Chemical weathering of limestone: my breath and rock chippings from the car park: teacher’s notes Level This activity is designed for students aged 11-14. It can be used to reinforce work on the reactions of carbonates with acids as well as the chemical weathering of rock. Topic The chemical attack on limestone by rain that is naturally acidic (containing dissolved carbon dioxide) and ‘acid rain’ (rain that is more acidic because of dissolved pollutants such as sulfur dioxide and nitrogen oxides). Description Students blow through a straw into water containing a little Universal Indicator and note that the water becomes slightly acidic because carbon dioxide from their breath dissolves in it. They then add limestone chippings to the water and note that the solution gradually becomes neutral as the calcium carbonate in the chippings reacts with the acid. Teachers may prefer to do the activity using ground limestone (available from garden centres) rather than limestone chippings. This has the advantage of speeding up the activity (the colour change occurs in a few minutes rather than an hour or so) but the disadvantage of using a somewhat less familiar material). Context Pupils should know the pH colour scale of Universal Indicator; that green is neutral, that yellow, orange and red colours indicate increasing acidity and that green / blue, blue and purple indicate increasing alkalinity. They should also know that the air they breathe out contains more carbon dioxide than the air that they breathe in. Teaching points Avoid using the term ‘acidic gases’. Acids are only formed when gases such as carbon dioxide are dissolved in water; the gases are not themselves acidic. To show that the colour of the indicator is changed by the limestone and is not a natural reaction with the atmosphere, you may wish to set up a control at the same time that contains no limestone fragments. Timing To carry out the initial part of the activity and complete the table takes about 15 minutes, depending on how quickly the class works. Colour changes due to the limestone reaction take up to an hour. Apparatus Each student (or group) will need eye protection 250 cm3 beaker drinking straw Chemicals Each student (or group) will need A few limestone chippings (these can often be found on drives or car parks – check that they are in fact limestone by confirming that they fizz when hydrochloric acid is added). Alternatively marble chips (calcium carbonate) can be used. If ground limestone is to be used, several grams of this will be required (available from a garden centre). A little Universal Indicator solution in a dropping bottle with a colour chart. Safety notes Wear eye protection. Only allow one pupil to blow down each straw – then throw the straw away. Emphasise that pupils should not ‘suck up’ the water – blowing only is allowed. It is the responsibility of the teacher to carry out an appropriate risk assessment. The activity Students blow through a straw into water containing a little Universal Indicator and note that the water becomes slightly acidic because carbon dioxide from their breath dissolves in it. They then add limestone chippings to the water and note that the solution gradually becomes neutral as the calcium carbonate in the chippings reacts with the acid. Points to bring out Carbon dioxide from the respiration of animals (and of ourselves) is one factor that increases the carbon dioxide content of the atmosphere. The burning of fossil fuels also does this. Carbon dioxide dissolves in rain forming very dilute carbonic acid (this is the reaction that turns the indicator yellow when you blow into water). The dilute carbonic acid then attacks limestone. The reaction is a neutralisation reaction that turns the indicator neutral (green) again. Possible extension Discuss what happens when water containing dissolved calcium carbonate evaporates in caves to form stalactites (on the roofs of caves) and stalagmites (on the floors). Answers to questions Q 1. The only way to protect limestone is to remove it from the weather – by moving it indoors or giving it a weatherproof coating. Q 2. Soil water containing more carbon dioxide attacks limestone faster than rain water. This is an example of increased concentration increasing the rate of a reaction. Natural limestone ‘pavements’, such as those in the Yorkshire Dales, were formed largely by soil water activity. Q 3. Rain made more acid by dissolved pollutants such as sulfur dioxide and nitrogen oxides attacks limestone even more quickly than naturally acidic rain.