Calcium Carbonate (CaCO3) is an abundant chemical compound found naturally in chalk, limestone, marble, and marine shells. This compound consists of a calcium cation bonded ionically to a carbonate anion, forming a white, crystalline solid. While large quantities of CaCO3 are quarried directly from the earth, synthesizing it in a laboratory allows for the production of a purer, finely-controlled product known as Precipitated Calcium Carbonate (PCC). Always exercise caution by wearing appropriate safety gear, including gloves and eye protection, and ensuring the work area is well-ventilated.
Synthesis Using Soluble Salts
The most straightforward method involves a double displacement reaction between two soluble salts. The reaction relies on the principle that when the two solutions are combined, the resulting calcium carbonate is highly insoluble and quickly forms a solid precipitate. A typical reaction pairs Calcium Chloride (CaCl2) with Sodium Carbonate (Na2CO3). To begin, dissolve equal masses of each reactant, such as one gram, separately in approximately 10 milliliters of distilled water until both solutions are clear. The chemical equation for this exchange is represented as CaCl2(aq) + Na2CO3(aq) → CaCO3(s) + 2NaCl(aq).
Slowly add the sodium carbonate solution into the calcium chloride solution while gently stirring the mixture. A white, cloudy suspension will form immediately, signaling the formation of the insoluble calcium carbonate. The solid CaCO3 is the desired product; the byproduct, sodium chloride (common table salt), remains dissolved in the water. Continuous agitation encourages the formation of uniform, fine particles of the precipitate.
Synthesis Using Carbon Dioxide and Limewater
Another method for synthesizing calcium carbonate involves the reaction of Carbon Dioxide (CO2) with limewater. Limewater is a saturated, clear solution of Calcium Hydroxide (Ca(OH)2), which is slightly alkaline. This solution is often used as a direct chemical test for the presence of carbon dioxide gas.
When CO2 gas is introduced into the clear limewater, it reacts directly with the calcium hydroxide to produce solid calcium carbonate and water. This reaction is represented by the equation Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l). The appearance of a milky white cloudiness visually confirms that the CaCO3 precipitate has formed. Carbon dioxide can be introduced by bubbling the gas from a cylinder or by exhaling through a straw into the limewater. Avoid introducing excess carbon dioxide, as this can cause the calcium carbonate to re-dissolve by forming soluble calcium bicarbonate, making the solution clear once again.
Collecting and Purifying the Precipitate
Once the calcium carbonate has been synthesized, the next step is to separate the solid precipitate from the liquid reaction mixture containing soluble impurities. Separation can be accomplished through simple decanting, where the bulk of the clear liquid is carefully poured off the top of the settled solid. A more efficient separation is achieved through filtration, using filter paper in a funnel to capture the fine white powder while the liquid passes through.
The collected solid, known as the filter cake, must then be purified, as it holds traces of soluble reactants and byproducts like sodium chloride. Purification is achieved by washing the precipitate multiple times with small amounts of distilled water. The water dissolves the unwanted soluble salts, which are carried away through the filter paper, leaving the insoluble calcium carbonate behind.
The final step is drying the purified precipitate to remove all residual moisture. The damp filter cake can be spread thinly on a non-reactive surface and allowed to air-dry over 24 to 48 hours. For faster drying, the solid can be placed in a low-temperature oven, typically set around 105°C, for a few hours. The resulting dry, fine white powder is the high-purity Precipitated Calcium Carbonate.
Common Uses of Synthesized Calcium Carbonate
The synthesized calcium carbonate, often referred to as PCC, is valued for its fine particle size and controlled structure, making it suitable for applications requiring high purity. One common use is in the pharmaceutical industry, where it serves as a dietary calcium supplement and is an active ingredient in antacid tablets. It neutralizes stomach acid by reacting to form water and carbon dioxide.
The fine white powder is also used extensively as a filler material in the paper and plastic industries. In paper manufacturing, PCC improves brightness and opacity, allowing for a higher quality print surface. It is incorporated into household products, including abrasive cleaners and toothpaste, where its mild abrasive properties aid cleaning. The compound can also be used in environmental applications, such as adjusting the pH of water or soil due to its alkaline nature.