Tuesday, January 26, 2016

Acids and Bases Article

Acids, Bases, and Neutrals
Name ______________________________   Date __________

For thousands of years people have known that vinegar, lemon juice, and many other foods taste sour. However, it was not until a few hundred years ago that it was discovered why these things taste sour – because they are all acids.

In the seventeenth century, the Irish writer and amateur chemist Robert Boyle first labeled substances as either acids or bases (he called bases alkalies), according to the following characteristics:

Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and become less acidic when mixed with bases.

Bases feel slippery, change litmus blue, and become less basic when mixed with acids.

While Boyle and others tried to explain why acids and bases behave the way they do, the first reasonable definition of acids and bases would not be proposed until 200 years later.

In the late 1800s, the Swedish scientist Svante Arrhenius proposed that water can dissolve many compounds by separating them into their individual ions. Arrhenius suggested that acids are compounds that contain hydrogen and can dissolve in water to release hydrogen ions into solution. For example, hydrochloric acid (HCl) dissolves in water as follows:

HCl            +   H2O    H+   + Cl-

Bases are substances that dissolve in water to release hydroxide ions (OH-) into solution. For example, a typical base according to the Arrhenius definition is sodium hydroxide (NaOH):

NaOH   +   H2O    Na+    +            OH-

Acids act like they do because all acids release H+ into solution (and all bases release OH-).

This idea, that a base can make an acid weaker, and vice versa, is called neutralization.

Neutralization

Acids release H+ into solution and bases release OH-. If we were to mix an acid and base together, the H+ ion would combine with the OH- ion to make the molecule H2O, or plain water.  Water is NEUTRAL.

H+            +            OH-                        H2O


The neutralization reaction of an acid with a base will always produce water and a salt, as shown below:

Acid                        Base                        Water                        Salt
HCl            +            NaOH                        H2O            +            NaCl
HBr            +            KOH                        H2O            +            KBr


We measure the strength of acids and bases using the pH scale.

pH = -log [H+]

This means that the LOWER the pH number, the STRONGER the acid.  The higher the pH, the stronger the base (more OH-, less H+). 

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic. Pure water is neutral.
Acids with a low pH of around 1 are very reactive and can be dangerous. The same is true for bases of a pH near 13.
There are many strong acids and bases in nature. Many plants have acids and bases in their leaves, seeds, or even their sap. Citrus fruits like lemons and oranges have citric acid in their juice. This is what makes lemons taste so sour.

Our bodies use acids and bases too. Our stomachs use hydrochloric acid to help digest foods. This strong acid also kills bacteria and helps to keep us from getting sick. Our muscles produce lactic acid when we exercise. Also, our pancreas uses a base called an alkali to help with digestion. These are just a few examples of how the chemistry of bases and acids help our bodies function.

ACID/BASE Indicators

If you want to know if a substance is an acid, base or neutral, you need to use an acid/base indicator.  An indicator is a substance that changes color in the presence of H+ or OH-. 

Some common indicators:

LITMUS

Red litmus paper turns blue in the presence of a base. (pH = greater than 7)
Blue litmus paper turns red In the presence of an acid. (pH = less than 7)
If neither color changes, it’s a neutral. (pH = 7)

Phenol Red

Phenol red liquid is red in a neutral, yellow in an acid, and magenta in a base.


Universal pH indicator

This liquid is green in a neutral, red in an acid and blue in a base.

Purple Cabbage Juice

Cabbage juice is purple in a neutral, reddish pink in an acid, and blue to greenish yellow in a base.