fizz'-end of story.
- A chemical reaction is not an interaction of ingredients, but one ingredient that plays an active role.
- Substances have an ongoing history, so that a gas formed in a chemical reaction is thought to have been present in some form in the initial ingredients, and that carbon formed from burning is in fact a burnt form of the original substance.
- A candle, when it burns, simply melts, or changes to vapour in the air. Thus, there is a tendency to think of reactions in terms of physical change.
- Oxygen or air is an enabling ingredient in the burning process, but is not consumed in the process.
- Reactants retain their identity in a chemical reaction.
- Gases have negative weight.
- Effects that are due to gases, such as carbon dioxide extinguishing flames, are due to more visible effects such as bubbles.
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