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Chemistry and Liquids Help (page 3)

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By — McGraw-Hill Professional
Updated on Aug 28, 2011

Vaporization

Liquids are affected by the amount of surface area exposed to air. Liquid surface molecules sometimes reach the energy needed to escape the rest of the liquid sample and become a vapor (gas). This is called vaporization .

Vaporization is the way that molecules change from a solid or liquid to a vapor (gas).

In science fiction movies, the alien monster is sometimes “vaporized” by the hero’s ray gun. This uses the same idea. The alien monster’s molecules go from a solid threatening form to being scattered harmlessly into the air.

Vaporization needs heat to occur. Some liquids can go to the vapor form at room temperature and use heat from the environment. When water or perspiration dries (turns to vapor) from the surface of the skin, it uses body temperature. Heat energy from the body gives water molecules the energy to break surface tension attractions and become vapor. The amount of heat that it takes to vaporize 1 mol of liquid at a constant temperature and pressure is called the molar heat of vaporization .

Condensation

When a closed container is completely full of molecules in the vapor form above the surface, those molecules become jammed together. In a closed container, vaporization goes on only until the space above the liquid is saturated , or so full of the vaporized molecules of the liquid that there is no more room to expand.

When a container’s air space becomes saturated, some of the vaporized molecules crash back into the liquid’s surface and are captured. When this happens, the liquid form is preserved.

Condensation is the opposite of vaporization. Molecules go from a vapor (gas) form back to a liquid state.

In a closed container, the rate of evaporation and condensation is not steady the whole time. It changes constantly. At first, the molecules slowly enter the vapor state. After a while, the closed air space is full of molecules and the liquid state grabs back the molecules that hit the surface. When this happens, the liquid sample has reached a state of equilibrium .

While the rates of vaporization and condensation are different at first, the rate of vaporization begins to slow down, while the rate of condensation begins to speed up. When the two rates become the same, dynamic equilibrium is reached. The exchange is dynamic because the molecules are not stuck, but continue to move back and forth between phases. The overall number of molecules in each phase is constant when the rates are the same.

Equilibrium of a liquid in a closed environment takes place when the rate of condensation and evaporation is balanced. Dynamic equilibrium comes about when both forward and reverse reactions happen at the same time.

Equilibrium can be easily remembered in the following way:

Liquid ↔ gas

Vaporization ↔ condensation

When liquid molecules turn to vapor (gas) at equal rates or molecules vaporize at the same rate that they condense, then dynamic equilibrium is accomplished. This dynamic equilibrium can continue to change. When heat is applied, molecules get more energy and vaporize quicker, but at the same time, condensation is speeded up and equilibrium is established at a higher temperature. This is why it is called dynamic, because it continues to move.

Solubility

Solubility takes place when one compound is dissolved into another. These compounds, looking for others to bind to and become more stable, separate into individual ions. In general, the solubility of any solute is written as the ratio of grams of solute per 100 grams of water at a specified temperature.

Polar liquids, like water, are able to dissolve polar and ionic solutes. Non-polar liquids, like gasoline and acetone, are able to dissolve non-polar solutes.

Most chemical reactions are done in solutions using the different properties that a solution has apart from its component parts. Often, the melting point or freezing point of a solution is lower for the solution than its parent liquids. For example, ethylene glycol (CH 2 OHCH 2 OH) when mixed with water serves as an anti-freeze in motor vehicles. The combined solution of water and ethylene glycol freezes at a temperature below water’s freezing point (−13°C instead of 0°C).

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