Carbonic acid

Some air turns into acid in water.

This happens when certain air meets water. It can make the water change. This change can make the water more acidic.

This is how fizzy water is made. People use pressure to put air in water. This makes drinks bubbly.

This process also happens in the ocean. As more air gets into the sky, more goes into the sea. This can change the water for all living things.

It is a very big job for nature.

Carbonic acid is a special chemical. It is made when carbon dioxide meets water. In water, it can quickly change back into water and gas.

This change is part of how many living things breathe. It also affects the water in our world. For example, people use it to make fizzy drinks. They use pressure to push carbon dioxide into the water. This makes the drink bubbly.

Carbonic acid can also change the pH of water. The pH is a way to measure how acidic a liquid is. In the ocean, more carbon dioxide in the air means more dissolves in the sea. This makes the ocean water more acidic. Scientists think the ocean's pH has already dropped by about 0.1.

In some places, carbonic acid works very fast. Living things have an enzyme called carbonic anhydrase. An enzyme is a tool that helps chemical changes happen quickly. This helps the body manage gas and water. Deep in space, this acid might exist on icy moons. On moons like Titan, high pressure keeps it there.

Carbonic acid is a special chemical compound. Its formula is H2CO3. This molecule is very interesting because it does not stay the same for long. When it meets water, it quickly turns into water and carbon dioxide gas. For a long time, people were not sure if it was a real molecule. However, many studies since about 1990 proved it is real. Scientists found it has its own distinct Raman spectrum. At 37 °C, it only lasts for about 20 milliseconds.

This chemical plays a huge role in how the world works. It is part of the breathing process for all aerobic living things. It also helps change the acidity of natural waters. In the beverage industry, people use a similar idea to make fizzy water. They dissolve carbon dioxide in water using a small amount of pressure. This is how many soft drinks get their bubbles.

In nature, the way carbonic acid works depends on what is around it. In pure water, it changes back into gas and water quite easily. In seawater, the balance is a bit different. Most carbon dioxide in the ocean stays as a dissolved gas. It does not turn into carbonic acid right away. This happens because the change is often very slow. The rate for turning into the acid is 0.039 s−1.

Living things have a way to make this happen much faster. They use a special tool called an enzyme. This enzyme is named carbonic anhydrase. It helps the chemical change happen very quickly. This is important for managing gases in biological solutions. Scientists also look at how this acid behaves under huge pressure. In the deep interiors of planets, carbonic acid might exist.

We can even find signs of this chemistry in outer space. Large icy moons like Ganymede, Callisto, and Titan have very high pressure. These pressures can reach up to 1.75 GPa at 300 K. In these cold places, carbonic acid might be found. It is denser than ice, so it might sink. It could settle between the ice and the rocky cores.

Carbonic acid is a chemical compound with the formula H2CO3. It is a highly unstable molecule that exists in a state of constant change. In the presence of water, it rapidly converts into water and carbon dioxide gas. For many years, scientists debated whether this molecule truly existed in a stable form. However, numerous studies since approximately 1990 confirmed it is a real molecule. Researchers identified it by its distinct Raman spectrum, which is a way to identify molecules using light. At a temperature of 37 °C, the molecule has a first-order lifetime of only about 20 milliseconds.

The behavior of carbonic acid depends heavily on its environment. In aqueous solutions, the molecule undergoes dehydration to become carbon dioxide and water. This process is often self-sustaining because the resulting water acts as a catalyst for further decomposition. In pure water at 25 °C, the hydration equilibrium constant is 1.2×10−3. In seawater, this constant is approximately the same. Because of these rates, most carbon dioxide at the interface of air and water stays as a dissolved gas. The natural, uncatalyzed rate of hydration is quite slow at 0.039 s−1. Meanwhile, the rate of dehydration is much faster at 23 s−1.

Biological systems have evolved ways to control these chemical reactions. All aerobic organisms rely on the interconversion of carbon dioxide and carbonic acid for breathing. To speed up the process, living things use a specific enzyme called carbonic anhydrase. This enzyme allows the chemical equilibrium to be reached very rapidly. In these biological solutions, the reaction follows a specific path where bicarbonate and protons convert to carbon dioxide and water. When the concentration of carbon dioxide becomes too high for the liquid to hold, the gas evolves from the solution. This gas release is governed by Henry's law, which relates the concentration of a dissolved gas to its partial pressure.

Carbonic acid also acts as a diprotic Brønsted acid. This means it can donate two protons during chemical reactions. It is the formal Brønsted–Lowry conjugate acid of the bicarbonate anion. In alkaline solutions, bicarbonate is quite stable. The way the acid dissociates into different parts depends on the pH and the ionic strength of the liquid. In the extracellular fluid of biological systems, the pH is about 7.4. At this specific level, carbonic acid is almost 50%-dissociated at equilibrium. This balance is vital for maintaining the chemical stability required for life.

Scientists have also studied carbonic acid in extreme physical conditions. While it is usually a liquid or gas in water, researchers have isolated solid anhydrous carbonic acid. This is pure carbonic acid without any water present. One way to produce it is by reacting hydrogen chloride and potassium bicarbonate at 100 K in methanol. Another method involves the proton irradiation of pure solid carbon dioxide. At room temperature, quantum chemical calculations suggest that pure carbonic acid should be a kinetically stable gas. Under high pressure, the substance changes its physical structure. At low temperatures and atmospheric pressure, it is an amorphous solid. However, under high pressure, it can crystallize into a structured form.

Using neutron diffraction, scientists have observed the geometry of carbonic acid at 1.85 GPa. In this state, the molecules are planar and form dimers. These dimers are joined together by pairs of strong hydrogen bonds. The C-O bonds in these molecules are roughly 1.34 Å, which is an unusual length. This length sits between a typical C-O single bond and a C=O double bond. This unique structure is caused by delocalized π bonding in the center of the molecule. This also results in a very short O—O separation of 2.13 Å. This is much shorter than the distances seen in other acids like oxalic acid.

This chemistry has massive implications for our planet and the solar system. In the beverage industry, carbonated water is made by dissolving carbon dioxide under positive pressure. This process creates the bubbles found in many soft drinks. On a larger scale, the increase in atmospheric carbon dioxide due to human industrialization is changing our oceans. As more CO2 dissolves in seawater, it forms more carbonic acid. This process is causing ocean acidification. The average surface pH of the ocean has already decreased by about 0.1 from pre-industrial levels.

Finally, carbonic acid may exist in the deep interiors of icy moons. In the cores of Ganymede, Callisto, and Titan, pressures reach between 0.6 and 1.75 GPa. In these high-pressure environments, significant amounts of carbonic acid can exist in aqueous solutions. Because pure carbonic acid is denser than ice, it is expected to sink. It might settle into a layer between the icy outer shells and the rocky cores of these moons. This demonstrates how a tiny molecule can influence the structure of entire worlds.