Slaked Lime (Ca(OH)₂): Calcium Hydroxide
The Nature and Production of Calcium Hydroxide
What Exactly is Calcium Hydroxide?
Calcium hydroxide is an inorganic compound. At room temperature, it appears as a soft, white, crystalline powder. It is sparingly soluble in water, meaning only a small amount dissolves to form a solution called limewater. This saturated solution is mildly basic or alkaline, which is a key property behind many of its uses. The compound is ionic, consisting of calcium ions ($Ca^{2+}$) and hydroxide ions ($OH^{-}$). Its formation and reactions are classic examples of acid-base and precipitation chemistry.
A great classroom example is the limewater test for carbon dioxide ($CO_2$). When you bubble $CO_2$ through clear limewater, a milky white precipitate of calcium carbonate ($CaCO_3$) forms. The chemical equation is:
The "milky" appearance is the solid $CaCO_3$ suspended in the water. If you continue to bubble $CO_2$, the solution clears again! This is because calcium carbonate reacts with more $CO_2$ and water to form soluble calcium bicarbonate: $CaCO_3(s) + CO_2(g) + H_2O(l) → Ca(HCO_3)_2(aq)$.
From Limestone to Lime: A Two-Step Journey
Slaked lime is not mined directly; it is manufactured through a simple but dramatic two-step process starting from a very common rock: limestone (calcium carbonate, $CaCO_3$).
Step 1: Calcination – Making Quicklime. Limestone is heated strongly in a special furnace called a lime kiln at temperatures above 825°C. This thermal decomposition drives off carbon dioxide gas, leaving behind calcium oxide ($CaO$), known as quicklime or burnt lime.
Step 2: Slaking – Making Slaked Lime. Quicklime is highly reactive with water. When water is carefully added to calcium oxide, a vigorous reaction occurs, releasing a large amount of heat (exothermic). This process is called "slaking," and the product is calcium hydroxide, or slaked lime.
The result is a fine, dry powder. If excess water is used, it forms a paste called lime putty, which has been used in construction for thousands of years.
Key Properties in a Nutshell
The diverse uses of calcium hydroxide are a direct result of its physical and chemical properties.
| Property | Description | Why It Matters |
|---|---|---|
| Solubility | Low solubility in water (~1.7 g/L at 20°C). Forms a saturated solution called limewater. | Creates a mild, controllable alkali. The undissolved solid can be suspended as a slurry (milk of lime) for easy application. |
| Alkalinity (Basicity) | A strong base. pH of saturated solution is about 12.4. | Neutralizes acids, which is crucial for soil treatment, waste neutralization, and chemical manufacturing. |
| Reactivity with $CO_2$ | Reacts with carbon dioxide to form calcium carbonate. | This "carbonation" reaction is the secret behind how mortar and plaster harden and gain strength over time. |
| Precipitation Reaction | Hydroxide ions can react with metal ions to form insoluble metal hydroxides. | Used in water treatment to remove impurities like magnesium and heavy metals by settling them out as solids. |
A Multitude of Modern and Traditional Uses
Building and Construction: The Ancient Binder
This is one of the oldest and most important uses of slaked lime. For millennia, it has been mixed with sand and water to create lime mortar and lime plaster. Here's how it works: The wet paste is applied between bricks or on walls. It doesn't "dry" like mud; it undergoes a chemical transformation. The calcium hydroxide slowly reacts with atmospheric carbon dioxide to re-form solid calcium carbonate crystals that interlock, binding the sand particles together. This process is called carbonation.
This type of mortar is more flexible and breathable than modern Portland cement, which is why it is still prized for restoring historic buildings. It's also used in whitewashing, providing a simple, inexpensive coating for walls and fences.
Environmental and Industrial Applications
Calcium hydroxide is an environmental workhorse due to its alkalinity and low cost.
- Water Treatment: It is added to both drinking water and wastewater. It adjusts pH to prevent pipe corrosion, precipitates out unwanted metals and phosphates, and softens water by removing calcium and magnesium ions[1].
- Flue Gas Desulfurization: In coal-fired power plants, "lime slurry" is sprayed into exhaust gases. The alkaline calcium hydroxide reacts with acidic sulfur dioxide ($SO_2$), a major cause of acid rain, to form harmless calcium sulfite or sulfate sludge. $Ca(OH)_2 + SO_2 → CaSO_3 + H_2O$.
- Soil Stabilization: In agriculture and civil engineering, slaked lime is mixed with acidic or clay-rich soils. It raises the soil pH (a process called liming), making it more suitable for many crops, and improves the engineering properties of weak soil for building roads and foundations.
From the Kitchen to the Science Lab
You might be surprised to find calcium hydroxide in food and science experiments.
- Food Processing: It is food-grade calcium hydroxide, often called "pickling lime" or "cal." It is used in traditional methods for making corn tortillas and tamales. Soaking corn kernels in a limewater solution (a process called nixtamalization) loosens the hulls, improves nutritional value, and gives the distinct flavor and aroma. It is also used in pickling to make cucumbers extra crisp and in some sugar refining processes.
- Chemistry Education: As mentioned earlier, limewater is the classic test for carbon dioxide. It's also used to demonstrate precipitation reactions and the concept of pH and basic solutions.
- Dentistry: A paste of calcium hydroxide is used in some dental procedures (like root canals) because of its antibacterial properties and ability to stimulate tooth repair.
A Practical Example: Making Traditional Mortar
A Step-by-Step Historical Process
Let's walk through how a builder in ancient Rome or a restoration expert today might make and use traditional lime mortar. This connects all the chemistry we've learned into a tangible, real-world application.
Step 1: Source the Materials. They start with chunks of high-quality limestone or seashells (both are $CaCO_3$).
Step 2: Make Quicklime. The limestone is stacked in a kiln with layers of fuel (like wood or coal). The kiln is fired to over 900°C for several days. The heat drives off $CO_2$, leaving behind brittle, white lumps of calcium oxide ($CaO$). This material is now highly reactive.
Step 3: Slake the Lime. The quicklime is carefully placed in a pit or trough. Water is added slowly. The mixture hisses, steams, and heats up dramatically—this is the exothermic slaking reaction in action. It can take weeks for the mixture to fully convert and cool into a smooth, putty-like slaked lime ($Ca(OH)_2$ paste). Aging this lime putty for months improves its quality.
Step 4: Mix the Mortar. The lime putty is mixed with clean, sharp sand. A typical historical volume ratio might be 1 part lime putty to 2 or 3 parts sand. Water is adjusted to get a workable, but not runny, consistency.
Step 5: Apply and Let Chemistry Work. The mortar is laid between bricks or stones. Initially, it holds things together by suction and physical setting. But the real magic happens slowly over months and years. The calcium hydroxide in the wet mortar reacts with carbon dioxide from the air. It transforms back into sturdy, interlocking crystals of calcium carbonate—the same mineral they started with, but now acting as a glue. This process gives the mortar its long-term strength and durability, allowing structures to last for centuries.
This entire cycle—from limestone to quicklime to slaked lime and back to limestone—is known as the lime cycle, a perfect example of a chemical cycle in materials science.
Important Questions Answered
Q1: Is slaked lime dangerous to handle?
Yes, it requires caution. Dry calcium hydroxide powder or its solutions (like limewater) are alkaline and can cause irritation to the skin, eyes, and respiratory system. If ingested, it can cause serious internal burns. This is why food-grade calcium hydroxide is used in precise, small quantities in food preparation, and why construction workers wear protective gear. In the school lab, it is treated with the same respect as any other chemical—no tasting, and using gloves and eye protection if handling the solid.
Q2: What is the difference between quicklime, slaked lime, and limestone?
These are three different compounds in the lime cycle:
- Limestone ($CaCO_3$): The natural starting material, a sedimentary rock.
- Quicklime ($CaO$): Calcium oxide. Made by strongly heating limestone. It is very reactive, especially with water.
- Slaked Lime ($Ca(OH)_2$): Calcium hydroxide. Made by reacting quicklime with water. It is less reactive than quicklime and is the form used in most applications.
Q3: Can I make limewater at home for a simple science experiment?
Important: Only with adult supervision and proper safety awareness. You can purchase small amounts of food-grade calcium hydroxide. Add a tiny spoonful (less than 1/4 teaspoon) to a liter of distilled or boiled (and cooled) water in a clear jar. Stir well and let it settle overnight. The clear liquid above the settled white powder is a saturated limewater solution. You can carefully pour some into a small glass. Use a straw to gently blow your breath (which contains $CO_2$) into it. You should see it turn milky, proving the presence of carbon dioxide! This is a safe and fascinating experiment.
Conclusion
Slaked lime, or calcium hydroxide, is a chemical bridge between the ancient and modern worlds. Its journey from a simple rock to a versatile powder encapsulates fundamental chemical principles like thermal decomposition, exothermic reactions, acid-base chemistry, and precipitation. From holding together the pyramids and the Colosseum to treating our drinking water and giving tortillas their unique taste, its applications are a testament to its utility. Understanding $Ca(OH)_2$ is more than memorizing a formula; it's about seeing how a single compound, through its unique properties, has shaped human technology, industry, and cuisine for thousands of years. It remains a cornerstone material, proving that sometimes the simplest chemistry is the most powerful.
Footnote
[1] Water Softening: A process that removes calcium and magnesium ions from "hard" water, preventing scale buildup in pipes and appliances. Calcium hydroxide aids in this by precipitating these ions out as solids.