Acids and Bases

How Acids and Bases are Made

Water will naturally "break up" into H⁺ and OH^- in a process known as dissociation. Water broken into an H and OH When water dissociates the hydrogen atom breaks its bond with oxygen and leaves behind its electron. The hydrogen atom is now positively charged and properly called a hydrogen ion. The remaining hydrogen is still connected to the oxygen, which now has an extra electron, giving this pair a negative charge. The OH^- molecule is properly called a hydroxide ion. The amount of dissociated water molecules in relation to all the water molecules is very small, and since the overall amounts of H⁺ and OH^- are equal, they cancel each other out.

If, for some reason, the H⁺ and OH^- are not balanced, an acid or base is formed. pH table The acidity or alkalinity (baseness) of the solution is rated on the pH scale. The range of this scale is from 0 to 14 with 7 being the neutral point. Numbers below seven are acidic, while numbers above seven are basic. It is important to note that this scale is logarithmic. Thus, a pH of 2 is not twice as acidic as a 4, but rather 100 times as acidic. That same pH of 2 is not three times as acidic as a pH of 6, but rather 10,000 times as acidic.

Acids are aqueous solutions that have more H⁺ than OH^-. This can be caused by adding H⁺ ions or by taking away OH^- ions (actually, the process is far more complicated than this, but for what you will need to do this definition will work fine). Acids are characterized by their sour taste and their ability to dissolve metals. Many of the foods you eat, such as oranges, green apples, and rhubarb, taste sour due to the acids which they form. Many cleaners have acids in them. It is important that you read the warning labels on your household cleaners as we do not advise that you use acidic solutions to clean water pipes in your home.

Bases are aqueous solutions that have more OH^- than H⁺. This can be caused by adding OH^- ions or by taking away H⁺ ions (again, the process is far more complicated than this, but for what you will need to do this definition will work fine). Bases are generally slippery and taste bitter. The scales of fish are coated with a base that makes them slippery. Soaps are also bases, and naturally are quite slippery. When you squeeze lemon on your fish before eating it, what you are actually doing is neutralizing the basic, bitter taste with an acid, thus making it more palatable. You may have also experienced this acid-base neutralization reaction if you have ever drank orange juice after brushing your teeth. Be advised that bases can be just as damaging as acids, and in some cases even more damaging (the slipperiness makes it hard to wash out of your eyes should it get into them). One advantage to using bases as cleaners is that they do not react with metals. The cleaners used to unclog sinks (e.g. Draino, Liquid Plumber) are strong bases that readily dissolve hair and grease, but leave the pipes unscathed.

Many common fluids are acids or bases, yet they do not harm us. Be advised that either an acid or a base, if too strong, can cause serious injury. Always use caution when working with either substance.

Link for Additional Study

A Tutorial on Acid-Base Balance

Chemistry Review Directory

CHEMystery: Acids and Bases: The pH Scale

In Class Activity

Identify Substances as Either Acid or Base

Materials Needed:

One pipet full of Universal Indicator (found in workshop materials packet)
Two empty pipets (found in workshop materials packet)
Common acids (lemon juice, vinegar, clear colored soda pop [Sprite], citric acid)
Common bases (ammonia, baking soda, soap, Tums/Rolaids)
Small clear container (plastic cup works great)

Procedure:

Fill the container about ½ full with water.
Put 4-6 drops of indicator into the water.
Put a small amount (1-2 ml) of acid or base into the water.

Helpful Hints:

The universal indicator will do a good job of telling if a substance is an acid or a base. The redder it gets, the more acidic; the bluer, the more basic. See the color chart to get an idea of the pH level.
You really don't need much acid or base to get the reaction and associated color change.
The clearer the solution you are testing, the more visible the results will be (i.e. don't use Pepsi or root beer to test the pH of soda pop; instead use Sprite, 7UP, lemon-lime, or Squirt).
Remember to make the link between acids tasting sour and bases tasting bitter. This will be easily recognized if the students taste the citric acid and the soap.
For a dramatic demonstration, place a small piece (about 2 cm³) of dry ice into the water. As the dry ice melts it will dissolve into the water and produce carbonic acid according to the following reaction:

Decomposition of dry ice into water producing carbonic acid: CO2 + H20 = H2CO3

As more dry ice dissolves the color of the water will begin to change. If you want to go through more colors, put a few grains of baking soda into the water (just enough to make it basic and change color, too much and the ice will not be able to neutralize it).

CAUTION: DRY ICE IS EXTREMELY COLD. DO NOT HOLD IT IN YOUR BARE HAND OR PUT IT IN YOUR MOUTH. IF IT CONTACTS YOUR SKIN IT WILL FREEZE IT AND THE TISSUE WILL DIE.

CAUTION: DRAINO, LIQUID PLUMBER, AND OTHER DRAIN CLEANERS ARE STRONG BASES. WE DO NOT RECOMMEND YOU USE THESE ITEMS IN THIS DEMONSTRATION.