Solid hydrogen

Hydrogen is a gas. It can turn into ice.

It must be very cold. This happens when the heat goes away.

This ice is very light. It is one of the lightest solids.

Space tools use this cold ice. It keeps them very chilly.

It helps the tools work well. It is a very cool thing to see.

Hydrogen is usually a gas. It can turn into a solid. We call this solid hydrogen. To make it, you must lower the temperature. It must get colder than 14.01 Kelvin. James Dewar first collected it in 1899. This ice is very light. It has a very low density. This means it does not weigh much for its size. Scientists study how it changes under pressure. Pressure is a push on the atoms. At low pressure, the molecules can spin freely. We call this Phase I. If you push harder, it changes to Phase II. In Phase II, the molecules cannot spin as easily. There are even more phases like Phase III and Phase IV. These happen at very high pressure. It is hard to see these parts. This is because hydrogen does not react much with X-rays. Scientists use special tools to study these changes. One tool is called Raman spectra. It helps them see how the hydrogen moves. Space tools also use it. A tank of frozen hydrogen keeps tools very cold.

Hydrogen is usually a gas. It can become a solid under the right conditions. This is called solid hydrogen. It is a very special material. One reason is that it has a very low density. This means it is very light for its size. Its density is only 0.086 grams per cubic centimeter. This makes it one of the lightest solids we know. Scientists find this material very interesting to study.

To make solid hydrogen, you must change the temperature. You must lower it below the melting point. That point is 14.01 Kelvin. You can also change it with pressure. Pressure is a strong push on the molecules. At low pressure, the molecules can spin freely. This is called Phase I. If you push harder, it turns into Phase II. In this phase, the molecules can no longer spin freely.

We have known about this since the late 1800s. A scientist named James Dewar collected it first. He did this in 1899. He shared his work in a journal. The title was "Sur la solidification de l'hydrogène." This means "On the freezing of hydrogen" in English. His discovery helped us understand how elements change. It was a big step for science.

There are even more phases at high pressure. Phase III happens at about 160 gigapascals. Phase IV happens at pressures above 220 gigapascals. These phases are hard to see. Hydrogen atoms do not react much with X-rays. Scientists must use small samples in diamond anvil cells. They also use a tool called Raman spectra. This helps them detect changes in the solid. They use computer models to help them too.

Solid hydrogen is useful for space tools. NASA uses it in a special way. The Wide-field Infrared Survey Explorer has a tank. This tank holds frozen hydrogen. The tank is called a cryostat. It keeps the infrared instrument very cold. Keeping things cold is vital for space science. This shows how a tiny molecule helps us see the stars.

Solid hydrogen is the frozen state of the element hydrogen. Most of the time, hydrogen exists as a gas. However, it can become a solid under specific conditions. This transition occurs by decreasing the temperature below its melting point of 14.01 Kelvin. Solid hydrogen is quite unique because it has a very low density. Its density is measured at only 0.086 grams per cubic centimeter. This makes it one of the lowest-density solids known to science. Understanding this material helps scientists study how atoms behave under extreme conditions.

Scientists study how hydrogen changes using different phases. A phase is a specific state of matter under certain conditions. At low temperatures and pressures, hydrogen forms molecular solid phases. These phases are made of individual H2 molecules. As you change the pressure, the way these molecules sit and move changes. This process is called a phase transition. Researchers use special tools like diamond anvil cells to create these high-pressure environments. These cells allow scientists to squeeze tiny samples of the material to see how it reacts.

There are several distinct phases of molecular solid hydrogen. Phase I occurs at low temperatures and low pressures. In this phase, the H2 molecules form a hexagonal close-packed array. These molecules are able to rotate freely within their structure. If you increase the pressure at a low temperature, the material moves into Phase II. This phase can occur at pressures up to 110 gigapascals. Phase II is a broken-symmetry structure. In this state, the H2 molecules are no longer able to rotate freely.

Further increases in pressure lead to even more complex structures. Phase III is encountered when the pressure reaches about 160 gigapascals. This phase is believed to have a C2/c symmetry. It consists of flat layers of molecules in a distorted hexagonal arrangement. If the temperature also increases, the material reaches Phase IV. This happens at temperatures of a few hundred Kelvin. Phase IV requires pressures above 220 gigapascals. This phase has a Pc symmetry. It features alternating layers of strongly bonded molecules and weakly bonded, graphene-like sheets.

Identifying these atomic structures is a very difficult task for researchers. Hydrogen atoms interact very weakly with X-rays. Because of this, X-ray diffraction provides only limited information about the structures. Scientists must rely on other methods to detect phase transitions. They look for abrupt changes in the Raman spectra of the samples. Raman spectra is a tool used to observe how molecules vibrate. They also use first-principles modelling to infer what the atomic structures look like. This involves using complex computer calculations to predict how atoms will behave.

This field of study has a long history of discovery. The first collection of solid hydrogen was achieved by James Dewar. He accomplished this in 1899. He published his findings in a journal called the Annales de Chimie et de Physique. His paper was titled "Sur la solidification de l'hydrogène," which means "On the freezing of hydrogen." Since his discovery, scientists have used advanced methods like Density Functional Theory. They also use Quantum Monte Carlo methods to study the energy of these different structures. These modern tools help create a theoretical phase diagram that matches what scientists see in real experiments.

Solid hydrogen also has important uses in space technology. NASA uses frozen hydrogen in the Wide-field Infrared Survey Explorer. This spacecraft carries an infrared instrument that must stay extremely cold. To keep it cold, engineers use a spherical tank called a cryostat. This tank contains solid hydrogen to maintain the necessary low temperatures. By studying the properties of solid hydrogen, we gain a better understanding of both deep-space technology and the fundamental laws of physics. The way molecules move and bond under pressure reveals much about the physical world.