# Planet Earth/3a. Gas, Liquid, Solid (and other states of matter)
## What is stuff made of?

Ancient classifications of Earth’s matter were early attempts to determine what makes up the material world we live in. Aristotle, teacher of Alexander the Great, in Ancient Greece in 343 BCE proposed five “elements”: earth, water, air, fire, and aether. These five elements were likely adapted from older cultures, such as ancient Egyptian teachings. The Chinese Wu Xing system, developed around 200 BCE during the Han dynasty, listed the “elements” Wood (木), Fire (火), Earth (土), Metal (金), and Water (水). These ideas suggested that the ingredients that make up all matter were some combination of these elements, but theories of what those elements were appeared arbitrary in early texts. Around 850 CE, the Islamic philosopher Al-Kindi who had read of Aristotle in his native Baghdad, conducted early experiments in distillation, the process of heating a liquid and collecting the cooled produced steam in a separate container. He discovered that the process of distillation could make more poignant perfumes and stronger wines. His experiments suggested that there were in fact just three states of matter: solid, liquid and gaseous.
Ancient early classifications of matter differ significantly from today’s modern atomic theory of matter, that forms the basis of the field of chemistry. Modern atomic theory classifies matter into 94 naturally occurring elements, and an additional 24 elements if you include elements synthesized by scientists. The atomic theory of matter suggests that all matter is composed of a combination or mixture of these 118 elements. However, all these substances can adopt three basic states of matter as a result of differences in temperature and pressure. Hence all combinations of these elements can exist theoretically in solid, liquid and gas phases dependent on their temperature and pressure. Most states of matter can be classified as solid, liquid or gas, despite the fact that they are made up of different elements.
A good example is ice, water and steam. Ice is a solid form of hydrogen atoms bonded to oxygen atoms, symbolized by H2O, as it contains twice as many hydrogens (H) as oxygen (O) atoms. H$_2$O is the chemical formula of ice. Ice can be heated to form liquid water. At Earth’s surface pressures (1 atmosphere) ice will melt into water at 0° Celsius (32° Fahrenheit). Likewise, water will freeze at the same temperature 0° Celsius (32° Fahrenheit). If you continue to heat the water it will boil at 100° Celsius (212° Fahrenheit). Boiling water produces steam, or water vapor, which is a form of gas. If water vapor is cooled below 100° Celsius (212° Fahrenheit), it will turn back into water.
One of the most fascinating simple experiments is to observe the temperature in a pot of water as it is heated to 100° Celsius (212° Fahrenheit). The water will rise in temperature until it reaches 100° Celsius (212° Fahrenheit), at that temperature it will remain until all the water is evaporated into steam (a gas) before the steam will rise any higher. A pot of boiling water is precisely at 100° Celsius (212° Fahrenheit), as long as it is pure water and is at 1 atmosphere of pressure (at sea level).
The amount of pressure can affect the temperatures that phase transitions take place at. For example, on top of a 10,000-foot mountain, water will boil at 89.6° Celsius (193.2° Fahrenheit), because it has less atmospheric pressure. This is why you often see adjustments to cooking instructions based on altitude, since it takes longer to cook something at higher altitudes. If you place a glass of water in a vacuum by pumping gases out of a container, you can get a glass of water to boil at room temperature. This phase transition happens when the pressure drops below about 1 kilopascal in the vacuum. The three basic states of matter are dependent on both the pressure and temperature of a substance. Scientists can diagram the different states of matter of any substance by charting the observed state of matter at any temperature and pressure. These diagrams are called phase diagrams.
A phase diagram can be read by observing the temperatures and pressures substances will change phases from solid, liquid and gas. If the pressure remains constant, you can read the diagram by following a horizontal line across the diagram, observing the temperatures a substance melts or freezes (solid↔liquid) and boils or evaporates (liquid↔gas). You can also read the diagram by following a vertical line across the diagram, observing the pressures that a substance melts or freezes (solid↔liquid) and boils or evaporates (liquid↔gas).
On the phase diagram for water, you will notice that the division between solid ice and liquid water is not a perfectly vertical line around 0° Celsius, at high pressures around 200 to 632 MPa, ice will melt at temperatures slightly lower than 0° Celsius. This zone causes ice to melt that is buried deeply under ice sheets, which increases the pressure on the ice. Another strange phenomenon can happen to water heated to 100° Celsius. If you subject normal water heated to 100° Celsius to increasing pressures, up above 2.1 GPa, the hot water will turn to solid ice and “freeze” at 100° Celsius. Hence, at very high pressures, you can form ice at the bizarrely hot temperatures of 100° Celsius! If you were able to touch this ice, you would get burned. Another strange phenomenon happens if you subject ice to decreasing pressures in a vacuum, the ice will sublimate, turn from a solid to a gas at temperatures below 0° Celsius in a vacuum. The process of a solid turning to a gas is called sublimation, and the process of a gas turning into a solid is called deposition. One of the most bizarre phenomena happens at a triple junction of the three states of matter, where the solid, liquid and gas phases can co-exist. For pure water (H2O) this happens at 0.01° Celsius and a pressure of 611.657 Pa. When water, ice or water vapor is subjected to this temperature and pressure, you get the weird phenomena of water both boiling and freezing at the same time!
What phase diagrams demonstrate is that the states of matter are a function of the space between molecules within a substance. As temperatures increase, the vibrational forces push the molecules of a substance farther apart, likewise as pressures increases, the molecules of a substance are pushed closer together. This balance between temperature and pressure dictate which phase of matter will exist at each discrete temperature and pressure.
More advanced phase diagrams may indicate different arrangements of molecules in solid states, as they are subjected to different temperatures and pressures. These more advance phase diagrams illustrate crystal lattice structural changes in solid matter that is more densely packed and can form different crystals arrangements.
Each substance has different phase diagrams, for example a substance of pure carbon dioxide (CO$_2$), which is composed of a single carbon atom (C) bonded to two oxygen atoms (O) is mostly a gas at normal temperatures and pressures on the surface of Earth. However, carbon dioxide when cooled down to −78° Celsius undergoes deposition, and turns from a gas to a solid. Dry ice, which is solid carbon dioxide, sublimates at room temperatures making a gas. It is called dry ice, because the phase transition between solid and gas at normal pressures does not go through a liquid phase like water. This is why dry ice kept in a cooler will not get your food wet, but will keep your food cold and actually much colder than normal frozen ice made of H$_2$O.
Strange things happen when gases are heated and subjected to increasingly high pressures. At some point these hot gasses under increasing compression will become classified as a super critical fluid. Super critical fluids act both like a gas and a liquid, suggesting an additional fourth state of matter. Super critical fluids of H$_2$O occur when water is raised to temperatures above 374° Celsius and subjected to 22.1 MPa or more of pressure, at this point the super critical fluid of water will appear like a cloudy steamy fluid. Super critical fluids of CO$_2$ occur at temperatures above of 31.1° Celsius and subjected to 7.39 MPa or more of pressure. Because super critical fluids act like a liquid and a gas, they can be used as solvents in dry cleaning without getting fabrics wet. Super critical fluids are used in the process of decaffeinating coffee beans, as caffeine is absorbed by super critical fluids of carbon dioxide when mixed with coffee beans.
Phase diagrams can get more complex when you consider two or more substances mixed together and examine how they interact with each other. These more complex phase diagrams with two different substances are called binary systems, as they compare not only temperatures and pressures, but also the ratio of two (and sometimes more) components. Al-Kindi when developing his distillation processes, utilized the difference in boiling temperature of water (H$_2$O) which occurs at 100° Celsius and alcohol (C$_2$H$_6$O) which occurs at 78.37° Celsius. The captured gas resulting from a mixture of water and alcohol heated up to 78.37° Celsius, would contain only alcohol. If this separated gas is then cooled, it would be a more concentrated form of alcohol, this is how distillation works.
Utilizing the knowledge of phase diagrams, the distribution of the different compositions of the 94 naturally occurring elements can be elucidated. And scientists can determine how substances can get enriched or depleted in these natural occurring substances as a result of changes in temperature and pressure.

## Plasma

Plasma is used to describe free flowing electrons, as seen in electrical sparks, lighting and found encircling the sun. Plasma is not technically a state of matter since it does not contain particles of sufficient mass. Although sometimes included as a state of matter, plasma, like electromagnetic radiation such as light which contains photons is best considered a form of energy rather than matter. Although electrons play a vital role in bonding different types of atoms together. In the next module you will be introduced to additional phases of matter at the extreme limits of phase diagrams.

## Density

Different phases of matter have different densities. Density as you may recall is a measure of a substance’s Mass per Volume. In other words, it is the number of atoms (mass) within a given space (volume). Specific gravity is the comparison of a substance’s density compared to water. It is a simple test to see if an object floats or sinks, such observations are measured as specific gravity. A specific gravity of precisely 1, means that the object has the same density as water. Substances whether solid, liquid or a gas with specific densities higher than 1 will sink, while substances with a specific gravity lower than 1 will float. Specific gravity of liquids is measured using a hydrometer. Otherwise, density is measured by finding the mass and dividing it by its measured volume (usually by displacement of water if the object is an irregular solid).
Most substances will tend to have higher density as a solid than a liquid, and most liquids have a greater density than in a gas phase. This is because solids pack more atoms together in less space, than a liquid, and much more atoms are packed in a solid phase of matter than a gas phase. There are exceptions to this rule, for example ice, the solid form of water floats. This is because there is less mass per volume in an ice cube than liquid water, as the crystal lattice of ice (H$_2$O) forms a less dense network of bonds between atoms and spreads out over more space to accommodate this crystal lattice structure. This is why leaving a soda can in the freezer will cause it to expand and burst open. However, most substances will be denser in the solid phase than their liquid phase.
Density is measured as kg/m$^3$ or specific gravity (in comparison to liquid water). Liquid water has a density of 1,000 kg/m$^3$ at 4° Celsius, and steam (water vapor) has a density of 0.6 kg/m$^3$. Milk has a density of 1,026 kg/cm$^3$, slightly more than pure water, and the density of air at sea level is about 1.2 kg/m$^3$. At 100 kilometers above the surface of the Earth (near the edge of outer space), the density of air drops down to 0.00000055 kg/m$^3$ (5.5 × 10$^{−7}$ kg / m$^3$).

## Mass

Remember, the acceleration of gravity (g) is dependent on an object’s mass, hence the denser an object is, the more gravitational force will be exerted on it. This previously came into discussion on calculating the density of the Earth, in refuting a hypothesis of a hollow center inside the Earth.
It is important to distinguish an object’s Mass from an object’s Weight. Weight is the combined force of gravity (g) and an object’s Mass (M), such that Weight = M × g. This is why objects in space are weightless, and objects have different weights on other planets, because the value of g differs depending on the density of each planet. However, Mass, which is equivalent to the total number of atoms within an object, remains the same no matter which planet you visit.
Weight is measured by scales that use springs pushing down which combines mass and gravity pushing an object toward the Earth and recording the displacement of the spring. Mass is measured by scales that compare an object to standards, like in a balance-type scale, where standards of known mass are balanced on the scale.
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