Chalk vs. Limestone
Limestones are a class of sedimentary rocks composed of the same mineral, calcite, which is made from calcium carbonate, or CaCO3. Despite this similarity in composition, however, not all limestone rocks are the same. Chalk is actually a special form of limestone that exhibits some differences from other rocks in the same class.
Composition and Reactivity
Limestone and chalk are both primarily calcium carbonate with a small amount of silicates in the form of clay or silt. Adding strong acids like hydrochloric acid to chalk or limestone causes them to bubble as the acid reacts with the calcium carbonate to form CO2. Heating limestone or chalk to high temperatures forms a useful industrial chemical called calcium oxide or quicklime, which can be added to water to make calcium hydroxide.
Some limestone has inorganic origins. Water in caves, for example, can leave deposits of inorganic limestone that form remarkable stalactites. Most limestones and all chalk, however, have organic origins. Many sea creatures have shells, teeth or other parts made from calcium carbonate. When they die, their remains settle to the seafloor. Over long eons of geologic time, layers of calcium carbonate are converted to rock as more sediment accumulates. Chalk is composed exclusively of shells of single-celled creatures, which is not true of all other limestones.
Chalk is grayish-white or yellowish-white and soft. The small remains of the tiny planktonic sea creatures form a fine-grained material that is porous and permeable to water. The properties of other limestones vary depending on the type of rock; travertine, for example, is a kind of limestone popular as floor tile, while coquina is a limestone formed from broken shell debris that has a more jumbled consistency. These other limestones are harder and less fine-grained than chalk.
Chalk's softness and the ease with which it leaves traces on hard surfaces have made it popular as a writing and drawing material. It is also used sometimes to prepare quicklime and on snooker cues. Different varieties of limestone have many uses, ranging from building or tiling materials to remediation of over-acidic soils -- the crushed limestone reacts to neutralize acid in the soil. One especially important application of limestone is in the manufacture of Portland cement, which is widely used in construction.