3. Investigating reactions of acids
One of the reasons that teachers sometimes give for not doing practical work in their classes is that it takes up too much time and they will not be able to finish all the work they need for the exam. But the exam questions often assume that the students have done, or at least talked about, practical work. Having the opportunity to handle equipment and different substances can help students to retain factual knowledge. A carefully designed experiment can be used to illustrate scientific ideas. Case study 3 and Activity 3 describe two slightly different experiments, but the principle is the same: the practical illustrates the theory you want them to learn. Case study 3 involves an experiment that is very relevant to the exam and shows how the teacher helps the students to make the connection between what they are learning in class and the exam questions they will have to answer. Activity 3 describes a class experiment in which your students make a sample of a salt. In both cases the emphasis is on following the instructions carefully, making observations and working accurately.
Case study 3: Observing acids and metals
Mrs Boke was going through Kenya Certificate of Secondary Education Chemistry past examination papers and came across a question on the reaction of metals with acids. She decided to investigate this reaction with her lower secondary class. She assembled a variety of metals available in the school laboratory and within her environs and organised her class into five groups according to ability level. She chose a leader for each group and asked the group leaders to collect the metals and acids from the front bench and distribute them between the groups. Mrs Boke wrote clear instructions on the chalkboard. She asked each group to follow the instructions and add a few drops of hydrochloric acid to the test tube containing the metal. As the students were performing the experiments, she wrote a number of questions on the chalkboard to guide them. For example, does it fizz? How fast does fizzing occur? What happens to the metals? Does the test tube get hot? Does the solution change colour? Do different metals react at the same rate?
After the experiment, she asked the group to record their observations for each reaction and used the observations to determine the rate of reaction of metals with acids.
She asked the groups to construct a reactivity series in their notebooks for the metals tested. She also asked the groups to test for the gas produced. While the students were carrying out the experiments, Mrs Boke spent most of her time with the group with the weakest students to ensure that they followed the instructions, that every student got involved and each recorded the observations in their exercise books.
Towards the end of the lesson, Mrs Boke picked up the chemistry examination paper and read to the class the question on the reaction of metals with acids. The class discussed the question by relating it to what they did in the experiment. Many of the students left the class satisfied with what they had done in the class practical and realised that most of their class activities are relevant to the KCSE examinations.
Activity 3: Think-pair-share to make a salt
When an acid reacts with a base, a salt is formed. Salts are useful substances (Resource 4) and your students will need to know how they are made. Before the lesson, write out the steps for the experiment on the board (Resource 5). Number each one, but write them in the wrong order. At the beginning of the lesson, ask each student to put the steps in the right order. Then get them to compare their answers with a friend and agree the correct order. Each pair should then compare with another pair and so on, until the class agree on the correct order for the steps. This will ensure that they really engage with the method and are more likely to do the experiment successfully and remember the method. This technique is called ‘think–pair–share’ and you will find that it is useful in many contexts.
If you have enough apparatus they could perform the experiment themselves, otherwise you could demonstrate the method, getting your students to take part. Make sure you ask lots of questions to keep them interested in the demonstration.