Atoms & Light

Part 2 - Gases

Solid, liquid and gas are the three states of matter normally occurring on Earth. The most familiar, water, can exist in all three states, (as ice, at temperatures below 0 °C, liquid between 0 and 100 °C and as a gas at temperatures above 100 °C). (There is a fourth state, plasma, which is an extremely hot, ionized gas where electrons are stripped from the atoms. This exists naturally in places like the sun).

On Earth, a gas may consist of individual atoms (like the noble gas neon), linked atoms of one type like hydrogen (which join together to form a molecule with two hydrogen atoms) or compounds of more than one element (like carbon dioxide). Molecules of gas move around with very little interaction with each other because of the huge separation between molecules. This also permits light to pass through with little interference, making them normally transparent. Gas molecules are generally deflected by solids or liquids but they can be absorbed (dissolved) by liquids and, less readily, by solids. 

In 1662 England Robert Boyle repeated Evangelista Torricelli's 1643 barometer experiment, with a J-shaped glass tube, sealed at the bottom of the J, and partly filled with Mercury. Then, he carefully added mercury to the open end as he measured the volume of air trapped in the closed end. The difference in the level of the mercury in both parts of the J indicated the pressure of the trapped air. As he added more mercury, the pressure varied inversely with the volume of the gas. If the pressure was doubled, the volume was halved and vice versa. Stated another way, the pressure (P) when multiplied by the volume (V) was a constant (k) for a mass of confined gas as long as the temperature was constant.

Therefore Boyle's Law is P V = k   Or    P 1 x V 1 = P 2 x V 2 .

Guillaume Amontons, in 1703, was attempting to improve his J-tube air thermometer, which was based on Torricelli's invention of the mercury barometer.  As he did not add mercury, he knew that the air, trapped in the closed end of the J, was at constant pressure but its volume shrank as the temperature fell. After careful measurements, he was astonished to realize that a fundamental (thermodynamic) temperature scale existed. The volume of air would ultimately become zero, if the temperature could be reduced sufficiently. He calculate that this would occur at a temperature equivalent to minus 240 °C. (The correct value of absolute zero, is now known to be −273.15 °C).Although he came close to finding absolute zero, his discovery would not be accurately confirmed for another century.

In 1699, Amontons also built a hot air engine he named a "fire mill" (moulin à feu). It used a thermodynamic cycle that, a century later, was known as the Stirling cycle. This engine used the expansion of heated air to drive a wheel that generated 39 hp, equal to the most powerful hot air engines of the 19th century, except Ericsson's "caloric engine" in 1833.In 1742 Anders Celsius (1701–1744) discovered that the melting point of ice did not vary with pressure but he also determined that the boiling point of water decreased with lower atmospheric pressure. So his temperature scale specified 100 degrees Celsius as the boiling point of water at a standard air pressure (at sea level) and zero at the melting point of ice.

In 1777, Johann Heinrich Lambert compared the volume of a gas over a range of temperatures and used a simple straight-line extrapolation to calculate that absolute zero was about −270 °C.

Jacques Alexandre César Charles, the French physicist and balloon pioneer (in 1787) and Joseph Louis Gay-Lussac (in 1802) both discovered that oxygen, nitrogen, hydrogen, carbon dioxide and air, all expand to the same extent over the same 80 degree Celsius range. They noted that, the volume of an ideal gas (at constant pressure) varied in direct proportion to its absolute (thermodynamic) temperature. This is now know as Charles's Law.    V1/T1 = V2/T2. 

Gay-Lussac also noted that the pressure a gas exerted on the sides of a container was directly proportional to its temperature.

Boyle's law and Charles' law are often stated together as;   -V1 x P1 = V2 x P2 T1 T2.

Which shows that when a volume of gas is constant, if the pressure drops, then the temperature must drop also. This means that compressed air, cooled to ambient temperature and released from a pressurized container, will absorb heat from the surroundings as the pressure drops. (One method of refrigeration).

William Thomson, ( Lord Kelvin) in 1848 calculated that absolute zero was equivalent to −273 °C while he also promoted the importance of a scale that had absolute zero as the point of infinite cold and which used the Celsius scale increments. Now known the kelvin thermodynamic temperature scale, this absolute measure of temperature, is a principal parameter of thermodynamics and is designated with the symbol K.

Absolute zero, at which particles of matter have minimal motion and can become no colder, is the lowest temperature theoretically possible as no further heat can be extracted. In the quantum-mechanical description, matter at absolute zero is in its state of lowest energy, its ground state. 

Humphrey Davy, in 1795 at the age 17, refuted Antoine Lavoisier's idea that heat was a material substance ("calorique"). He demonstrating that friction alone could melt ice by rubbing two blocks of ice together, thus supporting the idea that heat was caused by the motion of particles (molecules).  

Davy discovered that breathing nitrous oxide (laughing gas) gave people a strong feeling of happiness and giddiness. This became a popular form of entertainment in England but Davy failed to follow up on its use as a pain killer, and people continued to suffered pain for the next 40 years. Davy and his protégé, Michael Faraday, became absorbed with electricity, convinced it would be a key to solving the mystery of matter.

In 1811, Amedeo Avogadro discovered that the volume occupied by an ideal gas at standard temperature and pressure was proportional to the number of molecules (or moles) present in the container. Standard temperature is 0 °C, (+273.15 K). Standard Pressure is defined as exactly 100 kPa (750 mmHg or 14.5 psi). Standard Temperature and Pressure is normally indicate by STP.

In 1801, John Dalton discovered that the pressure of a mixture of non reactive gases is equal to the sum of the pressures of each of the constituent gases alone. His Law of Partial Pressures is as follows:-The absolute temperature of a gas particle (molecule) is proportional to its speed and therefore proportional to the average kinetic energy of all the gas particles at constant pressure in an ideal gas.

Gas particles are free to rotate, vibrate and move in any linear direction and the speed range of gas particles varies constantly due to collisions with other particles, or the walls of a container, which can also induce rotational and vibrational transfer of energy.

In contrast, a molecule in a solid can only vibrate when heated, as the lattice crystal structure prevents both linear and rotational motions. In a mixture of unreactive gases, heavier gases do not sink to the bottom (as do liquids) and the type of gas particles do not effect the pressure. They behave as if their size is negligible.

The main points of Dalton's atomic theory, as it eventually developed, are: 

Elements are made of extremely small particles called atoms.

Atoms of a given element are identical in size, mass and other properties;

atoms of different elements differ in size, mass and other properties;

Atoms cannot be subdivided, created or destroyed;

Atoms of different elements combine in simple whole-number ratios to form chemical compounds;

In chemical reactions, atoms are combined, separated or rearranged.