Lesson plan: KS3 science – potions and precipitates for chemistry

  • Lesson plan: KS3 science – potions and precipitates for chemistry

Inspire curiosity about – and enthusiasm for – chemistry with these fantastic activities for KS3 learners from Dr Joanna Rhodes…

Inspire curiosity about – and enthusiasm for – chemistry with these fantastic activities for KS3 learners from Dr Joanna Rhodes…

Get back to the basics of chemistry in this engaging KS3 chemistry lesson to develop students’ skills in qualitative analysis. Use brightly coloured transition metal compounds and test tubes to interest and enthuse students while developing their skills of describing the results of analytical tests and learning the vocabulary we use to describe reactions such as ‘solution’ and ‘precipitate’. Collect gases and test them and make interesting cross-curricular links with biology. Discover how our bodies hold the key to making some chemical reactions happen.

The aim of this interactive lesson is to engage and enthuse students with some classic hands on chemistry. It combines colourful reactions with test tubes and pipettes, setting up visually impressive glassware and heating with Bunsen burners and hands on experiences with the gases found in air. “Pocket chemistry” looks at the virtual world of the chemistry lab and showcases some of the clean and safe ways students can practice their chemistry skills while the section “How Can Chemistry” encourages your most able students to discover how chemists have been able to solve problems and explain how things work.

Why teach this?

Chemistry is an exciting, engaging and colourful subject. It is also a facilitator for numerous careers and qualifications. This KS3 lesson is designed to captivate the imagination and trigger a curiosity about chemistry and what chemists can achieve and could achieve in the future. Year 8 students currently studying at KS3 will be the first to complete the new style science GCSEs for first teaching in 2016. This lesson introduces them to analytical techniques and vocabulary that will be familiar when they encounter it again in these more demanding courses. The move away from controlled assessment paves the way for us to encourage a much broader range of experimentation to take place in the classroom.

Welcome to Potions Class

As a vibrant start to the lesson show a teacher demonstration of the formation of a bright yellow precipitate of lead iodide from colourless lead nitrate and potassium iodide solutions [AR1]. Invite students to make and record observations relating to the reactions of transition metal ions in solution [AR2]. Use 0.5 moldm-3 solutions of copper (II) sulfate (blue), cobalt (II) chloride (pink), iron (II) sulfate (green), iron (III) sulfate (yellow), manganese (II) sulfate (pale pink) and nickel (II) sulfate (green). Use nickel as the teacher demonstration (see CLEAPPS guidance). These colourful solutions make gelatinous solids, that we call precipitates, on the addition of sodium hydroxide and also with the addition of ammonia. Copper sulfate continues to react with an excess of ammonia to make a beautiful deep blue/purple solution. Students will enjoy setting up test tubes and using dropping pipettes to carefully add small amounts of hydroxide and ammonia. Ask students to design their own table to record their observations and encourage good practice and scientific vocabulary in the way they record their findings. Key vocabulary includes test-tube, solution, concentration, pipette, precipitate and dissolve. An example of a good observation for the addition of ammonia to copper sulfate could be as follows:

“The clear pale blue solution produced a pale blue precipitate when I added ammonia solution, when I added more the precipitate dissolved to make a deep blue solution”.

The word “clear” is often confused with “colourless” giving you a good opportunity to correct this misconception. Extend more able students by asking them to describe what happens to the precipitate of iron (II) hydroxide formed if left in the air. By comparing this to iron (III) hydroxide they might be able to conclude that it is slowly turning into this substance (being oxidised). More able students would also find the word equations for the processes stimulating and could use an example to work out what may be happening in the other reactions [AR3]. For the most able students you could compare and contrast the copper ion complex formed when surrounded by water molecules and explain that it becomes deep blue when ammonia molecules surround it instead. Ask them to design an experiment to determine whether this reaction is reversible? [AR5].

1 – The Black Cauldron

There can be lots of substances dissolved in water. However, because these compounds are colourless we cannot see them and we do not know what is there. In the first part of this practical activity students use a Bunsen burner, tripod, gauze and evaporating basin to gently evaporate the water from a salt-water solution. They will collect the salt residue and could measure the mass of salt dissolved in water in milligrams per litre if mass and volume are carefully measured. Ask students what they could do to collect the water that has been evaporated as pure water? This can be a great opportunity to introduce them to some more complex glassware by carrying out a distillation of water containing ink or copper sulfate. Alternatives, with and without a Liebig condenser, can be found here [AR6] and here [AR7]. The separation of clear water from the dark ink solution is captivating. Make your classroom even more vibrant by using a range of different coloured inks bubbling in round-bottomed flasks set up around the classroom. Extend your more able students by re-arranging the glassware into the form of a reflux by mounting the condenser vertically above the round-bottomed flask. As a hinge question ask them to speculate why this configuration would not separate the two components but why it might be useful for getting something with a low boiling point to react for longer.

2 – Invisible but Interactive

In this activity students investigate properties of different gases by collecting them and using gas tests to determine which gases they are. Use magnesium ribbon and hydrochloric acid in test tubes. Students place their thumbs over the test tubes to collect the gas and then a partner uses a lighted spill to demonstrate a squeaky pop. Who can spot the red flash that sometimes goes with it? Next use 20 vol hydrogen peroxide and a spatula of manganese (IV) oxide powder in a conical flask to test for oxygen which relights a glowing splint. Always competitive, students will enjoy seeing how many times they can relight the splint during one experiment! Investigate the gas produced by respiration (carbon dioxide) by blowing with a straw into limewater and observing the solution turning cloudy. Linking in with the starter activity ask students what might we call this solid being produced in solution? Demonstrate the connection between chemistry and biology by showing how liver can also catalyse the production of oxygen from hydrogen peroxide [AR8]. Finely chop a piece of liver for use in the experiment and see how long this will continue to produce oxygen for using a glowing splint. Catalase – the enzyme in liver is an antioxidant in our bodies, which prevents the cell damage that can be caused by chemicals such as hydrogen peroxide formed from some of our cell reactions.

How to be a Chemical Detective

Turn your classroom into a chemical forensic lab as students test a set of samples to help solve a crime. In the first crime, someone has been poisoned and the scientists are trying to work out the location of the poison. Provide a sample of drinking water found at the scene. Students should use an evaporating basin to evaporate the solution to dryness to see if some solid is left behind. In the second crime a cyclist has to have a urine drugs test following winning his Seventh tour de France. You suspect he might have dissolved some iron (III) chloride powder in water in the toilets (which looks like urine) to avoid giving a sample. Ask students how they could test this theory? Addition of sodium hydroxide solution would give a rusty coloured precipitate showing that this is definitely not urine! Finally you are an environmental officer and you suspect that a company has been allowing effluent containing large amounts of chloride to escape from 3 different pipes into a local river killing fish. Students need to use silver nitrate solution to see if the samples go cloudy. If they do then the samples contain chloride. To extend students further ask them to be the expert witness while other students cross-examine them on the chemical techniques.

Pocket Chemistry

If you have access to tablet computers or if your school has a bring-your-own-device policy then why not try out some of these simulations and apps? Interested students can mix and model safely on their phone even carrying out “controlled explosions”!

Chemist – this is a portable chemistry lab. The 3D virtual lab enables students to conduct chemistry experiments, explore chemistry reactions and use different laboratory tools. Try mixing chemicals by pouring them into beakers or test tubes. You can also heat the chemicals with a Bunsen burner or carry out a titration.

ChemCrafter – this app lets you build your own lab to run fun and creative experiments. Use the chem-o-converter to measure energy released by a reaction and gain points that unlock new experiments equipment, and chemicals. Use your new supplies to craft more gases, liquids and solids!

ChemDoodle – allows students to draw chemical structures and view them in 3-D. Structures cannot be saved on a guest account but can be displayed on the whiteboard or drawn out in books. Challenge students to draw a molecule of paracetamol or aspirin. They could research an interesting drug and then try to produce a 3D drawing on the app.

ChemSpider – this allows you to search the Royal Society of Chemistry’s database of thousands of chemical compounds. What will you find?

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