Acidic, Basic and Neutral SolutionsChapter 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 1000B 3C 30D 100
Lesson Review 9-2
2. When an acid and base are mixed, the products are ____ and ____.
A water; a baseB water; ammoniaC water; saltD 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 acidicB homogeneousC basicD polar
Lesson Review 9-2
4. Blue litmus paper turns ____ in acid solutions.A redB orangeC greenD 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 tasteB produces OH– in waterC produces H+ in waterD 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 hydroxideB hydrogenC hydroniumD oxygen
Lesson Review 9-2
8. What is the symbol for the hydrogen ion?A H3O+
B H2O
C H+
D OH–