Chemistry and Liquids Help (page 4)
Introduction to Liquids
Lava, a cold mountain stream, and mercury are all liquids. They are very different, with different compositions and different standard temperatures, but they are all members of the in-between element club, known as the liquids. The members of this club are affected by environment much more than solids. Sometimes the difference of a few degrees of temperature can cause an element to slip from being a member of the solid club to new membership in the liquid club. Cesium has a particularly great trick of switching from a solid to a liquid. It is solid at room temperature, but when held in the hand and heated to body temperature of 98.6°F, it melts and becomes a liquid.
A chocolate bar does the same thing. On the shelf at room temperature, it’s a solid, but when held in the hand and heated to body temperature, it melts.
Liquids are less dense than tightly packed solids. The density of a liquid affects the way a liquid acts. It has different characteristics from solids, like the ability to flow across a table top when spilled.
Table 16.1 gives some general characteristics of liquids.
Cannot be tightly compressed
Have different viscosities or resistances to flow
Held together by surface tension
Miscible or immiscible
Expand and vaporize when heated
Unequal molecular bonding
The density of water is 1.00 g/ml at 4°C. The metric system of measuring liquid density is based on this number. When comparing the density of liquids, generally they can be compared to water. This makes it easier to figure out whether liquids will mix or not, since two liquids of very different densities don’t usually combine.
There are exceptions. Very dense ionic solutions like salt water will dissolve in water since both are polar. Oil which is non-polar will not dissolve in water even if the densities were close to each other. Their failure to mix is due to their properties, rather than density.
For example, the densities of mercury (13.5 g/ml) and water (1.0 g/ml) are very different relative to each other. This relative density difference (sometimes called specific gravity) causes mercury to sink to the bottom of a container filled with water.
Relative density (specific gravity) is the ratio of the density of a sample at 20°C divided by the density of water at 4°C.
Some of the physical characteristics that affect liquids are viscosity, surface tension, boiling point, vaporization, condensation, and evaporation. These are described below.
The shape and combination of ions and molecules in solids determines a lot about their characteristics. The same is true of liquids. The size, strength, and shape of molecules along with the intermolecular forces in liquids have a big effect on the viscosity of different liquids.
Think of how ketchup pours when we are hungry for French fries. Ketchup is more viscous than water. Water and wine pour easily because they have a low viscosity.
Look at the following liquids. Can you name which have high viscosity (hv) and which have low viscosity (lv)?
(1) pancake syrup, (2) vinegar, (3) motor oil, (4) apple juice, (5) molasses, and (6) pine sap.
Did you get (1) hv, (2) lv, (3) hv, (4) lv, (5) hv, and (6) hv?
Viscosity is the capability of a liquid to flow or not flow freely at room temperature.
Think of the difference between water and honey. Hydrogen and oxygen contain covalent bonding, but there are intermolecular forces of attraction between water molecules called hydrogen bonding. Commonly, these are stronger than the forces between organic molecules, like proteins and the sugars in honey. However, if the molecules are big enough, there can be very strong intermolecular forces between organic molecules. This is what happens with honey, giving it a very high viscosity. The stronger the molecular forces between molecules, the thicker (or more like a solid) a liquid becomes. The weaker the molecular forces, the thinner or less viscous the liquid. Table 16.2 compares the different strengths of bonding interactions.
In the petroleum industry, the separation of different parts or fractions of naturally occurring crude oil allows the individual collection of many different products. The initial, thick, sticky crude tar is heated in a column to separate the different parts of the oil which boil at distinct temperatures. The heated products that rise up the column all have different lengths of carbon chains and can be pulled off as pure fractions. This is called fractionation . Methane (CH 4 ), propane (C 3 H 8 ), and butane (C 4 H 10 ) are purified separately of any impurities and then used as fuels. Gasoline and diesel fuels are separated as hydrocarbon products of 6–12 carbon atoms per molecule. Tar with hydrocarbons of 20–40 carbon atoms per molecule can be collected at a different time and level than propane, since the larger the molecule, the higher the boiling point.
A strange thing happens at the surface of many liquids. The forces that hold molecules together pull down and to the sides, but there is no equal pull from above, where the surface is exposed to air. Then, the intermolecular forces from the rest of the liquid molecules flatten its shape when pulling from below.
The flattening allows the molecules to “float” or “ride” the molecules just below them that still keep their structure. Then rather than sinking, they form a film on the surface. When a molecule is in the middle of the liquid, it is attracted equally in all directions by the intermolecular forces. When a molecule is at the top, the forces underneath pull it unequally.
The stronger the molecular forces of a liquid, the greater the surface tension. Look at the strong intermolecular forces of water. When dropped as rain through the air, water droplets are spherical due to surface tension, and stretched slightly longer by gravity. Water droplets in microgravity experiments in space take on the shape of perfect spheres.
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 .
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 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).
In an open container, vaporization is not limited by the amount of open space above the liquid. It can continue until all the molecules have gone into the air. When this happens, it is called evaporation . That is why fish tanks need to have water added every few days; the water molecules escape to the air and the liquid water level goes down.
Evaporation is different from vaporization in that it happens below the boiling point of a liquid, generally at room or outside temperatures.
Lakes and streams get smaller and smaller in hot weather if there is no rain. The water molecules evaporate into the atmosphere and don’t come back down again. Water from underground reservoirs, called aquifers, are used up and not replenished. In a drought, water rationing is a way to limit water use until the environment changes, condensation happens, and water is returned to the earth.
Temperature has a huge affect on solids and liquids. Since many elements are solid at room temperature (15–25°C), we normally think of them as solids. When solids are heated, they melt and become liquid. As we learned in Chapter 15, this temperature is called the melting point.
When we look at liquids, however, boiling point is like the melting point for solids. It is the specific point where enough heat energy has been added to the molecules of a liquid to allow it to vaporize and become a gas. This energy change is usually needed to cause the molecules to move from the in-between liquid form to the gas form.
Boiling point is the temperature at which the vapor pressure equals atmospheric pressure. This allows liquid molecules to escape intermolecular bonding forces and escape into the air (as vapor).
Table 16.3 shows the big range of boiling points for liquids. It makes it easy to see how the millions of different reactions that happen with liquids are affected by temperature.
Liquids have a lot more freedom to perform different activities that denser solids cannot.
Practice problems for these concepts can be found at –Chemistry and Liquids Practice Test
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