Latent Heat

A substance often undergoes a change in temperature when energy is transferred between it and its surroundings. There are situations, however, in which the transfer of energy does not result in a change in temperature. This is the case whenever the physical characteristics of the substance change from one form to another; such a change is commonly referred to as a phase change. Two common phase changes are from solid to liquid (melting) and from liquid to gas (boiling); another is a change in the crystalline structure of a solid.

All such phase changes involve a change in internal energy but no change in temperature. The increase in internal energy in boiling, for example, is represented by the breaking of bonds between molecules in the liquid state; this bond breaking allows the molecules to move farther apart in the gaseous state, with a corresponding increase in intermolecular potential energy.

 

If a quantity Q of energy transfer is required to change the phase of a mass m of a substance, the ratio { L = Q/m } characterizes an important thermal property of that substance. Because this added or removed energy does not result in a temperature change, the quantity  L  is called the latent heat  (literally, the “hidden” heat) of the substance. 

 

Latent heat of fusion  Lf  is the term used when the phase change is from solid to liquid.

Latent heat of vaporization Lv is the term used when the phase change is from liquid to gas.

 

 

 

Heat and Internal Energy

Heat is defined as the transfer of energy across the boundary of a system due to a temperature difference between the system and its surroundings.

When you heat a substance, you are transferring energy into it by placing it in contact with surroundings that have a higher temperature. This is the case, for example, when you place a pan of cold water on a stove burner—the burner is at a higher temperature than the water, and so the water gains energy.

Internal energy is all the energy of a system that is associated with its microscopic components—atoms and molecules—when viewed from a reference frame at rest with respect to the center of mass of the system. 

Internal energy includes kinetic energy of random translational, rotational, and vibrational motion of molecules, potential energy within molecules, and potential energy between molecules. It is useful to relate internal energy to the temperature of an object 

The Celsius, Fahrenheit, and Kelvin Temperature Scales

Tc = T - 273.15

This Equation shows that the Celsius temperature Tc  is shifted from the absolute  

(Kelvin) temperature T by 273.15°. Because the size of a degree is the same on the two
scales, a temperature difference of 5°C is equal to a temperature difference of 5 K. The
two scales differ only in the choice of the zero point. Thus, the ice-point temperature
on the Kelvin scale, 273.15 K, corresponds to 0.00°C, and the Kelvin-scale steam point,
373.15 K, is equivalent to 100.00°C.

A common temperature scale in everyday use in the United States is the Fahrenheit
scale. This scale sets the temperature of the ice point at 32°F and the temperature
of the steam point at 212°F. 

The relationship between the Celsius and Fahrenheit temperature scales is as follows in

the following equation

Tf = 9/5 Tc + 32 

 

Boiling point

The boiling point of a substance is the temperature at which it can change its state from a liquid to a gas throughout the bulk of the liquid.

A liquid may change to a gas at temperatures below the boiling point through the process of evaporation.

Any change of state from a liquid to a gas at boiling point is considered vaporization.

Absolute zero

Absolute zero is the lowest possible temperature where nothing could be colder and no heat energy remains in a substance.

Absolute zero is the point at which the fundamental particles of nature have minimal vibrational motion, retaining only quantum mechanical, zero-point energy-induced particle motion. By international agreement, absolute zero is defined as precisely; 0 K on the Kelvin scale, which is a thermodynamic (absolute) temperature scale; and –273.15 degrees Celsius on the Celsius scale.

Absolute zero is also precisely equivalent to; 0 degrees R on the Rankine scale (also a thermodynamic temperature scale); and –459.67 degrees F on the Fahrenheit scale. While scientists can not fully achieve a state of “zero” heat energy in a substance, they have made great advancements in achieving temperatures ever closer to absolute zero (where matter exhibits odd quantum effects).

In 1994, the NIST achieved a record cold temperature of 700 nK (billionths of a kelvin).

In 2003, researchers at MIT eclipsed this with a new record of 450 pK (0.45 nK).