2.6

## National STEM Learning Centre

Skip to 0 minutes and 10 secondsWe're now going to look at reacting hydrochloric acid and calcium carbonate together, sometimes known as marble chips or limestone. What we want to be able to do here is to allow students to start investigating the reactions where they can change three of the factors-- temperature of solutions, the surface area of the solid, and the concentration of the solutions. If students have carried out previous experiments on rates of reaction, they should be able to predict what will happen and explain why, and then can present results that either confirm or challenge their prediction. To change the surface area, we can use large marble chips, medium, or small. Sometimes you can get up to five different sizes of marble chips.

Skip to 0 minutes and 55 secondsWe can change the concentration of acids, starting at one level of hydrochloric and working down to 0.1 molar or less. For less able students, pre-preparing the solutions may be particularly useful. But for more able students, challenging them to produce their own serial dilutions, given a starting point of one molar, will help them get familiar with the mathematics behind it. Also, if they produce unusual solutions like 0.25, that will help them understand the ratios even better. When this reaction goes on, we produce carbon dioxide. So this can be measured in much the same way as we did with magnesium and hydrochloric acid. And shortly, we're going to show you three different ways of doing that practical.

Skip to 1 minute and 39 secondsHowever, be aware that carbon dioxide is slightly soluble in water, so there may sometimes be a delay before the reaction occurs. One word of warning. Don't do this with sulfuric acid. Sulfuric acid reacting with calcium carbonate produces an insoluble layer of calcium sulphate on the surface of the chips, and the reaction starts and stops almost immediately.

Skip to 2 minutes and 8 secondsIn this first reaction, we're going to perform the experiment on a balance. That means having a conical flask with about 30 centimetres cubed of our acid in, and the marble chips weighed out in a weighing boat on the balance to start off with. We will then pour the marble chips into the acid. We turn the weighing boat to the balance, and start the stop clock. Here we're measuring the mass loss as carbon dioxide is released from the open system into the surroundings. And this can be used as an equivalent to measuring the volume of gas.

Skip to 2 minutes and 46 secondsNow the setup we're using is very similar to the one that we used for magnesium and hydrochloric acid. We set up our measuring sound over our trough filled with water and our delivery cube inside. Our 30 centimetres cubed of hydrochloric acid is put into the conical flask. We add our calcium carbonate into our flask, put the bung in, and start the stopwatch. We're now going to measure how much carbon dioxide is produced per 10 seconds. So every 10 seconds, measure the volume of gases. We may not take the first two measurements, as the gas is slightly soluble, and so will not start coming out of solution immediately.

Skip to 3 minutes and 30 secondsIn the last version of this practical, we are using a glass gas syringe to measure more precisely the volume of gas produced. So in the reaction, we're using a conical flask with our 30 centimetres cubed of hydrochloric acid in, and a bung and delivery tube to the gas syringe, which is firmly mounted in a clamp and stand. Be aware that gas syringes are very fragile, and that the plunger can easily fall out. Here we've tied it on with some string, which limits its manoeuvrability, so it shouldn't drop out.

Skip to 4 minutes and 2 secondsHowever, as soon as the reaction reaches 100 centimetres cubed on our gas syringe , we need to remove the cork from the conical flask to prevent undue pressure building up in the system. Otherwise, we can record the volume every 10 or 20 seconds, much as we did in the last practical.

# What can students learn from effervescing limestone?

Reacting limestone (calcium carbonate) with acid on its own is a fairly simple reaction that students will have done earlier in their science education:

Carbonate + acid $\rightarrow$ carbon dioxide + a salt + water

CaCO3(s) + 2HCl(aq) $\rightarrow$ CO2(g) + CaCl2(s) + H2O(l)

However, without any context it’s just another reaction.

## Taking things further

If students have carried out previous reactions, and can safely use and manipulate the equipment, at 14-16 years now may be the time to set a challenge to ask students “what effects the rate of reaction between acid and limestone”?

They could investigate concentration of acid, surface area of the carbonate (big lumps, down to powder size) and the temperature of the reactants (by heating the acid, standing it in a bowl of hot water, for example).

Students can they apply their practical and science knowledge to give a quantitative prediction to how changing one factor at a time alters the rate. It also gives teachers a good idea of how students think scientifically: they could plan their practical in advance, possibly even for someone else to carry out, and receive feedback on how good their procedure was. This approach can help students with final assessment type questions, where application of knowledge is more important than just recall.

You could also ask them to show that the gas produced is carbon dioxide. You could ask them to link to both weathering of buildings from acid rain and the role of carbonates in holding onto carbon, particularly within rocks and the oceans.

Making links out to address the “why should I bother?” question, can help students better understand the science and its application to the world around them. Science needs to have authentic and relatable contexts, as we discussed last week.

Students can start to demonstrate their progression in understanding, by controlling more than one variable, such as investing the effects of temperature and concentration on a reaction, which will require more involved planning by the students.

## Share

We’d like you to inject a bit of creativity here. What else might you use limestone and acid reactions to teach? When you’ve come up with an idea, self-evaluate how effective that might be in conveying your chosen concept. Share your ideas and your reflection below.