Lime – a time-tested chemical
Pure lime, or quicklime, is calcium oxide. Its ease of manufacture and chemical properties make it an important industrial chemical.
Kakahu lime kiln
The lime kiln at Kakahu was built in 1882. Local limestone was placed in the kiln and burnt between layers of wood. The lime was then used as mortar and to fertilise soils.
Lime has a long history dating from the earliest of times. Its main uses were as an ingredient in mortar and as a soil fertiliser.
Lime manufacture
From the earliest of times, lime has been made by heating limestone to high temperatures. Production methods have evolved from heating limestone in open fires, to the use of brick lime kilns at the start of the 17th century, to today’s horizontal rotating kilns several metres in diameter and up to 100 metres in length. These modern kilns operate at a temperature of about 1100-1200°C, allowing rapid conversion of limestone into lime.
CaCO3(s) limestone → CaO(s) lime + CO2(g) carbon dioxide
Lime’s chemical properties
Lime (calcium oxide) is a white solid with strongly basic properties.
Lime reacts readily with water to produce slaked lime, which is the chemical compound calcium hydroxide. A considerable amount of heat energy is released during this reaction.
Calcination
Lime, produced by calcining limestone, plays a key role in a multitude of industrial, manufacturing and agricultural processes.
This interactive outlines the process steps taken at the McDonald’s Lime Limited Ōtorohanga plant.
Calcium hydroxide is sparingly soluble in water producing an alkaline solution known as limewater. When carbon dioxide gas is passed through or over limewater, it turns milky due to the formation of calcium carbonate.
CaO(s) lime
+
H2O(l) water
→
Ca(OH)2(s) slaked lime
Ca(OH)2(s) slaked lime
+
H2O(l) water
→
Ca(OH)2(aq) limewater
Ca(OH)2(aq) limewater
+
CO2(g) carbon dioxide
→
CaCO3(s) calcium carbonate
+
H2O(l) water
Lime reacts with acidic gases like sulfur dioxide.
CaO(s) lime
+
SO2(g) sulfur dioxide
→
CaSO3(s) calcium sulfite
Coal and gas-fired power stations produce large volumes of gaseous product, some of which is sulfur dioxide. Lime and slaked lime are both used to reduce these sulfur emissions.
Slaked lime reacts with chlorine gas to produce the bleaching agent calcium hypochlorite – a common form of ‘swimming pool’ chlorine.
2Ca(OH)2(s) slaked lime
+
2Cl2(g) chlorine gas
→
Ca(ClO)2(s) calcium hypochlorite
+
CaCl2(s) calcium chloride
+
2H2O(l) water
When heated with coke, a form of carbon, calcium oxide combines to form calcium carbide. When calcium carbide is mixed with water, a gas called acetylene is produced. This is the fuel of the oxy-acetylene gas torch used in the metals industries to cut and weld.
2CaO(s) lime
+
5C(s) coke
→
2CaC2(s) calcium carbide
+
CO2(g) carbon dioxide
CaC2(s) calcium carbide
+
2H2O(l) water
→
Ca(OH)2(s) calcium hydroxide
+
C2H2(g) acetylene
Lime mortar
Mortar is a workable paste used to bind construction bricks and blocks together. Lime mortar is made by mixing lime, sand and water. It is one of the oldest types of mortar dating back to ancient times. Nowadays, mortar is made by mixing cement powder, sand and water.
The setting of lime mortar into a hard, binding material involves reaction with atmospheric carbon dioxide to produce calcium carbonate crystals that lock the sand grains tightly together.
Ca(OH)2(s) slaked lime
+
CO2(g) carbon dioxide
→
CaCO3(s) calcium carbonate solid
Fresco is a painting technique that involves applying pigment to a fresh surface of lime mortar.
This technique was extensively used by Renaissance painters in 15th and 16th centuries – some of the works created such as Michelangelo’s painting of the Sistine Chapel ceiling are marvelled at by a continuous stream of visitors to the Vatican each year.
Sistine Chapel frescos
Michelangelo used the fresco painting technique to decorate the walls and ceiling of the Sistine Chapel. This technique involves applying paint pigments to damp lime mortar.
Lime – an interesting physical property
If a marble-sized piece of lime is heated to a high temperature, it emits a very bright white light. In the 1820s, British Army officer Thomas Drummond used this property of lime to develop a light that could be used in lighthouses and on the battlefield. Called Drummond lights, they eventually replaced the gas lights used in music halls and theatres. Performers and actors were now ‘in the limelight’ when on stage.
Limelight
Strongly heating a small piece of lime results in the production of bright white light. 19th century music hall spotlights were developed using this principle. Performers were now ‘in the limelight’ when on stage.
Rare-earth metal oxides (cerium and thorium oxides) also show this property.
