Heat in changes of state

When a liquid’s temperature is reduced to or below its freezing point, freezing, or solidification, occurs. With the exception of helium, all known liquids freeze at low temperatures. (At atmospheric pressure, liquid helium remains a liquid even at absolute zero, and can only solidify at higher pressures.)
The melting and freezing points of most substances are the same; however, some substances have different solid-liquid transition temperatures. Agar, for example, has a hysteresis in its melting and freezing temperatures: it melts at 85 degrees Celsius (185 degrees Fahrenheit) and solidifies between 31 and 40 degrees Celsius (89.6 degrees Fahrenheit to 104 degrees Fahrenheit).

Phases of matter and the phase changes

To cool a drink with ice cubes (solid to liquid), cool our bodies with perspiration (liquid to gas), and cool food inside a refrigerator, we use variations between the gas, liquid, and solid states (gas to liquid and vice versa). As a refrigerant (solid to gas), we use dry ice, which is solid CO2, and we make artificial snow for skiing and snowboarding by converting a liquid to a solid. This segment looks at what happens when one of the three types of matter is transformed into the other two. These state shifts are often referred to as phase shifts. Figure (PageIndex1) depicts the six most common phase transitions.
The enthalpy changes associated with various chemical and physical processes were previously described. Table (PageIndex1) lists the usual boiling points and enthalpies of vaporization ((H vap)) of selected compounds, as well as the melting points and molar enthalpies of fusion ((H fus)), the energy needed to transform from a solid to a liquid, a process known as fusion (or melting).

Aleks – identifying phase transitions on a heating curve

So far, we’ve discovered that applying thermal energy to a material by heating it raises its temperature. Surprisingly, there are times when adding energy has little effect on the temperature of a material. Instead, the extra thermal energy induces a phase shift by loosening bonds between molecules or atoms. Latent heat is described as energy that enters or leaves a system during a phase change without causing a temperature change in the system (latent means hidden).
Strong, liquid, and gas are the three states of matter that you are most likely to experience (see Figure 11.9). Solids have the least energetic state; atoms in solids are in close contact with one another, with forces between them allowing them to vibrate but not change direction with neighboring particles. (Think of these forces as springs that can be stretched or compressed but not broken easily.)
Plasma is the most energetic of all the states. While you may not be familiar with plasma, it is the most common state of matter in the universe—stars and lightning are both made of plasma. Heating a gas to the point that particles are torn apart, separating the electrons from the rest of the atom, produces the plasma state. This results in plasma, which is an ionized gas made up of negatively charged free electrons and positively charged ions.

Phase changes

Temperature changes may cause substances to change phase. Most substances are solid at low temperatures; as the temperature rises, they become liquid; and at even higher temperatures, they become gaseous.
Melting is the transformation of a solid into a liquid. (An older word you can come across is fusion.) Solidification is the result of a liquid turning into a solid. The temperature at which a pure material melts (also known as the melting point) is a property of that substance. The transformation of a solid into a liquid necessitates the use of energy. To change from a solid to a liquid, any pure substance requires a certain amount of energy. The enthalpy of fusion (or heat of fusion) of the material is denoted by the symbol Hfus. Table 10.2 “Enthalpies of Fusion for Different Substances” lists certain Hfus values, which are believed to be for the melting point of the material. Since the unit of Hfus is kilojoules per mole, we must know the amount of material to determine the amount of energy involved. The Hfus is always a positive number in the table. It can, however, be used for both melting and solidification processes as long as you remember that melting is always endothermic (i.e., H is positive), while solidification is always exothermic (i.e., H is negative).