Developing understanding of reacting ratios: gas volumes

Developing understanding is a series of resources that encourage learners to connect their thinking at the macroscopic, sub-microscopic and symbolic levels.

Learning objectives

1. Identify the gaseous product in a balanced chemical equation and describe how this will be observed.
2. Describe the ratio of the number of molecules or atoms reacting shown by the large numbers in a balanced chemical equation.
3. Compare the volume of product formed from different quantities (number of moles or mass) of a reactant.
4. Calculate the volume of product produced from a given volume of reactant or the volume of reactant needed to form a given volume of product.

How to use this resource

This resource aims to develop learners’ understanding of the how to calculate the volume of product formed from a given volume of reactant or vice versa. The questions encourage learners to think about what a balanced chemical equation means at the sub-microscopic level in terms of the ratio of atoms and molecules reacting. The questions then connect the idea of the ratio of moles of reactants and products to the calculation of the volume of reactants and products. As a result, learners should develop more secure mental models to support their thinking about this topic.

• When to use? Use after initial teaching or discussion of this topic to develop ideas further. You can also use as a revision activity.
• Group size? Suitable for independent work either in class or at home. Or use the questions for group or class discussions.
• How long? 15–30 mins

Johnstone’s triangle

Johnstone’s triangle is a model of the three different conceptual levels in chemistry: macroscopic, symbolic and sub-microscopic. You can use Johnstone’s triangle to build a secure understanding of chemical ideas for your learners.

Introduce learners to Johnstone’s triangle with our Decomposition of calcium carbonate Johnstone’s triangle worksheet which guides learners to use the relative formula mass of a reactant and the balanced chemical equation to determine the volume of gas produced.

Johnstone’s triangle and this resource

The icons in the margin indicate which level of understanding each question is developing to help prompt learners in their thinking.

• Macroscopic: what we can see. Think about the properties that we can observe, measure and record.
• Sub-microscopic: smaller than we can see. Think about the particle or atomic level.
• Symbolic: representations. Think about how we represent chemical ideas including symbols and diagrams.

The levels are interrelated, for example, learners need visual representation of the sub-microscopic in order to develop mental models of the particle or atomic level. Our approach has been to apply icons to questions based on what the learners should be thinking about.

Questions may be marked with two or all three icons, indicating that learners will be thinking at more than one level. However, individual parts of the question may require learners to think about only one or two specific levels at a time.

Support

This worksheet is ramped so that the earlier questions are more accessible. The activity becomes more challenging in the later questions. You can give extra explanations for the more challenging questions. If completing as an in-class activity it is best to pause and check understanding at intervals, as often one question builds on the previous one.

It is useful for learners to observe macroscopic properties first-hand. You could circulate examples of one mole of a substance in the classroom, run a class practical of a chemical reaction or show a teacher demonstration of properties.

Give learners physical models to use and manipulate, such as a Molymod™ kit or counters.

Additional support may be needed for any learners still lacking in confidence in the required symbolic representation, for example by sharing and explaining a diagram or a simulation that can show movement of the particles.