The chemical name for lime is calcium oxide, with the formula CaO. That said, “lime” is a broad term in chemistry, and it refers to several closely related calcium compounds depending on how the lime has been processed. The three most common are calcium oxide (quicklime), calcium hydroxide (slaked lime), and calcium carbonate (limestone or agricultural lime).
Quicklime: Calcium Oxide
When chemists or industrial workers say “lime” without any qualifier, they usually mean calcium oxide (CaO). It’s also called quicklime or burnt lime. It appears as an odorless, white or gray-white solid in the form of hard lumps and has a molecular weight of 56.08 g/mol.
Calcium oxide is produced by heating limestone (calcium carbonate) to extremely high temperatures, a process called calcination. The heat drives off carbon dioxide gas, leaving behind pure calcium oxide:
CaCO₃ → CaO + CO₂
Quicklime is highly reactive and caustic. When it contacts water, it releases significant heat and transforms into a different compound entirely, which brings us to the next form of lime.
Slaked Lime: Calcium Hydroxide
When calcium oxide reacts with water, it produces calcium hydroxide, Ca(OH)₂. This process is called slaking, which is why calcium hydroxide goes by the name slaked lime or hydrated lime. Commercially, hydrated lime is made by adding two to three times more water than the weight of calcium oxide in industrial hydrators.
CaO + H₂O → Ca(OH)₂
Calcium hydroxide is a strong base and is the form of lime most commonly used in water treatment. Municipal water systems add it to raise pH and precipitate the calcium and magnesium ions that cause hard water. It’s also widely used in construction (as a component of mortar and plaster) and in food processing.
Limestone: Calcium Carbonate
Limestone is the raw material that the other forms of lime come from. Its chemical name is calcium carbonate, CaCO₃. Technically it’s not “lime” in the strict chemical sense, but in everyday use, especially in agriculture, “lime” very often refers to ground-up limestone.
Agricultural lime (sometimes called aglime) is simply crusite calcium carbonate spread on fields to neutralize acidic soil. It can be calcitic, meaning it’s mostly calcium carbonate, or dolomitic, meaning it also contains at least 6% magnesium in the form of magnesium carbonate. Pure calcium carbonate is the reference standard for all liming materials, with a neutralizing value of 100%. Oyster shells and other shellfish are also mainly calcium carbonate, and ground shell is sometimes used as a liming material too.
The Lime Cycle
These three compounds are connected through a series of chemical reactions known as the lime cycle. It starts with calcium carbonate (limestone), which is heated to produce calcium oxide (quicklime) and carbon dioxide. Adding water to calcium oxide produces calcium hydroxide (slaked lime). And if calcium hydroxide is exposed to carbon dioxide in the air, it slowly converts back into calcium carbonate and water:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
This final step, called carbonation, is what makes lime mortar harden over time. The calcium hydroxide in wet mortar gradually absorbs carbon dioxide from the atmosphere and turns back into solid calcium carbonate, essentially re-forming limestone within the wall.
Which “Lime” You’re Looking For
- Quicklime / burnt lime: calcium oxide, CaO
- Slaked lime / hydrated lime: calcium hydroxide, Ca(OH)₂
- Limestone / agricultural lime: calcium carbonate, CaCO₃
If you’re answering a chemistry question, the expected answer is almost always calcium oxide (CaO). If you’re reading about water treatment or construction, the product in use is typically calcium hydroxide. And if the context is farming or geology, the substance is calcium carbonate.