 Basically, evaporation is a type of phase transition. It is the process by which molecules in a liquid state spontaneously become gaseous.  Evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed to a significant volume of gas

 Molecules do not have enough heat energy to escape from the liquid. But, when the molecules will bump into each other, they transfer energy to each other. The transfer of energy is so one- sided for the molecules near the surface that they end up to be the first to escape from the liquid  Liquids that do not evaporate visibly at a given temperature in a given gas have molecules that do not tend to transfer energy to each other. However, these liquids are evaporating, it's just that the process is much slower and thus significantly less visible

 To evaporate, molecules of a liquid must fulfill some criteria…… - Must be located near the surface - Need to move in the proper direction - Need to have sufficient kinetic energy But, only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited.

 The kinetic energy of molecules is proportional to its temperature. Thus, evaporation proceeds more quickly at higher temperatures. Evaporation also proceeds more quickly with higher flow rates between the gaseous liquid phase and in liquids with higher vapour pressure  As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy and the temperature of the liquid will decrease. This phenomenon is also called evaporative cooling.

» Boiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure » Liquid may also boil when the pressure of the surrounding atmosphere is sufficiently reduced, such as the use of a vacuum pump or at high altitudes

» Boiling occurs in three characteristic stages…… - nucleate - transition - film These stages generally take place from low to high surface temperatures, respectively

Nucleate boiling  Characterized by the growth of bubbles on a heated surface, which rise from discrete points on a surface, whose temperature is only slightly above the liquid’s  The number of nucleation sites are increased by an increasing surface temperature

 An irregular surface of the boiling vessel can create additional nucleation sites, while an exceptionally smooth surface, lends itself to superheating Under these conditions, a heated liquid may show boiling delay and the temperature may go somewhat above the boiling point and never boil

Transition boiling May be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling The formation of bubbles in a heated liquid is a complex physical process which often involves cavitations and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface

Film boiling o If a surface heating the liquid is significantly hotter than the liquid then film boiling will occur, where a thin layer of vapour, which has low thermal conductivity insulates the surface. This condition of a vapour film insulating the surface from the liquid characterizes film boiling

Freezing, also known as solidification is the process in which a liquid turns to a solid when it’s cold enough. The freezing point is the temperature at which melting happens. All known liquids undergo freezing when the temperature is lowered enough, except the liquid helium that can only be solidified under pressure. For most substances, the melting and freezing points are the same temperature, however some substances possess differing solid-liquid transition temperatures. This process is known as hysteresis.

Crystallization х Most liquids freeze by crystallization х Crystallization consists of two major events……  Nucleation - the molecules start to gather into clusters, arranging in a defined and periodic manner that defines the crystal structure  Crystal growth - The subsequent growth of the nuclei that succeed in achieving the critical cluster size

Supercooling Crystallization usually begins at lower temperature than the melting point, due to high activation energy of homogeneous nucleation Some energy is expended to form this interface If a hypothetical nucleus is too small, the energy that would be released is not enough to create its surface, and nucleation does not proceed

Freezing does not begin until the temperature is low enough to provide enough energy to form stable nuclei In presence of indiscretion on the surface of the containing vessel, solid or gaseous contamination, pre-formed solid crystal or other nucleates, heterogeneous nucleation may occur, where some energy is released by the partial destruction of the preceding interface, raising the supercooling point to be close or equal to the melting point

BASIC KNOWLEDGE Gas Liquid Solid Evaporation Condensation Melting Freezing Deposition Sublimation * Please note that I’m not going to talk about Deposition and Sublimation

BoilingEvaporation Takes place only at a particular temperature at which the vapour pressure of the atmosphere Occurs spontaneously at all temperatures Involves the formation of bubbles of the vapour throughout the bulk of the liquid Takes place only at the surface of the liquid Occurs at a faster rate than evaporation Occurs in a slower rate than boiling The motion of particles is very fast The motion speed of the particles is not constant, some move faster while some move slower

Water's boiling point is at 100 degrees Celsius ONLY if the pressure is at its standard point. The standard point of pressure is one atmosphere or kilo Pascal. Because of this relationship, the boiling point of water at the summit of a tall mountain is significantly lower than the boiling point of water at sea level. This can be explained by the universal gas law. Because PV=nRT, the pressure and temperature are directly proportional. If one goes up, the other must go up as well.  It is because the Celsius system is based on water.  It is because we, the humans set the scale for all temperatures.  It is because enough energy has been added to it, to create a state change from a liquid to a gas. Anonymous said:

 The freezing point is the same as the melting point because the two differ only in the direction from which the phase change is approached. For instance if you have ice you warm it up to 0°C so it can melt, if you have water you cool it down to 0°C so it freezes. Why melting point and freezing point are in the same temperature?

 As the molecules bump into each other and transfer heat energy, the heat energy slowly breaks the bonds that hold water molecules  Actually, it’s that expansion takes place and breaks the bonding between water molecules  So, they loosen and then transformed into gases The force to break bonds between water molecules

As I mentioned before, even the molecules have bonds between themselves, they still can move slightly. Sometimes, they bump into each other and thus there’s heat energy transferring to each other. Then, expansion will take place and it will break the bonds between water molecules. But, the expansion is one-sided because the transfer of energy is one-sided too. So, the molecules near the surface will definitely be those that escape first.

Based on the last slide, someone may ask the question above…… I’ll explain this with pictures. = water molecule When water molecules bump into each other, they transfer heat energy…… Eg. But how about those that are near to the surface? Those at the top will be bumped into more. Thus, they’ll receive more heat energy and escape faster.

 nce-between-evaporation-and-boiling/ nce-between-evaporation-and-boiling/  em03054.htm em03054.htm   melting-point-freezing-same-temperature-for melting-point-freezing-same-temperature-for 

Done by: Yap Wei Lok (32) 1O2