Learning Objectives

Physical States

The compound water, H2O, can exist as a solid, liquid or gas depending on the temperature and pressure. In order for compounds to be in the liquid or solid states, the molecules must form attractions to each other.

The dipoles in a water molecule give the hydrogen atoms some positive character and the oxygen atom some negative character. This factor alone would make water molecules attract each other like small magnets. However, dipole-dipole attractions are fairly weak and would not account for the strong interaction that is observed between water molecules.

 

 

 

 

There is another, even stronger, attraction between water molecules that explains the unique character of water. Since the hydrogen atoms have positive character they are attracted to those orbitals of other molecules that contain non-bonding electron pairs. These attractions are called hydrogen-bonds.

It is these attractions that hold water molecules together in the solid or liquid states. Water is amazing because most compounds that are as small as water are gases at room temperature. Hydrogen-bonds give water a boiling point of 100oC, which allows it to exist as a liquid in our environment.

 

 

 

 

 

 

 

When water is in the form of ice, virtually all of the hydrogen atoms and oxygen atoms are involved in hydrogen-bonds.

If energy is applied in the form of heat, many of these hydrogen-bonds are disrupted and the water molecules are free to move about.  When this happens, ice becomes a liquid. If more heat is applied all of the hydrogen-bonds are disrupted and the molecules of water take the form of a gas.

Density

Density is the term chemists use to express how heavy matter is for a given volume. Density equals mass per volume. We tend to think of density as weight per volume since we weigh objects to determine their mass. The definition of density can be written as a mathematical expression. 

When we measure quantities of matter, we find that liquids are easier to measure by volume than they are by weighing the liquid, therefore we use density to convert the mass of a given liquid to volume. In the metric system of measurement, the density of water is one gram per milliliter.

Density as a Conversion Factor

Density problems are readily solved by dimensional analysis. The definition of density is grams per milliliter which can be thought of as so many grams equals one milliliter. Thus the conversion factor for water can be written either as 1gram/1mL or 1mL/1gram. These factors can be used directly to solve density problems. How many grams are present in 100 mL of water if the density of water is 1g/mL?

The Importance of Water

We personally use about 300 liters of water a day. However, domestic use of water only represents 9% of the total per capita consumption.

Agricultural use represents 34% of the water used and industrial use accounts for the remaining 57%.

The per capita consumption of water in the United States is the highest in the world at about 7,200 liters per day. Japan uses about 2,600 liters per day and China uses only 1,200 liters per day.

It is interesting that 97.4% of all the water is in the oceans. Of the remaining 2.6%, all but 0.014% is in groundwater and ice caps. It is this remaining 0.014% that is present in rivers, lakes and the atmosphere.

Water is a precious commodity and converting seawater into pure water is very important in many parts of the world. Seawater is a mixture of water and dissolved salts.

Mixtures

A mixture is a physical combination of two or more substances. The substances in a mixture may be present in any amounts.

Air is an example of a mixture of compounds. Nitrogen, N2, is the major component of air at about 78%. Oxygen, O2, is the next major component of air at about 21%. The remaining 1% consists mostly of argon (an inert gas). It also contains molecules of water (0-4%), carbon dioxide (0.035%) and about ten other compounds.

Percentage is not commonly used to describe very low concentrations. Instead, low concentrations are often given in parts per million or ppm.

Air can also contain compounds that we consider air pollutants, such as sulfur dioxide, carbon monoxide, ozone and nitrogen oxides. Very small amounts of these pollutants can cause serious health problems.  The permissible limits of these compounds are shown in the following table.

Compounds Sulfur Dioxide Carbon Monoxide Ozone Nitrogen Oxides
Limits in PPM 0.3 9 0.12 0.053

Sulfur dioxide and nitrogen oxides irritate the respiratory tract, affect breathing and increase our susceptibility to respiratory infections. Carbon monoxide is absorbed into the blood and interferes with oxygen uptake. This affects manual dexterity, learning, visual perception and cardiovascular function. Ozone is quite toxic and reduces lung function. Amazingly low concentrations of these compounds in our air can cause serious health problems. The source of most of these pollutants is exhaust from internal combustion engines and coal fired electric power generation...the production of energy.

Ionic Compounds in Aqueous Solutions

The human body is essentially 70-80% water so the behavior of ionic compounds in water is very important. For example, K+ is a neurotransmitter and Ca+2 is involved in many processes including vision.

When ionic compounds (salts) dissolve in water, the ions are solvated or blanketed with water molecules…usually by hydrogen bonds or electrostatic attractions.

Not all salts are soluble in water. The solubility of a salt is governed by the energy requirement of staying together versus the energy requirements of being solvated by the water molecules.

Electrolytes

Ionic solutions are strong electrolytes, which means that they conduct electricity very well. Even a partially soluble salt will be an excellent conductor of electricity. Pure water will not conduct electricity.

Colligative Properties

Colligative properties are properties of a solution that depend on the concentration of a solute in a solution and not on the identity of the solute. Osmosis is an example of a colligative property.

In the case of colligative properties, the concentration of the solute is actually expressed in terms of total solute particle concentration (TSPC). For example, if sugar were dissolved in water, the solution would contain one sugar particle for each molecule of dissolved sugar. If sodium chloride was dissolved in water, the molecule would completely ionize to form sodium ions and chloride ions. Therefore, in the case of NaCl, the solution would contain twice the number of particles as the number of molecules dissolved in the solution. The sugar will not ionize; therefore there is no increase in particle concentration. If sodium nitrate (NaNO3) is dissolved in water, how many solute particles will be present?

Freezing Point Depression

A substance freezes to form a solid when the interactions between the molecules of liquid become strong enough to prevent them from breaking apart at a given temperature. In other words, the thermal energy of the system is too low to break up the interactions between the molecules.

When a solvent interacts with and dissolves a solute, many of the molecules of the solvent are used in these solvation interactions and thus these molecules are not available to interact with other solvent molecules to form a solid. In order for the solvent of a solution to freeze, the temperature must be lowered considerably below the normal freezing point for the pure solvent. The freezing point of a solution will be lowered.

Boiling Point Elevation

The boiling point of a solution is also a colligative property as long as the solute is not volatile. A liquid will boil when the molecules of the liquid have enough energy to break free of the interactions that hold these molecules together. Boiling occurs when the pressure of the vapor from the liquid equals the outside air pressure on the liquid. The reasons for this phenomenon involve the energy of disorder (entropy). A solution is more disordered than the pure liquid and that is favored by entropy…it has a lower energy. When a liquid boils, it becomes more disordered (The gas molecules are more random than the somewhat ordered liquid molecules.) but since a solution is already in a more disordered state, it takes more energy to boil a solution.

Problems

1. Describe how water molecules are attracted to each other.

2. Why is distillation an impractical method for the conversion of seawater into pure water?

3. What is the mass of 100 mL of sulfuric acid (battery acid) if the density of the acid is 1.84 g/mL at 20oC?

4. What is the volume in mL of 100 g of gold metal if the density of gold is 19.3 g/mL at 20oC?

5. How much would 1000 mL of mercury weigh if the density of mercury is 13.55 g/mL at 20oC?

6. How many mL would 454 g of lead occupy if the density of lead is 11.3 g/mL at 20oC?

7. The density of mercury is 13.55 g/mL at 20oC and the density of gold is 19.3 g/mL at 20oC.  Mercury is a liquid at 20oC.  Will gold float or sink in mercury?

8. The concentration of argon in air is 0.9%.  Express this concentration in parts per million.

9. Which of the following salts will cause the greatest lowering of the freezing point of water? Assume these salts have the same concentration in the water and all are equally soluble.

NaCl

CaCl2

AlPO4

Return to the Reviews of Concepts

Copyright © January 2001 by Richard C. Banks...all rights reserved.