Throughout my teaching career, I have often found myself doing demonstrations because they are in the unit plan. I have, at times, not really used them with great intentionality. One of my goals this year is to be much more intentional with the demonstrations I use. This includes thinking carefully about the conceptual understanding I want students to reach, the learning progression, the questions I need to ask and the awareness I need to raise, and the things I need to take out or change in order to reduce cognitive load.
This activity uses the concept of ionisation energy and the demonstration of the reaction of the group 1 metals with water. The intention is to allow students to apply their understandings of electrostatic forces of attraction with respect to the conceptual understanding given below:
“The strength of the electrostatic force between charged particles is dependant upon the size of the charges and the distance between the charged particles”
For this activity to be successful, students should have an understanding of what electrostatic forces are and why might affect their strength between particles. They should also have an awareness that electrons exist in energy levels based on the strength of the forces of attraction between the electrons and the protons in the nucleus. The concept of first ionisation energy should be spoken about in very simple terms so that they can access the graph. You can simply explain that it is the amount of energy required to remove the valence electron from the atom.
Part 1: Using understandings to make a prediction
Students complete use their prior knowledge and the graph shown in figure 1 below to answer the questions on the slide in the same figure.
They then use this to make a prediction about how the reactivity of group 1 metals with water will change as you go down the group and add this to the table shown in figure 2.
Part 2: Making observations to test the predictions
1. The teacher should then carry out a demonstration of the group 1 metals reacting with water (make sure you are trained to do this and carry out a full risk assessment). Students are asked to write down their observations (what they see and hear) as they watch the demonstration. It is made clear that they can also write down observations made by the teacher, who is closer to the reaction.
2. The teacher should start by cutting the metals one after the other, starting with lithium and ending in potassium. As the teacher, you might need to state the following as you do this:
- The metals are all kept in oil as they will react with moisture in the air and oxidise easily
- The metals are easy to cut. Lithium is the hardest, sodium is next and then potassium is the easiest to cut.
- They are shiny on the inside.
The students should then be given time to write down any observations and discuss these with a partner.
3. The teacher should then start with the reaction of lithium with water. Remind the students to write down everything they see and here. It should be clear that bubbles are being produced but you may need to say this if the reaction is not vigorous enough. It should also be clear that the lithium is moving around on the surface of the water. You might also need to make it clear to the students that the lithium retains its shape (i.e does not turn into a ball) as they may not be able to see this. Give students time to write down their observations and discuss with a partner and repeat the demo with lithium, just in case students missed anything.
4. Now repeat this process with sodium. Once the demonstration is finished, you might like to ask students what observations they have made. Depending on their answers, you might need to bring to their attention that following:
- The sodium turned into a ball.
- There was a time delay on the sodium producing a flame (and you might need to make it clear that this is an observation. This is an important teaching point that not all observations are obvious. This means we can often miss important information)
At this point, you might like to ask students to talk about the differences between their observations for lithium and sodium.
5. Next, the teacher carries out the reaction of potassium with water. Follow the same principles as previously mentioned. However, this time, you might need to point out that the potassium catches fire almost immediately!
Give students a couple of minutes to write down their observations and discuss.
6. Finally, ask students to discuss and write down an answer to the question in the final box in figure 2: To what extent do your observations support your prediction? It is important that students explain their reasoning. Some supporting evidence they might make are:
- Lithium produced bubbles slowly when compared to sodium and potassium which suggests it reacted slower
- Lithium did not turn into a ball whereas sodium and potassium did. This means it might have released less energy which might mean it is less reactive.
- Potassium caught fire much quicker than sodium, suggesting that it released energy much more quickly.
There are probably more points. The important thing here is that students recognise the order of reactivity and can support this using their understanding of electrostatic attractions.
Reducing cognitive load
Since the main teaching point in this activity is to allow students to apply their understanding of how the strength of the electrostatic force of attraction between valence electrons and the protons in the nucleus affects reactivity, it is useful to remove extraneous ideas.
- For this demo, teachers often add universal indicator. For this activity, it has been deliberately left out. This is because talking about the production of alkaline substances is not useful to this teaching point. The demo can be repeated at a later more relevant date with this teaching point in made
- Teachers also talk about the production of hydrogen gas and the metal hydroxide. Again, this is not useful for the main teaching point and so is not touched upon here (other than the observation that bubbles are produced during the reaction).
- Teachers often ask students to write the balanced symbol equations for this reaction. Again, this is not supportive of the main teaching point. If you wish students to write the balanced symbol equation, it may be more useful to revisit the demonstration at a more suitable point.
Thanks for taking the time to read this post. I would love to hear some feedback from anyone trying this out. I would also be interested to hear from anyone who is interested in conceptual understandings in chemistry and how we might reduce cognitive load during lessons. Please, comment away! Constructive criticism always welcome.