Exploring a Method of Carbon Sequestration With Chalk (Calcium Carbonate)

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The production of Calcium Hydroxide (Ca(OH)2 or lime water through the thermal decomposition of Calcium Carbonate for demonstration in the sequestration of CO2 from the atmosphere. When Calcium Carbonate (CaCO3) is heated to extreme temperatures, it decomposes forming calcium oxide and producing CO2. The calcium oxide (CaO) will react with water, forming calcium hydroxide.  The research will test the application of calcium hydroxide in sequestering CO2 from the environment. It may prove to be an efficient way of removing the CO2 with which industrial practices pollute the air.

Calcium Carbonate

Calcium Carbonate, a salt of carbonic acid and calcium, has three main forms. Most notably, calcium carbonate forms chalk, a substance used as a writing implement for thousands of years. Chalk is a loose, microcrystalline form of calcium carbonate, created in the calcification of deceased microorganisms. Deposits of calcium carbonate, if subject to geologic heat and pressure will become the metamorphic rocks limestone and marble. Pure marble, the likes of which Michelangelo used in his sculptures, is very rare and precious. Marble is often used as kitchen counter surface and as high-end outdoor flooring, decoration and construction.

Calcium Carbonate is said to be the most versatile and useful material known to man. Its powdered form has applications as an additive to paint, plastic, adhesives and paper to enrich color, texture and smoothness as well as a filler to increase volume and perfect consistency. It is often taken in pill form as a dietary calcium supplement and applied to toothpaste, baking powder and many other products. Structurally, calcium carbonate is used not only in the form of marble but as an additive to cement, mortar, rubber. When thermally decomposed, calcium carbonate forms lime, and important element in the production of glass and steel. 

Calcium Hydroxide

Calcium Hydroxide, also known as slaked lime or hydrated lime, is formed by the addition of a hydrogen atom to calcium oxide. This is achieved simply by mixing calcium oxide with water  { CaO + 2H2O → Ca(OH)2 +H2 } . Calcium Hydroxide is often used in whitewash, mortar and plaster. It is an antacid and is injected into the waste of metal machining to neutralize acidic byproducts.


            An emerging use of Calcium Hydroxide is as an additive to seawater to absorb the CO2 the most prominent greenhouse gas. It has been suggested that this process could help lower atmospheric CO2 levels to pre-Industrial Revolution rates. This research will investigate the most efficient way to employ this characteristic of Calcium Hydroxide by cyclically reforming it from the calcium carbonate produced in this reaction. The CO2 produced by the decomposition of calcium carbonate will be contained and in a real life application, sequestered in one of the many emerging methods of burrowing away atmospheric CO2.


  1. Place about 0.200g of calcium carbonate (CaCO3) in a large test tube. Using stoichiometry, calculate the expected mass of calcium oxide (CaO) that would form if the CaCO3 were to decompose entirely.

  2. Apply extreme, direct heat from a Bunsen burner for 10 minutes. No obvious change may take place but if the CaCO3 is solid, it may crack and pop as CO2 escapes.

CaCO3(s) + (heat) è CaO(s) + CO2(g)

  1. At the end of the heating period, re-mass the test tube to find the mass of the product. This mass should be half way between the initial mass of CaCO3 and the mass of the product or less to confirm that there’s a sufficient amount of CaO produced.

  2. Fill the test tube ¾ of the way and use a stirring rod to stir for about 2 minutes. This will ensure that all CaO has reacted with the water to form Calcium Hydroxide (Ca(OH)2).

CaO(s) + H2O(l) è Ca(OH)2(aq)

  1. The remaining CaCO3 is causes the cloudiness in the solution and must be filtered out. Use filter paper and a funnel to drain the solution into another test tube. The resulting solution should be transparent. Test this solution with pH paper.

  2. Prepare the apparatus diagramed below in order to bubble CO2 from dry ice through the solution.

  3. Test the pH of the solution every 30 seconds. Record observations.

 Data and Results

Thermal Decomposition of Calcium Carbonate

Initial CaCO3

0.193 g

Stoichiometric expectations of CaO Product

0.108 g


0.140 g

CO2 lost

0.053 g

CaO produced

0.067 g

Ca(OH)2 produced

0.088 g

The above calculations show that 0.067 g of CaO was produced in the thermal decomposition of Calcium Carbonate. When reacted with water in step 4, 0.88 g of Ca(OH)2 are formed. After filtration, testing this solution with pH paper reveals an extremely basic pH of 13.

Carbonation of Calcium Hydroxide

Time (min)




















The data show that as acidic CO2 was bubbled through the solution, the pH dropped to 7, neutral.

During step 7, a cloudy white precipitate was observed in the test tube.


The results show conclusively that each stage of the demonstrated cycle was a success and proceeded as expected. The amount of CaO produced was calculated to be 0.067 g and it’s presence was confirmed when the pH of the resulting solution of Ca(OH)2 was found to be extremely basic. In the carbonation of the Ca(OH)2 solution the pH was observed to decrease to neutral after 2.5 minutes. This suggested that the following reaction was taking place. The reaction was confirmed by the presence of a cloudy precipitate: CaCO3.

Ca(OH)2(aq) + CO2(g) èCaCO3(s) + H2O(l)

            The intriguing part of this study is in its application in the real world. Carbon sequestration. Carbon sequestration is a chemical process that would allow humans to take responsibility for the exacerbation of global warming. The goal of is to remove carbon from the atmosphere and in some way, facilitate its placement back into the earth. This may involve injection into subterranean aquifers, oil wells, landfills, etc. The intent is to make oxidation of the carbon impossible so it can’t be released into the atmosphere again.

Many different methods can be used to achieve this goal. This research postulates that the exploitation of the demonstrated cycle may be an even more efficient and practical way of carbon sequestration.

Calcium Carbonate is an extremely abundant compound that has already found uses in manufacturing many common items and materials. Thus obtaining some for this purpose wouldn’t be very costly, although mining and shipping the compound would counter the cause of CO2 reduction.

Thermal decomposition is the greatest road bump in this theory, as burning the fuel or using the electricity require to produce a given amount of CaO would certainly offset the benefits of the cycle. This problem may be overcome either by utilizing wasted heat or useful byproducts of other industrial processes or by using renewable wind or solar energy to heat the Calcium Carbonate. Also, a more efficient heating method, such as an oven would be a must. The Bunsen burner method used in this experiment allows too much energy to escape. Where as decomposition in this experiment allowed the CO2 produced in the decomposition reaction to escape, it is at this stage that the CO2 must be contained and stored, as is the final goal of carbon sequestration.

The production of Ca(OH)2 from CaO shouldn’t produce any energy problems, as no activation energy is required. However, this will produce an extremely basic, from which point, strong precautions must be taken with handling the substance.

In the actual sequestration stage, many methods could be designed for interfacing atmospheric air with the solution. One idea involves misting the solution into the air. This would allow for very rapid absorption of CO2 as the surface area of the water as a whole would increase drastically. The only issue would be containing recapturing the Calcium Carbonate produced so that it may be dried and reused in the experiment.


            Much to the surprise of the researcher, an article was found online in research concerning carbon sequestration with calcium hydroxide. The method described was not cyclic but simply involved adding great amounts of Ca(OH)2 to the ocean. It cited the ocean as being the greatest carbon sink of the world, as millions of algae and phototrophic plankton carry out photosynthesis, producing oxygen. It cited Ca(OH)2 as having two benefits. Of course, the benefit demonstrated above is the same being exploited in the ocean, but the alkaline water absorbs acidic CO2 more readily, as well, expediting the process.

            My two concerns about this method are that the extreme alkalinity of Ca(OH)2 would have negative effects on the marine ecosystem and the failure to exploit the cycle I’ve demonstrated. While calcium carbonate is not harmful to the ocean (it’s even produced by microscopic shellfish and deposited on the sea floor when they die), the continued mining and extracting processes to produce calcium hydroxide for this task only work to diminish its effects.


“A Dash of Lime — a New Twist That May Cut CO2 Levels Back to Pre-industrial Levels.” PhysOrg.com – Science News, Technology, Physics, Nanotechnology, Space Science, Earth Science, Medicine. 21 July 2008. Web. 09 Mar. 2011. <http://www.physorg.com/news135820173.html> .

“Calcium Carbonate.” Home. Web. 20 Mar. 2011. <http://www.ima-na.org/calcium-carbonate>.

“Calcium Hydroxide — Infoplease.com.” Infoplease: Encyclopedia, Almanac, Atlas, Biographies, Dictionary, Thesaurus. Free Online Reference, Research & Homework Help. — Infoplease.com. Web. 20 Mar. 2011. <http://www.infoplease.com/ce6/sci/A0809855.html>.

“Thermal Decomposition of Calcium Carbonate.” Home – Practical Chemistry. 29 Oct. 2008. Web. 09 Mar. 2011. <http://www.practicalchemistry.org/experiments/thermal-decomposition-of-calcium-carbonate,282,EX.html>.


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