Phases of Matter
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- Last Updated: Tuesday, 26 November 2024 02:23
- Published: Thursday, 06 July 2023 19:21
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States/Phases of matter:
There are three common states of matter: solid, liquid, and gas. (Plasma is another state of matter but is beyond this discussion.)
Libretexts has this excellent material on it: https://chem.libretexts.org/Courses/Bellarmine_University/BU%3A_Chem_104_(Christianson)/Phase_1%3A_The_Phases_of_Matter
A solid will maintain its shape (ice, wax, steel), while a liquid will flow and take the shape of its container (water, mercury). A gas or vapor will fill all available volume (steam, air, mercury). There is a 4th state which is plasma, also known as ionized gas state. It's a gas state where atoms lose their electrons in presence of very high electromagnetic field, which causes these electrons to start conducting electricity, which isn't typical of gases. In nature, lightning is the most common example of plasma. It's the most common state in the universe, since all stars are in plasma state. However, in real life we don't encounter this state, so we don't discuss the plasma state.
States of matter are generally based on the form of the substance at room temperature and pressure. Things like air are gaseous at room temperature while water is a liquid. When heated, water become gaseous, but is referred to as a vapor. At atmospheric temperature (25C) and pressure (1.0 atm), most of the elements are found in solid state. Mercury (Hg) and Bromine (Br) are the only 2 elements found in liquid state. All noble gases and H, N, O, F and Cl are the only ones found in gaseous state.
Pressure: Pressure is Force per unit area. i.e P = F/A. SI unit of P is Pascal (Pa) which is 1N/m^2. 1 N is the force equiv to holding 100g of weight. This doesn't sound like too much weight. Now if we spread this weight over 1m^2, that's almost negligible Pressure. 100g of weight over 100cm*100cm = 0.01g/cm^2 of weight is 1 Pa. It's more convenient to use 1kPa (1 kilo pascal) or 1 bar (=100 kPa). 1kPa is 10g/cm^2, while 1 bar is 1kg/cm^2. Atmospheric pressure due to air is measured in atm (atmospheric pressure), where pressure at sea level is defined as 1 atm=101,325 Pa, which is ~1 bar (To be precise, 1 atm is slightly greater than 1 bar). Pressure is also measured in inches/mm of Mercury (Hg) where pressure at sea level = 760mm = 29.9 inches = 1 atm pressure.
In USA, pressure is measured in pounds of force per square inch (psi)—for example, in car tires. 1 lb=453.6g => 1 lb force = 453.6g*9.8N/kg = 4.445N. So 1 psi = 4.45N/(0.0254)^2=6897.5N/m^2 = 6.9kPa. => 1 bar = 100/6.9=14.5 psi. So, 1 atm=14.7 psi. Pressure readings in excess of atmospheric pressure are usually more relevant in measurements, so we use psig (pounds per square inch gauge) in such cases. PSIG is measured relative to atm pressure, i.e 1 PSIG = 1 PSI - 14.7, so at sea level PSIG =0. PSI measures pressure relative to vacuum.
Barometer: Barometers are used to measure pressure. We tke a tube closed at end end, and fill it with liquid as Mercury or water. Now, if we invert and immerse it in container of same liquid, then liquid inside the tube will rise until the point where pressure due t weight of liquid inside is same as pressure due to air outside. Mercury rises to 760 mm (or 2.5 ft), while water rises to 34 ft (since Hg is ~13,6 times denser than water). P = h*ρ*g.
Manometer: Thet are similar to Barometer, and are used to measure pressure of gases. Manometers are used in measuring blood pressure using device called a sphygmomanometer (Greek sphygmos = “pulse”).
GASES:
Ideal gas law: This provides a relation between P, V, T of an ideal gas (ideal gas is one with no interaction among gas particles and assuming gas particles having zero size) with the number of gas molecules for an ideal gas. There were multiple laws discovered by various scientists, but they were all combined into one once "Ideal gas law" was found. Link: https://en.wikipedia.org/wiki/Ideal_gas_law
The law states that
P*V = n*R*T => where P,V,T are pressure (in pascal), Volume (in m^3), Temperature (in Kelvin) of gas and n=amount of gas particles (in moles), R= universal gas constant = 
The equation above is written in many different forms. The one above is the molar form. Under STP (T=273K, P=atmospheric pressure = 1 bar = 101kPascal), 1 mole of gas occupies V=nRT/P = 8.3*273/(101*10^3) m^3 = 0.0224 m^3 = 22.4 L (since 1L=1000 cm^3 = 1/1000 m^3). So, 1 mole of any ideal gas is always 22.4L in volume under STP. What is very surprising is that no matter what gas, it always occupies the same volume under same conditions of P, T. Whether the gas is water or oxygen, they both occupy 22.4L for 1 mole of gas under STP.
Derivation: Ideal gas law is easy to derive. See wiki link above. Simple proof:
Consider a container of Volume V with N gas particles moving with rms velocity v. Assuming random movement is equally likely in all 3 directions, x,y and z, 1/3 of the particles move in x direction (1/6 th move in +X, while 1/6 th move in -X dirn), strike against the container wall of Area S, and bounce back with reverse velocity v. Momentum change = 2mv for each particle, Force due to each particle=Momentum change/t = 2mv/t. For "q" particles hitting in time t, F=2mv/t*q. In time t, volume of particles striking the wall are = v*t*S. If there are N particles in Volume V, then number of particles in Volume v*t*S = v*t*S*N/V. But only 1/6th are striking. So, number of particles striking wall in time t = v*t*S*N/V *1/6. This is "q". So, P=F/S = 2mv/t*q*1/S = 2mv/(tS)*1/6*v*t*S*N/V=1/3m*v^2*N/V. So, PV=1/3Nmv^2
Using Maxwell-Boltzmann distribution, any gas particle having speed b/w v and dv can be found out. We use this to calculate rms velocity (v^2) which comes out to 3k*T/m. So, PV=1/3N*m*3kT/m=NkT => PV=NkT. This is the molecular formula of ideal gas law.
Phase changes: When a substance changes from one state of matter to another, it experiences a phase change. For instance, ice melting into water is a phase change from solid to liquid. Water changing into steam is a phase change from liquid to vapor. Energy in the form of heat is required to create a phase change. The elements in periodic table change state, as well as the compounds formed from them. Infact most matter made up of any number of compounds has to exist in all of the 3 states. It's just that at room temperature and pressure, it exists in one of the 3 states. But by changing temp and pressure, it can be converted to other states too.
There are forces within an atom that holds electrons together bounded to the +ve charged proton in nucleus. However this force is not the one that gives rise to phases. The force between atoms/molecules, which is called "internmolecular force" is the one that causes phases. Read about these intermolecular bonds in "Atomic bonds" section.