The chemistry of chalk Arthur B (L6)

I would like to start with what chalk is. Chalk, also referred to as calcium carbonate, has the molecular formula CaCO3. This displays a lot more about chalk than one might first anticipate.

What do these letters mean? Firstly, ‘Ca’ is calcium. Calcium is a group 2 element also commonly referred to as the earth metals. Ca has an oxidation number of 2 which means it wants to lose 2 electrons to obtain a noble gas configuration. This is a state where an atom has a full outer shell of electrons and is therefore more stable. This forms a strong ionic bond with the carbonate ion (CO3) due to the fact that after electron transfer, they have 2- and 2+ charges. These different charges attract very strongly and so the two differently charged ions interact and stick together. CO3 has an oxidation number of 2 as well. However, the carbonate ion wants to gain 2 elections rather than lose them. The ionic compound chalk is commonly known for its large white cliffs, but it is also extremely exciting in terms of chemical reactions.

Chalk (CaCO3) is a base. This means it is an insoluble alkali with pH above 7. As a base it has useful properties such as the fact it neutralizes acids. An example of this in the real world is Rennie tablets that are used to cure indigestion or heart burn. This works as the base doesn’t dissolve in your saliva and damage your oesophagus, but only reacts with hydrochloric acid once the tablet has reached your stomach. The equation of the reaction between hydrochloric acid and calcium carbonate goes as follows:

2HCl + CaCO3

CaCl2 + H2O + CO2

This basic part of chemistry has allowed a simple cure for a common discomfort.

The importance of chalk’s insolubility can be underestimated. Another example of this importance is the test for carbon dioxide. It is fairly common knowledge that if you were to pass CO2 through limewater or calcium hydroxide (Ca(OH)2) that the limewater would go cloudy. This is to do with calcium carbonate. As carbon dioxide is passed through the limewater it forms CaCO3. This is incredibly valuable as it is a definite proof that CO2 is present. The chemical equation for this reaction is:

CO2 + Ca(OH)2

CaCO3 + H2O

This is one of the most common chemical tests and is incredibly valuable throughout testing chemicals. The fascinating thing about this experiment is that if carbon dioxide is passed through the limewater continuously then the white precipitate (CaCO3) will disappear. This may appear to be a reversible reaction but that is not the case. This is because in excess carbon dioxide calcium hydrogen carbonate forms. This is soluble and as more CaCO3 is converted into Ca(HCO3)2 the solution turns colourless again. This reaction has the chemical equation:

CaCO3 + CO2 + H2O

Ca(HCO3)2

Chalk’s chemistry is also important for the formation of chalk streams. These are incredibly pure sources of water as chalk is a very porous and pure form of limestone. This means that it filters out other sediments whilst the water rises to the surface from chalk bedrock. Chalk is a very pure calcite which is made up of fragments of small sea creature shells. This makes it soft and porous which is why any sediment is filtered out and this creates the clean and pure water that chalk streams are renowned for.

For all these reasons chalk’s chemistry should not be overlooked in its importance as it has an interesting set of reactions that, upon further understanding, can provide great use in daily life, nature, and science.