Differences Between Acids & Bases

Acids and bases have some superficial differences, such as a difference in taste (acids are bitter and bases are sour). They also react with different classes of chemicals. However, they also have some mirror properties. For example, they both react with each other to produce salts, and they both conduct electricity.

Definition

Acids are substances that, in water solution, increase the concentration of H+ (hydrogen ion). Hydrochloric acid (HCl) is an example, because its hydrogen dissociates from the chlorine in solution. A base increases the concentration of OH- (hydroxide) in solution. Sodium hydyroxide (NaOH) is an example, because the OH- dissociates from the Na+ in solution.

The pH Scale

Acid and base solutions are differentiated on the pH scale. The autoionization of water leaves a portion of H2O molecules dissociated as OH- and H+. They have equal amounts, so their pH is 7.0. Why 7.0? Because the concentration of H+ in pure water at normal temperatures is 10^-7 moles per liter (M). pH is defined as the negative of the base-10 logarithm of the concentration of H+.

Since the scale uses a base-10 logarithm, one point on the scale refers to a change in the H+ concentration by a factor of 10. Values less than 7 are acidic. Values above 7 are basic.

There is no particular reason why the scale is built around H+. Since the concentrations of OH- and H+ multiply to equal 10^-14 moles per liter, we could just as easily define a pOH scale, where pOH + pH = 14.

Reactivity

What is the difference in how acids and bases act when in contact with other substances? There is a conception that acids are dangerous and bases aren't. But there is symmetry in their behavior. When they combine they can react violently. Consider for instance the combination of baking soda (a base) and vinegar (an acid). The resulting reaction produces tremendous foaming, but the volatility isn't owed to either the base or the acid alone, but instead both combined.

Aside from combining with bases, acids additionally react with metals in the electromotive series to produce hydrogen gas. They also react with carbonates to produce carbon dioxide gas.

Bases and Acids on Your Skin

Bases and acids feel different on your skin. The OH- of bases breaks down the oil on your skin, creating a slippery feeling. The H+ from an acid reacts in a variety of ways with the skin. HCl, for instance, causes a burning sensation even at low concentration, whereas carbonated sodas and lime juice have pH of around 2-3, but cause no such burning sensation. Nitric acid causes no burning sensation either, but it rapidly reacts with skin to leave a brown stain.

Common Misconception

Many beginning chemistry students think that pH is a measure of charge in the water, since it measures the ratio of OH- to H+. These are not the only charged particles in an acid or base solution though. In pure water (pH = 7.0), the number of OH- and H+ ions are equal. But in hydrochloric acid, for example, the greater number of H+ come from the HCl. The Cl- ions don't enter into the pH calculation, but they do balance out the charges so that the solution stays neutral.

To expand on this point, the charge of the solution remains neutral. Electrostatic forces are a billion-billion-billion-billion (10^36) times stronger than the gravitational force. Therefore, pH level is not an electricity or electrostatics problem. Even a deviation in charge of 1 percent from neutrality would not be allowed, because the force of attraction by the solution on surrounding charges to neutralize it would be enormous. In fact, if the solution had the mass of the Earth, the force to lift it would be comparable to the force the 1 percent charge imbalance created to bring about its return to neutrality.

References

• Raymond Chang; Chemistry; 1984
• Acid Strength and Skin Interaction