Conservation of mass | Structure strip

Structure strips can be used to promote independent writing. Learners write independently about mass using key terms, calculate the mass of substances, explain changes in mass and conservation of mass.

Learning objectives

1. Define the term conservation of mass.
2. Calculate the mass of substances using the law of conservation of mass.
3. Explain changes in mass when gases are reactants or products in a reaction.
4. Write independently about conservation of mass to describe and explain the results from chemical reactions.

Introduction

The law of conservation of mass is a key concept when learning about chemical reactions. It states that no atoms are lost or made during a chemical reaction. This means that the total mass of reactants equals the total mass of products. Sometimes, the mass might appear to have increased or decreased during a chemical reaction.

The mass may seem to increase when one of the reactants is a gas and isn’t included in the initial measurement of mass, e.g. the oxidation of a metal:

reactants → products

magnesium + oxygen → magnesium oxide

The mass may appear to decrease when one of the products is a gas and isn’t included in the final measurement of mass, as it has escaped the reaction vessel, e.g. the thermal decomposition of a metal carbonate:

copper carbonate → copper oxide + carbon dioxide

The law of conservation of mass has many real-world applications, from making drugs in the pharmaceutical industry to monitoring the release of gases that can pollute the environment.

How to use structure strips

Structure strips are a type of scaffolding you can use to support learners to retrieve information independently. Use them to take an overview at the start of the topic, to activate prior knowledge, or to summarise learning at the end of a teaching topic. For more ideas on how to use structure strips with your learners, visit 5 ways to use structure strips effectively. 

Structure strips have sections containing prompts, sized to suggest the amount that learners must write. Learners glue the strips into the margin of an exercise book and write their answers next to the sections, in full sentences. When learners have finished using the structure strip, they should have an A4 page set of notes and examples.

The strips are printed five to a page and will need to be trimmed to size. Find them on the second page of the student sheet.

Scaffolding

• Encourage learners to use the suggested key words and phrases in their answers.
• To further support learners, include additional prompts in the structure strip. If learners are struggling to engage with the task, supply them with sentence starters created from the model answers.
• As learners grow in confidence, ask them to attempt the extension question first and then use the structure strip to improve or self-assess their answer.

Key words

Mass, conservation of mass, reactant, product, word equation, balanced symbol equation.

Metacognition

This resource supports learners to develop their metacognitive skills in three key areas.

• Planning: the strips provide scaffolding to plan the written response. Learners will decide where to gather information from (textbooks, own notes, revision websites). Ask learners: is the source of information you are using reliable?
• Monitoring: learners are prompted by the questions in the structure strip and can check their own answer against the prompts. Ask learners: have you covered all of the prompts in the space provided? Do you need to change anything to complete the task?
• Evaluation: learners can self-assess or ask a peer to check their work against the answers. Ask learners: did you achieve what you meant to achieve? What might you do differently another time?

Answers

Suggested answers for the structure strip activity are given in the downloadable teacher notes.

Answers to extension question

The answer could be written as a social media post, a script to a podcast or even in the style of an email, using professional greetings and full sentences.

Find below examples of points that could be included. These have been separated into sections about each individual reaction:

To understand your results, you need to be aware of the law of conservation of mass. This states that no atoms are lost or made during a chemical reaction, so the total mass of reactants equals the total mass of products.

1. Thermal decomposition of copper carbonate: copper carbonate → copper oxide + carbon dioxide

In this reaction the mass has decreased by 44 g. The mass of reactants and products is expected to be the same, however in this reaction a gas is produced. Carbon dioxide will leave the reaction vessel, so those atoms are no longer contributing to the mass you have measured. This means that 44 g of carbon dioxide was produced.

2. Reaction of an acid with an alkali: hydrochloric acid + sodium hydroxide → sodium chloride + water

In this reaction the mass has remained constant. The mass of reactants and products are the same, as none of the reactants or products are gases. So all of the atoms remain in the reaction vessel.

3. Oxidation of magnesium: magnesium + oxygen → iron oxide

In this reaction the mass has increased by 1.4 g. The mass of reactants and products is expected to be the same, however in this reaction oxygen is a reactant and is a gas. This means it is not included in the initial measurement as the atoms are in the air. The product is a solid however, so all the atoms of oxygen which have reacted are now in the crucible. This means that 2.4 g of magnesium reacted with 1.4 g of oxygen.