NaCl dissolves easily in water, while CaCO3 does not. Table salt breaks apart into ions that mix evenly with water, but chalk and limestone remain mostly undissolved unless the water is acidic.
Why NaCl dissolves in water
NaCl, or table salt, dissolves quickly in water because water molecules are polar. The oxygen end of water attracts the positively charged sodium ions (Na⁺), while the hydrogen end pulls on the negatively charged chloride ions (Cl⁻). This pulls the ions away from the crystal and surrounds them in a hydration shell, allowing the salt to dissolve completely.
The energy released when water molecules form hydration shells around the ions is greater than the energy holding the crystal together. This makes the process spontaneous and efficient. At room temperature, about 360 grams of NaCl can dissolve in one liter of water, forming a saturated solution.
Why CaCO3 does not dissolve in pure water
Calcium carbonate (CaCO3), found in chalk, limestone, and seashells, has a very stable ionic lattice. The calcium ion (Ca²⁺) and carbonate ion (CO₃²⁻) are strongly attracted due to their double charges, making the crystal hard to break apart.
Water molecules cannot provide enough energy to overcome this strong lattice. As a result, only a tiny amount dissolves—about 0.013 grams per liter. In everyday terms, this means CaCO3 appears completely insoluble in pure water.
The role of water’s polarity in dissolving salts
Water’s bent shape and uneven electron sharing create a polar molecule with a positive and negative end. This polarity allows water to interact strongly with charged particles in ionic compounds.
For NaCl, this interaction is strong enough to dissolve the salt. But for CaCO3, the carbonate ion spreads its charge over multiple atoms, reducing the attraction to water molecules. This makes hydration less effective, so the compound stays solid.
How acidity allows CaCO3 to dissolve
While CaCO3 doesn’t dissolve in neutral water, it reacts with acidic solutions. Rainwater, which is slightly acidic due to dissolved CO₂, can slowly dissolve limestone over time.
When vinegar (acetic acid) or stomach acid (hydrochloric acid) contacts chalk, it produces carbon dioxide gas and soluble calcium salts. This is not simple dissolution but a chemical reaction: the carbonate ion reacts with hydrogen ions to form CO₂ and water, freeing calcium ions into solution.
Environmental and biological importance
CaCO3’s low solubility makes it a key building block in nature. Coral reefs, seashells, and limestone formations are made of it. Its stability helps buffer ocean pH by neutralizing added acids.
However, rising CO₂ levels are making oceans more acidic, which increases CaCO3 dissolution. This threatens marine organisms that rely on it for shells and skeletons, especially those using the more soluble form aragonite. In contrast, NaCl’s solubility supports fluid balance in living organisms and contributes to ocean salinity.