Limewater is a clear, colorless liquid known in chemistry as a saturated solution of calcium hydroxide (\(\text{Ca}(\text{OH})_2\)). This compound is only slightly soluble in water. Limewater is primarily used as a reagent—a substance employed to cause a chemical reaction—making it an important tool in laboratory settings.
Required Materials and Safety Precautions
Creating this solution requires a clean, airtight container with a lid, distilled water, and calcium hydroxide powder (slaked lime or hydrated lime). Use only calcium hydroxide, not calcium oxide (quicklime), as the latter reacts vigorously with water and generates significant heat. Distilled water is preferred over tap water to avoid introducing dissolved minerals that could interfere with purity. Calcium hydroxide is an alkaline, caustic substance that can cause irritation and burns. Wear appropriate personal protective equipment, including chemical splash goggles and gloves, and work in a well-ventilated area to avoid inhaling the fine powder, which can cause respiratory irritation. If powder or solution contacts the skin or eyes, flush the affected area immediately with water; seek medical attention for eye contact.
The Process of Creating Limewater
Begin the preparation by mixing the calcium hydroxide powder with the distilled water inside the clean container. Because the goal is to create a saturated solution, a small excess of the solid must be added; a rough guide is about one teaspoon of the powder per liter of water. The mixture should be shaken or stirred vigorously for several minutes to ensure that the maximum possible amount of calcium hydroxide dissolves into the water. Since calcium hydroxide is only sparingly soluble, most of the powder will remain undissolved, giving the mixture a milky, opaque appearance.
After thorough mixing, the container must be sealed and left undisturbed for 12 to 24 hours. This allows the undissolved solid particles to settle completely to the bottom. The clear liquid at the top is the actual limewater, which must be separated without disturbing the settled solid.
The most critical step involves carefully transferring the clear liquid, either by decanting or filtering, into a new, clean storage container. Decanting involves slowly pouring the clear liquid off the top, taking care to leave the sediment behind. If filtering, a funnel lined with filter paper or a clean coffee filter can be used to capture any remaining fine particles, ensuring the final limewater is perfectly clear. Finally, the limewater must be stored in an airtight container to prevent atmospheric carbon dioxide from dissolving into the solution, which would react and compromise the reagent’s purity.
Practical Uses for the Solution
The most common application for limewater is as a simple chemical test for the presence of carbon dioxide gas (\(\text{CO}_2\)). When carbon dioxide is bubbled into the clear limewater, it reacts with the dissolved calcium hydroxide to produce a white, insoluble precipitate. This precipitate is calcium carbonate (\(\text{CaCO}_3\)), which is essentially chalk, and its formation causes the solution to immediately turn cloudy or milky.
This visible reaction is a standard tool in science education and laboratory analysis. Beyond the chemistry lab, limewater has traditional applications, particularly in construction and food preparation. Historically, it has been a component in certain lime washes used as a protective and decorative coating for walls. It is also occasionally used as an additive in mortars and plasters, where the calcium hydroxide aids in the material’s setting and hardening process.