“Acids, Bases, and the pH Scale

Acids and Bases are Everywhere

• Most substances you see are either an acid or a base. • Distilled water is an exception. It is completely neutral. • Pure distilled water has a pH value of exactly 7. But

most common substances are fairly close to neutral.• Substances at the far ends of the pH scale are

considered dangerous (strong acids and bases).

ACID (O-7 pH) NEUTRAL (7 pH) BASE (7-14 pH)

What are Acids?• An acid is a substance

when dissolved in water produces hydrogen ions H+.

• Acids have pH values less than 7.

ACID (O-7 pH) NEUTRAL (7 pH) BASE (7-14 pH)

What are Acids? (continued)

• Acid comes from the Latin word acidus that means "sharp" or "sour".

• Acids are common• Some are dangerous and

can burn your skin• Some are safe to eat and

drink• Stomach acid helps digest

food

What are Bases?• A base produces

hydroxide ions, OH-

when dissolved in water.

• Bases have pH values greater than 7 (7-14).

ACID (O-7 pH) NEUTRAL (7 pH) BASE (7-14 pH)

What are Bases?• Another word for base is

alkali.• In our home we often use

bases to clean things like bleach and toothpaste

• Some bases are safe to use (like soap or toothpaste) and some are dangerous like strong ammonia or drain cleaner.

• When acids and bases are mixed and react with each other they form water

• This reaction is called neutralization.

• Hydrogen ions, H+ from the acid combine with the hydroxide ions of the base, OH- to form water HOH or H2O (water).

WaterH (+)O

H (-)

Acid Base

H H O

Water

Neutralization

H H O

What is an indicator?

• To decide if something is an acid or a base we can use an indicator

• Indicators are weak acids and bases that change color due to the pH of the substance to which they are added.

• Red Cabbage juice is a natural indicator and will turn a specific color at a specific pH.

Other Acid-Base or pH Indicators• Introduction to Indicators1. Phenoyl Red – turns yellow in the presence

of an acid and pink in a base.

2. Phenolphthalein – turns pink for a base (remember the Sodium Demo)

3. pH paper turns various colors to give you a measure of how acidic or basic a substance is and tells you the pH pf the substance.