When a pure substance is placed in contact with water, there are three possible outcomes. The substance may do nothing that is, the substance does not dissolve and no visible change takes place. The substance may visibly react with the water producing a gas, a temperature change, a color change, or other physical manifestations indicating that a new substance has been formed. The substance may dissolve in the water with no visible change in the system except the dissolution. Which of these three processes takes place depends on the structure and properties of the substance (solute) and the dissolving medium (solvent).
"Like dissolves like" is a general rule frequently stated by chemists. The rule means that the more similar substances are in their properties then the more likely the substances are to dissolve in one another. Polar compounds are more soluble in polar solvents and less soluble in nonpolar solvents, and nonpolar compounds are more soluble in nonpolar solvents and less soluble in polar solvents.
What is polarity? In a covalent bond, where electrons are shared some atoms attract the shared electrons of the bond more than others. These atoms are said to be more electronegative. The differences in electronegativity of atoms in a bond cause a partial negative charge on one end of the bond and a partial positive charge at the other end of the bond. The bond has two poles or is said to be polar. If the atoms in the bond are the same or have very similar abilities to attract electrons, the electrons in the bond would be evenly distributed between the two atoms and there would be no separation of partial charges. The bond would be nonpolar.

A molecule like water which has an angular shape - the bonds form an angle of 105º - and which has polar H-O bonds, has a partial positive charge at the hydrogen end and a partial negative at the O end. Water therefore has two poles (dipole) and is a polar molecule. The polar nature of the water molecule is largely responsible for its ability to dissolve ionic compounds and other polar covalent compounds. Many chemical reactions and most biochemical reactions take place in water.
On the other hand molecules such as methane (CH4) or hexane (C6H12) which have polar C-H but have symmetrical spatial distribution of the C-C and the C-H bonds are not dipole molecules. They are nonpolar. Kerosene, oil and gasoline are also nonpolar compounds.

Soluble ionic compounds, such as sodium chloride (NaCl), contain positive and negative ions that are attracted to polar compounds like water, but not to nonpolar compounds like hexane or kerosene. If ionic compounds dissolve in water, the charged ions are free to move about in the solution independent of each other. These solutions will conduct an electric current. An electrolyte is a substance whose aqueous solution conducts electricity well. A nonelectrolyte is a substance whose aqueous solution does not conduct electricity well. Conduction of electricity is the result of a substance "breaking up" into charge components called ions. In the case of molecular compounds, this "breaking up" process is called ionization; in the case of ionic compounds, the term dissociation is used. For example:
IONIZATION OF A MOLECULAR COMPOUND
HCl(aq) ï‚® H+(aq) + Cl-(aq)
DISSOCIATION OF IONIC COMPOUNDS
NaCl(aq) ï‚® Na+(aq) + Cl-(aq)
NaOH(aq) ï‚® Na+(aq) + OH-(aq)
Aqueous solutions of nonelectrolytes do not conduct electricity. Nonelectrolytes include (1) insoluble ionic and molecular compounds, and (2) soluble, ionic, and molecular compounds which do not ionize or dissociate in water. Nonelectrolytes should not be written as separate ions in solution. Some common molecular compounds which are soluble nonelectrolytes are:
Water* H2O
Ethanol(common alcohol) CH3CH 2OH
Glucose C6H12O6
Acetone (nail polish remover) CH3COCH3
Isopropyl alcohol (rubbing alcohol) CH3CH(OH)CH3
*You may note that, except for water, all of these are organic compounds (contain carbon).
In order to adequately describe the behavior of a compound in contact with water, it is necessary to determine whether the compound actually dissolves in water. Experimental observations have led to guidelines for predicting solubility for ionic compounds. Memorizing solubility is easier than it might first seem because a lot of data for common ionic compounds can be summarized in a few solubility guidelines as follows:
1. Almost all nitrates, acetates and perchlorates are soluble.
2. Most salts of GroupIA metals (Li+, Na+, K+, Rb+. Cs+) and the ammonium ion (NH4+) are soluble.
3. Most chlorides, bromides, and iodides are soluble except those of Ag +, Pb2+, and Hg22+.
4. Most sulfates are soluble except SrSO4, BaSO4, PbSO4, and Hg2SO4. CaSO4 is slightly soluble.
5. Carbonates, hydroxides, phosphates and sulfides are insoluble except ammonium and GroupIA metal salts of these anions. Hydroxides and sulfides of Ca2+, Sr2+, and Ba2+ are slightly to moderately soluble.
Hard water contains appreciable amounts of calcium (Ca2+) and magnesium (Mg2+) ions. In the presence of bicarbonate (HCO3-) and a pH>7, massive scale formation occurs due to formation of insoluble carbonates:
Ca2+(aq) + 2HCO3-ï‚®CaCO3(s) + CO2(g) + H2O(l)
Mg2+(aq) + 2HCO3-ï‚®MgCO3(s) + CO2(g) + H2O(l)
When hard water reacts with soap, the ions in the hard water and some of the soap molecules form insoluble salts (soap scum). The soap molecules tied up in the scum are not free to perform their cleaning function. Initially soap is used to remove these ions, and then more soap needs to be added to produce more suds and better cleaning. The reaction of a soap solution with the ions in hard water can be used to compare hardness of water samples.
A. SOLUBILITY
Materials Needed
Test Tubes
Isopropyl alcohol (rubbing alcohol)
Vegetable oil (corn oil, olive oil, or safflower oil)
Fat or lard (Crisco)
Sodium Chloride (common salt)
Sodium bicarbonate (Baking Soda)
Distilled water
Digital Scale
Plastic Pipette
CAUTION: ALCOHOL, KEROSENE AND LAMP OIL ARE FLAMMABLE. DO NOT USE THESE SUBSTANCES NEAR OPEN FLAMES, HOT GRILLES OR HEATING ELEMENTS.
Procedure
1. To test the solubility of a liquid in a liquid (miscibility), mix approximately 1 mL of each liquid in a test tube. Shake the test tube or stir with a glass rod and observe the formation of two layers (or not) after being left standing a few minutes.
2. To test the solubility of a solid in a liquid, add a small amount of the solid about the size of a grain of rice or pea (0.1 g to 1.0 g) - to approximately 1mL of liquid in a test tube. Shake the test tube or stir with a glass rod.
3. Record and rationalize your observation on the REPORT FORM.
B. HARDNESS OF WATER
Materials Needed
125-mL flask
Plastic pipette
Liquid soap or shampoo
Distilled water
Tap water
Other water samples
Procedure
4. Place 50 mL of the water sample to be tested, (begin with distilled water) in a 125-mL flask.
5. Using a plastic pipette, add 0.1 mL of liquid soap or shampoo. Stopper the flask and shake for 10 sec. With distilled water, you should see a thick layer of suds. If you don't, add another 0.1 ml of liquid soap or shampoo and shake for 10 sec again. The suds that form in the distilled water sample will serve as your reference sample. Save for comparison. Shake again if necessary.
6. Add 0.1 mL of the same liquid soap or shampoo to another water sample (tap water). Shake. If no suds form, add another 0.1 mL of liquid soap or shampoo and shake. If no suds form, continue adding liquid soap or shampoo in 0.1 mL volumes and shaking after each addition until the sample forms an amount of suds similar to that in the distilled water sample. Stop if no suds are formed after you have added 10 mL of liquid soap or shampoo. Test an assortment of water samples available from your home (garden hose, pool, well, softened tap water, rain water, etc.).
7. On the REPORT FORM, record the volume of liquid soap or shampoo needed to form a thick layer of suds (reference distilled water sample) for each water sample tested.

Transcribed Image Text:

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