Carbonate

A carbonate is a type of chemical compound that comes from carbonic acid. It contains a special group of atoms called the carbonate ion, which has the formula CO₃^2–. You might have heard of carbonates because they are in many everyday things, like fizzy drinks. When we talk about carbonates in drinks, we mean the process of adding carbon dioxide to water, which makes it bubbly. This is called carbonation.

In nature, carbonates are very important. They make up many kinds of rocks and minerals. One common carbonate is calcite, which is mostly calcium carbonate. This mineral is a big part of limestone, the rock that makes up many hills and mountains. It’s also what forms the shells of molluscs and the skeletons of corals living in the ocean.

People have used carbonates for thousands of years. For example, sodium carbonate, also called soda or natron, was used for cleaning and preserving things long ago. Today, carbonates are used to make glass, cement, and even in special batteries called molten carbonate fuel cells. They are very useful in many parts of industry and science.

Structure and bonding

The carbonate ion is made of one carbon atom and three oxygen atoms arranged in a flat triangle. It has a mass of 60.01 g/mol and a charge of −2. It is connected to the hydrogencarbonate ion, which comes from carbonic acid.

The bonds between the carbon and oxygen atoms share their length due to a process called resonance, making all three bonds appear the same. This is similar to what happens with the nitrate ion.

Chemical properties

Metal carbonates break down when heated, releasing carbon dioxide and leaving behind a metal oxide. This process is known as calcination. For example, calcium carbonate turns into calcium oxide and carbon dioxide.

Carbonate can bind to many metal ions. Some carbonates, like those of lithium, sodium, and potassium, dissolve in water, while others, like calcium carbonate, do not. Calcium carbonate can build up in pipes, which is why hard water often needs treatment to softens it. When acid is added to carbonates, carbon dioxide gas is released, which is why acid can remove limescale buildup.

Organic carbonates

In organic chemistry, a carbonate can mean a special part inside a bigger molecule. This part has a carbon atom connected to three oxygen atoms, with one oxygen double bonded to the carbon. These are called organocarbonates or carbonate esters. Some important ones are dimethyl carbonate, and cyclic compounds like ethylene carbonate and propylene carbonate.

Buffer

Our blood has a special system that helps keep its pH just right, like a buffer. This system uses three important reactions involving carbonates. When we breathe out carbon dioxide, it changes how these reactions work, helping to keep the blood balanced.

The same kind of buffer system is also important in the oceans. It affects our climate and the carbon cycle because many sea creatures, like corals, are made of calcium carbonate. When the ocean gets warmer, it can dissolve more carbonate, which can change the amount of carbon dioxide in the air and affect Earth’s temperature.

Carbonate salts

Carbonates are salts made from carbonic acid. They contain a special group of atoms called the carbonate ion, which has the formula CO₃^2–. These salts are found in many places, like rocks and minerals.

We also use the word "carbonate" to talk about adding carbon dioxide to water, which makes fizzy drinks. This process increases the amount of carbonate and bicarbonate ions in the water.

Presence outside Earth

Carbonates in rocks often suggest the presence of liquid water. Scientists have found signs of carbonates in space, specifically in the planetary nebula NGC 6302, even though water might not be there like it is on Earth.

Small amounts of carbonate have also been discovered on Mars through special imaging and in Martian meteorites. There may have been groundwater in places like Gusev and Meridiani Planum on Mars.