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7: Year Acids and alkalis About this unit In this unit pupils: Learn about acids and alkalis as classes of chemicals with distinct properties and uses Use indicators to classify solutions as acidic, alkaline or neutral Use the pH scale to compare the acidity and alkalinity of different solutions Begin to explore neutralisation In scientific enquiry pupils: Recognise hazards and use information sources to assess risks associated with acids and alkalis Make and present qualitative observations Interpret qualitative observations, making comparisons and identifying simple patterns Investigate the effectiveness of different antacids, controlling appropriate variables This unit is expected to take approximately 7 hours. Section 1: What are acids and alkalis like and where do we use them? Objectives Children should learn: That many household materials are acids and are not hazardous Activities Outcomes Children: Elicit pupils' ideas about acids by asking them to work in pairs or small groups Identify some to suggest words they would use to describe an acid and examples of where acids, eg acids are used. vinegar, lemon juice Show pupils a range of household acids that can be safely handled, eg vinegar, fruit juices, and ask them to explore and describe the appearance and smell of these. If appropriate, pupils could be asked to taste drops of the Identify some solutions that are food products. Provide packaging or labels from additional everyday uses household materials (that do not have hazard warnings) and ask pupils to find of acids, eg in the names of acids contained in them. foods, medicines, cleaning Ask pupils to compare what they found with the ideas they had about acids, materials and to highlight differences. Points to note This activity could be used to reinforce the idea that not all colourless liquids are water. Safety o Eye protection should be used when working with acids and alkalis. Teachers should follow school procedures for dealing with spills and splashes Scrupulous hygiene must be observed in all tasting activities o Section 2: What are acids and alkalis like and where do we use them? Objectives: Children should learn: To recognise and interpret common hazard signs To select key ideas from written material How to deal with acids or alkalis if they are spilt or splashed on the skin That adding water to an acid or alkali solution dilutes it and makes it less hazardous Activities Outcomes: Children: Provide pupils with containers for household and laboratory acids Identify hazard symbols and alkalis with hazard warning labels, student safety sheets, for harmful, irritant and Hazcards and information about hazard symbols and ask them to corrosive substances describe the distinctions between them and why the materials they Describe how to work used in the previous activity did not have hazard labels. Ask pupils to safely with acids and suggest how the acids and alkalis could be made less hazardous. alkalis and what to do if a Consider hazard warning signs used when acids and alkalis are spill occurs transported by road. Explain that if water is Ask pupils to suggest why different acids and alkalis are labelled added to an acid or alkali differently and to suggest safety procedures for working with acids it dilutes it and the and alkalis. Agree a common set of procedures, which should solution becomes less include use of eye protection. hazardous Points to note At this stage common names, eg caustic soda, bicarbonate of soda, can be used where appropriate. Adding water to concentrated sulphuric acid is hazardous because the reaction is highly exothermic. Concentrated sulphuric acid should be diluted by adding the acid to water. Section 3: How can acids and alkalis be identified and distinguished from each other? Objectives: Children should learn: To devise a table to show results effectively and to identify patterns in these That acids and alkalis can change the colours of some dyes and that this can be used to classify them The names of some common laboratory acids and alkalis To classify solutions as acidic or alkaline, using indicators Activities Establish, by quick questioning, that pupils recall key ideas about safety. Provide pupils with solutions of dyes extracted from plant material, eg red cabbage, raw beetroot, blackcurrant, litmus, and ask them to explore and record the effect of adding household and laboratory acids and alkalis to the dyes. Compare the results with different dyes and establish, by comparing results from the class, that there are two classes of solution and that the dyes can indicate which is which. Introduce the terms 'indicator' and 'alkali'. Outcomes: Children: Present their results in a way which helps them identify patterns Recognise that solutions of dyes which show one colour in acids and another in alkalis are called indicators Recall the names of some common laboratory acids and alkalis Provide pupils with a range of acidic and alkaline solutions and Use their record of results to indicators and ask them to use the indicators to identify and identify which solutions are record whether the solutions are acidic or alkaline. acidic and which are alkaline Points to note Pupils could extract the dyes from plant materials themselves if they are familiar with the techniques required. Safety Avoid the use of bleach, drain cleaner or descaler. 0.4 mol dm-3 laboratory acids and alkalis can be used for this activity. None are hazardous although some are irritants, and eye protection is needed. In general, alkalis are more hazardous to skin and eyes than acids of a similar concentration Section 4: Is there a range of acidity and alkalinity? Objectives: Children should learn: That universal indicator gives a range of colours in acidic and alkaline solutions That pH numbers indicate how acidic or alkaline a solution is That neutral solutions are pH7, acidic solutions below 7 and alkaline solutions above 7 Activities Demonstrate the use of universal indicator paper or solution to obtain a pH number for a few solutions previously tested. Ask pupils to explore the range of pH of solutions previously tested (including some neutral solutions) and to see if they can relate them to earlier work on hazard labels. Outcomes: Children: Identify the pH of a given solution from an appropriate colour chart Classify the solution as strongly or weakly acidic or alkaline, or neutral Relate their classification to the use of the acid or alkali and associated hazards Points to note Full-range universal indicator pH0-14 should be used for this activity. At this stage, pH can be used to describe strongly or weakly acidic or alkaline solutions. It is not necessary to discuss the difference between weak and strong, and dilute and concentrated, although the appropriate terms should be used. Extension: pupils could asked be to predict the effect on pH of making an acid more dilute or more concentrated and how this would affect its corrosiveness. Section 5: What happens when an acid is added to an alkali? Objectives Children should learn: That acids and alkalis are used in a range of everyday situations How to find information, eg using contents, index, glossary, key words, hotlinks To select and note appropriate information about uses and effects of acids and alkali Activities Outcomes Children: Ask pupils to use a range of reference sources including advertisements Identify uses of to find information about the use of acids and alkalis in a range of acids and alkalis everyday situations, eg hair and skin care, treatment of stings and bites, and the benefits treatment of indigestion, food preservation, treatment of soil. Ask them to and potential find out how living things use acids, eg ants, nettles, humans in hazards of these digestion. Agree with pupil's key words they need to use and remind them how to use an index, glossary or search facility. Ask them to Communicate identify and note key points about the range of pH used and potential clearly key points harmful effects, and to explain these to other pupils either orally or by about a particular contributing to an information folder. use of acids or alkalis Points to note Advertisements for skin-care and hair-care products could be used to introduce this activity. Most pupils will need to be guided towards suitable sources of information, eg websites such as www.miamisci.org. Section 6: What happens when an acid is added to an alkali? Objectives Children should learn: That when an acid is added to an alkali, it lowers the pH That a neutral solution can be obtained by adding an acid to an alkali Activities CD-ROMs, or from databases Points to note ICT: pH logging using ICT could be used to record changes and generate a graph. Teachers may wish to emphasise that acidity and alkalinity are measured on a continuous scale. Extension: pupils could be asked to describe what computer-generated graphs show about the way pH changes as more alkali is added. Extension: pupils could be asked to investigate changes in temperature during neutralisation and be introduced to the idea that a chemical reaction is taking place. Safety o 0.4 mol dm-3 solutions of acid and alkali can be used. These may be irritant, so eye protection is needed Outcomes Children: Ask pupils to explore what happens to the pH when Describe that when an acid is added to a solution of an acid is added drop by drop to a an alkali, the pH of the mixture falls and solution of an alkali. Challenge pupils to predict what vice versa will happen if more acid is added, or if alkali is added Explain how to obtain a neutral solution to an acid, and test their predictions using a pH monitor and data logger. Find information in reference books, on Section 7: Checking progress Objectives Children should learn: To summarise key ideas about acids, alkalis and neutralisation Activities Outcomes Children: Help pupils to work in groups to produce questions about acids, Identify key questions alkalis and neutralisation, which will help clarify their thoughts. Pupils about acids and alkalis ask others to answer or find the answers to their questions. Use all the questions and answers and, together with the pupils, agree a Use correct scientific summary of key ideas. terminology in answering questions Points to note Later activities in this unit offer the opportunity for consolidation of key ideas for pupils who need this. Extension: pupils could also be asked to explore whether mass is conserved as neutralisation takes place. This idea is covered more fully in unit 9 'Using chemistry'. Section 8: Where is neutralisation important? Objectives Children should learn: How to frame a question that can be investigated How to use preliminary work to help decide what to measure or to observe To choose appropriate techniques and equipment To compare their investigative method and evidence collected with those of others To work with others in summarising infor...