![]() ![]() Note that both the x and the y axis are labeled with a logarithmic scale (data source Wikipedia - Calcium Carbonate Solubility In the end one molecule of chalk, when dissolved with carbonic acid, creates one calcium ion and two bicarbonate ions.įigure 3 - The CO2 pressure needed to dissolve a certain concentration of chalk in water. Because this proton was lost to dissolving chalk we can say that the one molecule of chalk was also responsible for the creation of that bicarbonate ion. The other bicarbonate ion comes from the carbonic acids which lost a proton. The chalk molecule fell apart into a calcium ion and a bicarbonate ion. H 2CO 3 → HCO 3 - + H + HCO 3 - + H + + CaCO 3 → Ca 2+ + 2HCO 3. When carbonic acids becomes bicarbonate it looses a proton which can convert the carbonate ions added by the chalk to bicarbonate. If the bicarbonate or carbonate content of the water doesn't change its alkalinity won't change either.Īcids other than carbonic acid don't work. One of the reasons why we want to dissolve chalk in water is increasing its alkalinity which is caused by bicarbonate and carbonate ions. Looking at these results, we could have done this water modification much simpler by just adding calcium chloride as a salt. The result of the reaction between hydrochloric acid and the chalk is water, CO 2 (which escapes), chloride (Cl -) and calcium (Ca 2+). ![]() Hydrochloric acid for example which, in water, falls apart into a proton (H +) and a chloride ion (Cl -):ĢH + + 2Cl - + CaCO 3 → H 2O + CO 2 + 2Cl - + Ca 2+ Let's have a look at what happens when we add an acid other than carbonic acid. ![]() For acids we have basically two choices carbonic acid or other acids. After all that's what an acid does, it adds protons and as a result lowers the pH of a solution. To get these protons we need to add an acid. It is these protons that we have to supply to dissolve more chalk than the water can hold without any help. This is done by converting the carbonate into bicarbonate. The EPA (Environmental Protection Agency) recommends drinking water to have a pH between 6.5 and 8.5 īut how does this tie into dissolving chalk? Well, earlier we saw that we need to remove the carbonate that dissolving chalk contributes to the water in order to allow the chalk to dissolve. Figure 2 plots these relative concentration as % of the total carbonic acid + bicarbonate + carbonate amount over a pH range of 0 to 14.įigure 2 - The relative concentration of carbonic acid, bicarbonate and carbonate based on the pH of the solution. The more there is the more carbonic acid will exist and the less there is the more carbonate will be present. As mentioned earlier, the relative concentration of these 3 carbonate species, as they are called, depends on pH since it is pH which determines how much H + is available. When a bicarbonate ion looses a proton it becomes carbonate: if a bicarbonate ion picks up a proton it becomes carbonic acid. When a carbonic acid molecule looses one proton it becomes a bicarbonate ion: We looked at weak acids in Weak Acids and Bases where we saw that the loss or acceptance of protons depends on the pH of the environment. The interesting thing about carbonic acid is that it is a weak acid that can loose up to two protons (H +). But on average there is always a specific portion of CO 2 bound as carbonic acid. carbonic acid is formed and falls apart into CO 2 and water constantly. The "↔" means that there is a constant back and forth between both sides. Not all the dissolved carbon dioxide (CO 2) will form carbonic acid but some does according to this chemical equation:įigure 1 - The transformation of carbonic acid to carbonate over bicarbonate and back to carbonic acid Carbonic acid is formed when carbon dioxide is dissolved in water. To understand this we have to take a look at carbonic acid and the carbonate system. If the product is greater that that calcium carbonate will precipitate until enough calcium and carbonate have been removed from the water to satisfy the equation.īut how can we get calcium carbonate dissolved in water? We do this by removing the carbonate that chalk adds when it dissolves. What that formula means is that water can only hold calcium and carbonate in solution as long as the product of their concentrations is less or equal to 3.36×10 -9 (0.00000000336). ![]() The names in brackets () stand for the concentration of the particular ions. This is because the solubility product between calcium and carbonate is a very small number. If we know one thing about chalk in brewing it is that it doesn't dissolve very well in water. It's ions are calcium (Ca 2+) and carbonate (CO 3 2-). If you do not want to be bothered with the chemistry behind dissolving chalk you can safely skip ahead.Ĭhalk, also known as calcium carbonate (CaCO 3), is a salt of carbonic acid.
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