C3 05 Lesson Plan Continued

c3_05 Lesson plan continued

Resources

Student Book pages 180−181 § Interactive Book: Practical investigations ‘PH meter’ § Interactive Book: Drag and drop ‘How is glass made?’ § Homework pack c3_05

Files on Teacher Pack CD: c3_05_practical; c3_05_technician

Equipment for main practicals

Learning outcomes

C3.3.1 recall that, even before industrialisation, alkalis were needed to neutralise acid soils, make chemicals that bind natural dyes to cloth, convert fats and oils into soap and to manufacture glass

C3.3.2 recall that traditional sources of alkali included burnt wood or stale urine

C3.3.3 understand that alkalis neutralise acids to make salts

Literacy focus: Following written instructions.

In this lesson students are learning to:

§ understand that alkalis have been used for many purposes for centuries

§ remember that early sources of alkalis were wood and urine

§ explain that alkalis neutralise acids to make salts

Key vocabulary

alkali § indicator § acid § salt § neutralisation

Obstacles to learning

There is a considerable amount of practical work in this lesson – students will need patience and skill to achieve the anticipated results.

Stimuli and starter suggestions

§ Display the term ‘alkali’ and ask students to write down a sentence saying what the term means to them. They may then read out their sentences. Note correct definitions and observations and misconceptions. Note the origin of the term ‘alkali’, given on Student’s Book p. 182, from the Arabic for a plant, ‘kali’, which was a source of alkali.

Learning activities practical c3_05

Low demand § If possible, all Foundation students should carry out the first experiment, Testing alkalis. However, if time is limited this can be left till the next lesson because the main experiment here, Making soap, takes some time. Or it could be done after the Making soap experiment has been set up, during the stirring period (see ‘Standard demand’ notes below). The results of the Testing alkalis experiment should show that alkalis are substances that turn litmus blue and that they neutralise acids. In addition, students find that wood ash is an alkali. Draw their attention to the ammonia solution – they may have already noted its smell – and explain that an early source of ammonia was urine, collected from households. Explain that these naturally occurring alkalis were found to have useful properties. Explain the neutralisation reaction of hydrochloric acid and sodium hydroxide using a word equation. You could demonstrate the formation of salt by heating a mixture of the acid and alkali to dryness. Students should write their own glossary of the new terms used in this lesson.

Teaching and learning notes: Students will have met acids and alkalis earlier but their knowledge will need reinforcing. Use different indicators to show the same pattern of behaviour.

Standard demand § All students should start the Making soap experiment as soon as possible. Make sure that attention is drawn to the hazard of the sodium hydroxide solution – see the technician sheet. The mixture will need to be stirred frequently for about half an hour. Foundation-tier students can do the Testing alkalis activity during this time; Higher-tier students can start the Dyeing activity (see ‘High demand’ notes below). When the experiment has been completed, discuss the students’ observations. They should have seen that the oil/alkali mixture turns into a paste and that solid bits of soap form when the salt is added. This soap should form a lather in distilled water. It may still contain sodium hydroxide so it is not a good idea to try it out.

Discuss the early sources of the alkali for the soap boilers and note that wood ash (potash or potassium carbonate) produces a soft, liquid soap while seaweed ash (soda, sodium carbonate) produces a hard soap. Also note that the soap was not used particularly for personal hygiene. Explain that limestone is another source of alkali (lime), after being heated in lime kiln. Discuss the applications of lime – on acid soils, to make mortar, as whitewash, and in glass making. (It was also put in the graves of plague victims and was an effective disinfectant.) Students can write their own accounts of the sources, properties and uses of common alkalis.

Discuss the cottage industries and skills that existed prior to the birth of the ‘modern’ chemical industry – i.e. soap boilers, dyers, lime burners, glass makers. Students could investigate these further.

Teaching and learning notes: Question students to assess their knowledge and understanding of acids and alkalis. This can be covered in the next lesson if students are not doing Higher-tier work on the reactions of alkalis.

Health and Safety: The soap-making activity presents hazards. Students must wear goggles and chemical-resistant gloves and lab coats are advisable. Refer to the technician sheet for details.

High demand § Students taking the Higher tier need not do the Testing alkalis exercise because this will be part of the practical work done in the next lesson. While these students are carrying out the Making soap exercise they can also perform the Dyeing experiment. This should show that cloth dipped in the ‘urine’ and alum solutions binds to the dye better than an untreated cloth. The treatment causes aluminium hydroxide (a ‘mordant’) to bind to the fibres of the cloth, and the dye then attaches to the aluminium. There may also be a colour change because the dyes often act as indicators. Students should make a record of their observations in all the experimental work.

Plenary suggestions

Quick-fire responses – ask students to name sources of alkalis (ashes of land plants and seaweeds, limestone (and chalk), urine) – and industries that used the alkalis (soap makers – for weavers, farmers – acid soils, dyers, glass makers) and name common alkalis (sodium, potassium and calcium hydroxides, ammonia, sodium and potassium carbonate).

Student Book answers

Q1 Ashes of burnt plants; urine.

Q2 The acid is neutralised and a salt is formed.

Q3 Making soap; making glass; treating acid soils; dyeing cloth.

Q4 Was required by soap boilers and glass makers.

Q5 Test with a suitable indicator; add it to an acid and see if the acid is neutralised.

Q6 Test-dye cloth with ammonia alone, alum alone, and both together – see which produces the cloth with the fastest dye.

Practical sheet answers

Testing alkalis

When the alkali is added to the acid, the colour of the litmus should change when approximately equal quantities have been mixed.

Q1 a) Red; b) blue

Q2 Answers include sodium hydroxide, sodium carbonate, ammonia, limewater (calcium hydroxide).

Q3 The acid is neutralised by the alkali forming a salt and water.