Acids & BasesSciencePower 10 (p210-235)

Acids, Bases, and Salts

• Scientists often refer to substances as being either acids, bases, or salts.

• During scientific experiments, it is important to know what type of substance you are working with, so that you have an idea of how it might react with other elements and chemicals.

Acids, Bases, and Salts

• Acid: Sour-tasting compound that produces hydrogen ions (H-) when it dissolves in water.

• Base: Bitter-tasting, slippery-feeling compound that produces hydroxide ions (OH-) when it dissolves in water.

• Salt: An ionic compound that results from the reaction between an acid and a base.

• pH (“potential Hydrogen”): The relative acidity (how acidic something is) or alkalinity (how basic something is) of a substance. pH refers to

What is pH?• The relative acidity or alkalinity of a substance.

• Acidity How acidic something is• Alkalinity How basic something is

• pH stands for “potential Hydrogen”: The ability of molecules to attract hydrogen ions.

• An acidic molecule (acid) has a low ability to attract hydrogen ions pH of 0-6

• An alkaline molecule (base) has a high ability to attract hydrogen ions. pH of 8-14

Characteristics of Acids

• Low pH (below pH of 7)– Turns blue litmus paper to red

• Sour taste

• Reacts with metals to produce salt & hydrogen

• Reacts with carbonate salts to produce carbon dioxide

Characteristics of Bases

• High pH (above pH of 7)

• Turns red litmus paper blue

• Slippery to the touch

• Taste bitter

• Produces hydroxide ions (OH-) when it dissolves in water.

Characteristics of Salts

• A salt is formed when an acid and a base are mixed.

• The acid releases H+ ions while the base releases OH- ions. This process is called hydrolysis.

• The pH of the salt depends

on the strengths of the original acids and bases:

Acid pH Base pH Salt pH

Strong Strong pH = 7

Weak Strong pH > 7

Strong Weak pH < 7

Weak Weak depends on which is stronger

The pH Scale

                                                                                                                                                                                                   

      

Common Acids pH Common Bases pH

Hydrochloric acidSulphuric acidStomach juiceLemonsVinegarApplesOrangesGrapesSour milkWhite breadFresh milk

0.10.31-32.32.93.13.544.45.56.5

Human salivaDistilled waterBlood plasmaEggsSeawaterBoraxMilk of magnesiaAmmonia waterLimewaterCaustic soda

6-877.47.87.99.210.511.612.414

Litmus Paper – Universal pH Indicator

Acids and Bases

• When acids and bases react with each other, they form salts.

• In the late 1800s, 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 (H+) into solution.

• For example, hydrochloric acid (HCl) dissolves in water as follows:

In H2O (water)…

HCl H+(aq) + Cl-(aq)

Acids

• Arrhenius defined bases as 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):

In H2O (water)… NaOH Na+(aq) + OH-(aq)

Bases

• The Arrhenius definition of acids and bases explains a number of things about acids and bases:

• Why all acids have similar properties to each other.

• Why all bases are similar.

• That all acids release H+ into solution and all bases release OH-.

• That acids and bases counteract each other. This means that a base added to an acid can make the acid weaker, and that adding an acid to a base can make the base more acidic. This is called neutralization.

Acid, Base, or Salt? – Activity

• Classify each of the following substances as either an acid, a base, or a neutral substance.

• Lemon Juice (pH 2.3)• Eggs (pH 7.8)• Blood Plasma (ph 7.4)• Apples (pH 3.1)• Water (pH 7)• Limewater (pH 12.4)• Stomach juice (pH 1-3)• Vinegar (pH 2.9)