There are three states of matter, whose structure and behaviour are explained by the kinetic theory: the atoms in any substance have energy proportional to the temperature. This energy causes motion, and the degree of movement the atoms or molecules have depends on the strength of the attractive forces, or bonds, between them.
Since all substances have different bonding strengths, the amount of energy needed to break these bonds is different. This is why, for example, it is necessary to heat iron to 1538 °C to melt it, while ice melts at 0 °C.
Solids are strongly bonded. This means the atoms attract each other so strongly they cannot move around, and vibrate within a rigid structure, with the atoms are fixed in the one place.
The kinetic theory says that atoms vibrate back and forth within a structure. How much energy is needed to break these bonds determines the melting point of the substance. Every substance has its own unique melting point, which depends on the elements which compose the solid.
Liquids are weakly bonded. This means they still attract each other, but less than in a solid. They have too much kinetic energy for the attractive forces to hold them in the one place, so are able to move around and past each other without being held together. Since the molecules are free to move around, they take the shape of a container they are in.
Gas has so much kinetic energy that the molecules are not bonded together at all. This high kinetic energy causes them to push against anything they hit, such as the walls of their container. This creates a pressure which is proportional to their thermal energy (temperature).
The gas pressure depends on the amount of gas (number of molecules), not what type of molecule they are.
The pressure will increase if the temperature increases. Conversely, if the volume the gas occupies increases, the pressure and the temperature will decrease. That is why it is colder on top of a mountain than at sea level, and the air is 'thinner' (lower pressure).
The kinetic theory states that all atoms and molecules which compose a substance have some thermal energy, causing them to vibrate in situ or move.
Ice has a temperature below 0°C, and forms a rigid structure of the $H_2O$ molecules. As you can see, ice has a lower density than liquid water, so floats. It has come in from a colder location and is melting.
The water around it has a temperature above 0°C. In time the water and the ice will reach the same temperature. Either the water freezes and forms more ice, or the ice melts and joins the water.
The clouds are not gas. They are composed of tiny droplets of of water. However, there is invisible water gas in the air. All liquids exchange molecules with the air as they try to balance out the temperature difference.
There are three temperature scales in common usage, all named after their inventors.
The Kelvin scale was invented by Scottish physicist William Thomson, also known as Baron Kelvin (1824-1907). It is based on a scale where its zero (0 K) is absolute zero (-273.15°C), and each increment is equivalent to 1 degree on the Celcius scale. Kelvin are not 'degrees': water freezes at 273.15 K.
Named after the Swedish astronomer Anders Celsius (1701–1744), from its inception in 1743 till 1948 it was known as centigrade, after which it was renamed Celcius and adapted by the SI. Until 1948, the scale was defined by the freezing and boiling points of water, but since then it has been defined by absolute zero (-273.15°C) and the triple point of water (273.16°C).
Named after physicist Daniel Gabriel Fahrenheit (1686 - 1736). The zero on the fahrenheit scale is the lowest temperature he could obtain experimentally (using salt water). Water freezes at 32°F and boils at 212°F, while the average human body temperature is around 96°F. There exists the Rankine temperature scale with equivalent degree intervals as Fahrenheit, and which has its zero as absolute zero (0°R = −459.67°F).
Usage: the Kelvin scale is the SI unit for temperature, so is used for most scientific and engineering purposes. Celcius is also commonly used in both scientific and for general purposes in most countries. Very few countries retain Fahrenheit as the official temperature scale, with the USA being the only major country to do so. Great Britain and Canada also use Fahrenheit, as they do some other non-metric and imperial units, but in a secondary or unofficial capacity.
F = (9/5)C + 32
C = 5/9(F - 32)
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