Introduction to Calcium
Calcium, represented by the symbol Ca, is an alkaline earth metal with atomic number 20. It is a silvery-white, relatively soft metal that is highly reactive. Calcium is the fifth most abundant element in the Earth’s crust and is an essential element for all living organisms, playing crucial roles in bone structure, nerve transmission, and muscle function.
Reactivity with Water
Calcium reacts with water, though less vigorously compared to alkali metals like sodium or potassium. When calcium metal is added to water, it reacts to produce calcium hydroxide and hydrogen gas.
The chemical equation for this reaction is: Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)
The hydrogen gas produced can be observed as bubbles. The reaction is exothermic, meaning it releases heat, but typically not enough to ignite the hydrogen gas unless the metal is in a finely divided form or the water is hot. The resulting solution of calcium hydroxide, also known as limewater, is alkaline. This reactivity explains why calcium metal is not found in its elemental form in nature but rather in compounds like limestone (calcium carbonate), found extensively in regions like Rajasthan and Madhya Pradesh in India.
Reactivity with Air
Calcium metal is reactive towards air. When exposed to air, it readily tarnishes, losing its characteristic metallic luster. This tarnishing is due to its reaction with oxygen and nitrogen present in the air.
With oxygen, calcium forms calcium oxide: 2Ca(s) + O₂(g) → 2CaO(s)
With nitrogen, it forms calcium nitride: 3Ca(s) + N₂(g) → Ca₃N₂(s)
Due to its reactivity with atmospheric gases, elemental calcium is typically stored under an inert atmosphere, such as argon, or submerged in paraffin oil to prevent oxidation and maintain its metallic properties.
Characteristics of Calcium
Toxicity
Elemental calcium is not considered toxic in its pure metallic form under normal conditions. In fact, calcium is an essential macro-mineral required for various biological processes in humans and animals, including bone and teeth formation, blood clotting, and muscle contraction. While excessive intake of calcium compounds can lead to health issues like hypercalcemia, the elemental metal itself does not pose a direct acute toxic threat.
Radioactivity
Naturally occurring calcium is not radioactive. Its most abundant and stable isotope is Calcium-40 ($^{40}$Ca), which constitutes over 96% of natural calcium. There are a few artificially produced radioactive isotopes of calcium, such as Calcium-41 and Calcium-45, but these are not naturally occurring and are primarily used in scientific research.
Flammability
Calcium metal is flammable, particularly when in powdered form or when heated. It can ignite and burn in air with a bright, often reddish-orange, flame to produce calcium oxide. The finely divided powder form presents a greater fire hazard due to its larger surface area.
Illustrative Chemical Reaction Example
One of the most famous and widely utilized chemical reactions involving a calcium compound is the “slaking of lime.” This reaction involves calcium oxide (CaO), commonly known as quicklime, reacting with water to form calcium hydroxide (Ca(OH)₂), also known as slaked lime.
The chemical equation is: CaO(s) + H₂O(l) → Ca(OH)₂(s)
This reaction is highly exothermic, releasing a significant amount of heat. Slaked lime is a fundamental material with extensive applications. It is used in agriculture for neutralizing acidic soils, in water treatment to remove impurities, and most notably, in the construction industry as a key component in the production of mortar, cement, and concrete. The widespread construction activities across India, from housing to infrastructure projects, rely heavily on these calcium-based materials. Calcium hydroxide is also traditionally used for whitewashing walls in many Indian households.