Acidic, Basic and Neutral Solutions

Chapter 9, Lesson 2

Acids

• An acid is a substance that releases a positively charged hydrogen ion, H+, in water.– Acids taste sour and turn blue litmus paper

red.– The hydrogen ion from an acid combines

with a water molecule in solution to form a hydronium ion, which has the formula H3O+

– Because acid solutions contain ions, they conduct electricity.

Hydronium ion

• When an acid dissolves, it releases a hydrogen ion.

• The H+ ion combines with water to form H3O

+, the positively charged hydronium

ion.

Hydrogen ion Water molecule Hydronium ion

What is a hydronium ion?

Uses of Acids

• Acids are important in several body processes, including breaking down food in the stomach.

• Acids are also used in making many products, such as fertilizers, detergent, and cleaners.

Bases

• A base is a substance that produces hydroxide ions when dissolved in water.– The formula for the hydroxide ion is OH-– Bases feel slippery on your skin, taste bitter,

and turn red litmus paper blue.– Most bases are ionic compounds.

Hydroxide ion

• In water, bases separate into a positive ion and the hydroxide ion (OH–).

• All the properties of bases are a result of the presence of hydroxide ions that form when the base is dissolved in water.

Uses of Bases

• Common bases include baking soda and cleaning agents.

• Bases are also used to produce new products, such as soap, paper, and plaster.

Acids and Bases

What is pH?

• pH is a numerical scale used to indicate how acidic or basic a solution is.

• The pH scale runs from 0 to 14.

– Acids are below 7; strong acids are near 0.

– Bases are above 7; strong bases near 14.

– Neutral solutions have a pH of 7.

pH and Hydronium ion Concentration

• The higher the concentration of hydronium ions, the more acidic a solution is.

• All acid and base solutions contain both hydronium ions and hydroxide ions.

• In a neutral solution (pH=7) the hydronium and hydroxide ion concentrations are equal.

• A change in one pH unit represents a tenfold change in the acidity or alkalinity (how basic) of a solution.

Neutralization

• When an acid and a base are mixed, a neutralization reaction occurs.

• A neutralization reaction produces water and a salt.

• Stomach acid reacts with magnesium hydroxide in the following equation:

2HCl + Mg(OH)2 → MgCl2 + 2H2O

• Neither of the products is acidic or basic.

How is pH measured?

• An indicator is a compound that changes form one color to another within a particular pH range.– Indicators can be used to determine

approximate pH, simply whether something is acidic or basic.

How is pH measured?

pH strips

• Litmus is one of the simplest indicator test papers but also the least accurate.

• pH testing strips are a quick way of determining the approximate pH of a solution.

pH Meters

• pH meters are electronic instruments with an electrode that is sensitive to hydronium ions present in a solution.

– Accurately measures pH

– Requires an electric source to operate

pH Meters (cont’d)

Lesson Review 9-2

1. An acid with a pH of 3 is contains how many times as many hydronium ions as an acid with a pH of 6?

A 1000

B 3

C 30

D 100

Lesson Review 9-2

2. When an acid and base are mixed, the products are ____ and ____.

A water; a base

B water; ammonia

C water; salt

D ammonia; salt

Lesson Review 9-2

3. Red litmus paper is dipped in a solution. The paper turns blue. The solution must have been ____.

A acidic

B homogeneous

C basic

D polar

Lesson Review 9-2

4. Blue litmus paper turns ____ in acid solutions.

A red

B orange

C green

D blue

Lesson Review 9-2

5. Which of the following is a hydroxide ion?

A H3O+

B H+

C OH–

D NH4+

Lesson Review 9-2

6. Which is a property of a base?

A sour taste

B produces OH– in water

C produces H+ in water

D reacts with metal to produce hydrogen gas

Lesson Review 9-2

7. What ions are present in the greatest amount in a solution with a pH of 2.5?

A hydroxide

B hydrogen

C hydronium

D oxygen

Lesson Review 9-2

8. What is the symbol for the hydrogen ion?

A H3O+

B H2O

C H+

D OH–