Properties of Water:

Acids and Bases

Molecules “break apart” in water to Form IONS:

• IONS are atoms or molecules that havean overall net charge

• In water, „break apart‟ in a reaction iscalled DISSOCIATION .

water is the universal solvent necessary to transport materials to and from all cells of the body.

The chemistry that happens in solution is

very important to the function of all living

systems.

water

NaCl

Na+

Cl-

water

Acids, bases and salts all dissociate into ions when placed in water.

*Usually, but not always

BubblesNRBaking soda

BubblesNRMagnesium

*Yellow*BlueBromothymol

*Cloudy/

white*PinkPhenolphthalein

RedBlueLitmus (blue or red)

114pH (# from the key)

not slipperySlipperyFeel (choose slippery or

not slippery)

SourBitterTaste

HCl(aq)NaOH(aq)

The Chemistry of Acids and Bases

Holt – Chapter 19

Acids

Some Important Properties of Acids for living systems:

Produce H+ ions in water

Electrolytes

React with bases to form a salt and water

pH is less than 7

Some Common Acids

HCl hydrochloric acid stomach acid

CH3COOH acetic acid vinegar

C6H8O7 citric acid lemon juice

BasesAlmonds contain Amygdalin which,

under certain conditions can

produce hydrogen cyanide. In fact,

cyanide gas has an almond scent.

Some important Properties of Basesin living systems:

Produce OH- ions in water

Are electrolytes

React with acids to form salts and water (neutralize)

pH greater than 7

Some Common Bases

NH3 hydrogen nitride ammonia

KOH potassium hydroxide liquid soap

Mg(OH)2 magnesium hydroxide Milk of magnesia

Al(OH)3 aluminum hydroxide Maalox (antacid)

Acids and Bases in Water

When an acid is poured into

water, it gives up H+

(hydrogen) to the water.

When a base is poured into

water, it gives up OH-

(hydroxide) to the water.

Solutions with excess H+ ions are called “Acids”.

Solutions with excess OH- ions are called “BASES”.

Basic Acid/Base definition*

Acids – increase [H+] in aqueous solutions

Bases – produce OH- ions in solution

*Arrhenius (traditional)

acids are measured by [H+]

bases are measured by [OH-]

When acids are combined with bases in solution, they neutralize each other, forming SALTS!

Neutralization Reactions

Acidic compounds react with basic compounds

and neutralize each other producing a salt (and

usually gas and water too!)

NaHCO3 + HCl NaCl + H2(g) + H2O Base Acid Salt

What happens when HCl and NaOH are combined in water?

NaOH + H2O dissociates to produce Na+ and OH- ions

HCl completely ionizes in H2O to produce H+

and Cl- ions

The equation of the reaction:

H+ + Cl- + Na+ + OH- NaCl + H2O

If the solution evaporates, an solid product is formed: NaCl – a salt - an ionic compound formed of a cation from a base and an anion from an acid.

pH

• from French – pouvoir hydrogenemeaning hydrogen power

another way to indicate [H+]

What is pH?

• a measure of how acidic, neutral or basic a

substance is.

• It measures the amount of H+ ions in a liquid.

• A substance with an = # of each is neutral.

The pH scale goes from 1 to 14.

• solutions with a pH of 1 to just less than 7 are acidic,

• solutions with pH just over 7 to 14 are basic

• solutions near 7 are neutral.

Pure water

As acids get stronger: [H+], pH A pH of 1 is a very strong acid.

As bases get stronger: [H+], pH A pH of 14 is a very strong base.

The pH scale is a way of expressing the strength of acids and bases.

NOTE: this is an inverse relationship

pH of Common Substances

Why is pH important?

• Humans depend on water.

• The pH of water must be within a range of 5 to 9 in order for body cells to survive.

• Liquids with a pH less than about 5 are too acidic for humans to drink.

• Waters that are too basic can also be harmful. Water with a pH of greater that 9 can dissolve materials and tissues such as bone and skin.

pH testing

There are many ways to test pH

Litmus paper (red = acid)

Red litmus paper (blue = basic)

Blue litmus paper (red = acid)

pH paper (multi-colored)

pH meter (7 is neutral, <7 acid, >7 base)

Universal indicator (multi-colored)

Chemical Indicators like phenolphthalein

Organic (natural) indicators like red cabbage,

radishes

Paper testing - litmus

Paper tests like litmus paper and pH paperPut a stirring rod into the solution and

stir.

Take the stirring rod out, and place a drop of the solution from the end of the stirring rod onto a piece of the paper

Read and record the color change. Note what the color indicates.

You should only use a small portion of the paper. You can use one piece of paper for several tests.

Paper Testing – pH paper

Put a toothpick into the solution.

Take the toothpick out, and place a

drop of the solution from the end of

the toothpick onto a piece of the

paper

Compare the color to the color

chart and note what pH the color

indicates.

You should only use a small portion

of the paper. You can use one piece

of paper for several tests.

indicators

• Indicators are dyes that can be added that will change color in the presence of an acid or base.

• Some indicators only work in a specific range of pH

• Once the drops are added, the sample changes color.

• Some dyes are natural, like radish skin or red cabbage

34

Buffers•Buffers resist changes to the pH of a solution when H+ or OH- is added to the solution.

•Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted.

Biological Buffers*

*A buffer resists changes in pH when small quantities of an acid or base are added to it.

Every life form is extremely sensitive to slight pH changes. Human blood for example needs to remain within the pH range of 7.38 to 7.42.

How do buffer solutions work?

A buffer solution has to contain things which will remove any hydrogen ions or hydroxide ions that you might add to it - otherwise the pH will change.

A buffer contains a weak acid and a weak base.

They do not react with each other or the water

But will react with strong acids or bases

If a strong base is added to a buffer, the weak acid part of the buffer will give up its H+ in order to transform the base (OH-) into water (H2O). Since the added OH- (from the base) is consumed by this reaction, the pH will change only slightly.

If a strong acid is added to a buffer, the weak base (in the buffer) will react with the H+ from the strong acid to form a weak acid. The H+

gets absorbed, so the pH changes only slightly.

H2CO3 H+ + HCO3-

Carbonic acid bicarbonate ion

Two especially important biological buffers are the phosphate and bicarbonate systems.

Living systems have built in biological buffers.

You can also find chemical (artificial) buffers.