Properties of MatterProperties of Matter

Investigation 4Investigation 4

Acids and BasesAcids and Bases

An An acid acid is a compound that releases is a compound that releases hydrogen ions in water. hydrogen ions in water.

The word “acid” comes form the Latin The word “acid” comes form the Latin word word acidus, acidus, which means sour. which means sour.

When you think of an acid, you might a When you think of an acid, you might a picture a scary smoking liquid eating picture a scary smoking liquid eating away at a piece of metal. away at a piece of metal.

Some acids do react that way, but many Some acids do react that way, but many acids are much more common and safer. acids are much more common and safer.

Common AcidsCommon Acids

Grapefruit, lemons, and vinegar taste sour Grapefruit, lemons, and vinegar taste sour because they contain acids. because they contain acids.

AcidsAcids All acids contain hydrogen atoms. All acids contain hydrogen atoms. Hydrogen atoms are neutral, that is they have equal Hydrogen atoms are neutral, that is they have equal

numbers of positive and negative charges. numbers of positive and negative charges. When some elements combine, atoms of one element can When some elements combine, atoms of one element can

lose negative charges to atoms of the other element. lose negative charges to atoms of the other element. That leaves the atoms of one element with more negative That leaves the atoms of one element with more negative

charges and atoms and the other element with more charges and atoms and the other element with more positive charges.positive charges.

These charged atoms are called ions. These charged atoms are called ions. When an acid compound is added to water, the acid When an acid compound is added to water, the acid

dissolves in water to form an acid solution that contains dissolves in water to form an acid solution that contains hydrogen ions. hydrogen ions.

Acids cont. Acids cont.

The more hydrogen ions the acid releases when The more hydrogen ions the acid releases when it dissolves in water, the stronger the acid is. it dissolves in water, the stronger the acid is.

Strong acids burn your skin and are poisonous. Strong acids burn your skin and are poisonous. They react quickly with many metals, releasing They react quickly with many metals, releasing

hydrogen gas. hydrogen gas. Weak acids give foods the sour, sharp taste that Weak acids give foods the sour, sharp taste that

you find in vinegar, soda water, spinach, apples, you find in vinegar, soda water, spinach, apples, limes and lemons. limes and lemons.

Acids cont. Acids cont.

Your stomach contains an acid that Your stomach contains an acid that dissolves your food. dissolves your food.

If you eat too many pickles or other sour If you eat too many pickles or other sour foods, the acid can become too strong, foods, the acid can become too strong, giving you a “sour” stomach. giving you a “sour” stomach.

BasesBases

A A base base is a compound that releases is a compound that releases hydroxide ions when dissolved in water. hydroxide ions when dissolved in water.

Just as with acids and hydrogen ions, the Just as with acids and hydrogen ions, the more hydroxide ions that are released into more hydroxide ions that are released into the water as a base dissolves, the the water as a base dissolves, the stronger the base is. stronger the base is.

Bases taste bitter and feel slippery. Bases taste bitter and feel slippery.

Common BasesCommon Bases

Soap is made from a base. Soap is made from a base. Weak bases are found in baking soda and some Weak bases are found in baking soda and some

antacids. antacids. Strong bases are often found in cleaning Strong bases are often found in cleaning

products, such as ammonia. products, such as ammonia.

The pH ScaleThe pH Scale

Because acids and bases can be very Because acids and bases can be very dangerous, you shouldn’t touch them or dangerous, you shouldn’t touch them or taste them. taste them.

So how do you know if something is an So how do you know if something is an acid or a base?acid or a base?

A simple test using litmus paper can tell A simple test using litmus paper can tell you if a substance is an acid or a base. you if a substance is an acid or a base.

Litmus paper is a paper that changes color Litmus paper is a paper that changes color when it contacts acids or bases. when it contacts acids or bases.

Litmus PaperLitmus Paper

If you place a strip of blue litmus paper into an If you place a strip of blue litmus paper into an acid, the paper changes to a pink or red color.acid, the paper changes to a pink or red color.

In a base, pink litmus paper changes to blue. In a base, pink litmus paper changes to blue.

pHpH

Litmus paper shows whether you have an acid Litmus paper shows whether you have an acid or a base but doesn’t show how strong the acid or a base but doesn’t show how strong the acid or base is.or base is.

The strength of an acid or a base is measured The strength of an acid or a base is measured using a set of numbers from 0-14.using a set of numbers from 0-14.

This set of numbers is called the This set of numbers is called the pH scale.pH scale. The strongest acids are found at the low end of The strongest acids are found at the low end of

the pH scale.the pH scale. For example, if a compound has a pH of 1, it is a For example, if a compound has a pH of 1, it is a

very strong acid. very strong acid.

pH continuedpH continued

As the pH number increases, the strength As the pH number increases, the strength of the acid becomes weaker.of the acid becomes weaker.

An acid with a pH of 3 is weaker than an An acid with a pH of 3 is weaker than an acid with a pH of 1. acid with a pH of 1.

A solution with a pH of 7—halfway A solution with a pH of 7—halfway between 0 and 14– is neutral. between 0 and 14– is neutral.

Substances with pHs beyond 7 are bases. Substances with pHs beyond 7 are bases. The higher the number, the stronger the The higher the number, the stronger the

base. base.

pH scalepH scale

IndicatorsIndicators

Litmus paper can’t tell you how strong an acid or Litmus paper can’t tell you how strong an acid or a base is, but certain other substances called a base is, but certain other substances called indicators, can.indicators, can.

Indicators Indicators are substances that change color at are substances that change color at a certain range of the pH scale.a certain range of the pH scale.

Some indicators change color many times as the Some indicators change color many times as the pH of the solution changes. pH of the solution changes.

For example, red cabbage juice can change For example, red cabbage juice can change from red to green to greenish yellow as the pH from red to green to greenish yellow as the pH changes. changes.

Effects of pH ChangesEffects of pH Changes

The pH of soil, rivers, lakes, and even rain The pH of soil, rivers, lakes, and even rain is very important to life on Earth.is very important to life on Earth.

Many of the plant foods that you eat grow Many of the plant foods that you eat grow well in soil with a pH range between,5 and well in soil with a pH range between,5 and 7 which is slightly acidic. 7 which is slightly acidic.

Some of the acid gets into the soil as Some of the acid gets into the soil as plants decay and acid forms. plants decay and acid forms.

Rain washes the acid into the soil. Rain washes the acid into the soil. This process keeps the soil slightly acidic. This process keeps the soil slightly acidic.

Effects of pH Changes cont. Effects of pH Changes cont.

Farmers also add materials to the soil to Farmers also add materials to the soil to keep its pH in the correct range.keep its pH in the correct range.

In dry areas, the soil tends to be basic, In dry areas, the soil tends to be basic, and very few plants grow. and very few plants grow.

What can be done to change What can be done to change substances that are too acidic or basic?substances that are too acidic or basic?

Plain water is a neutral substance. Plain water is a neutral substance. When an acid reacts with a base, water is When an acid reacts with a base, water is

always one of the products. always one of the products. The other product depends on which acid The other product depends on which acid

and base you combine. and base you combine.

NeutralizationNeutralization

NeutralizationNeutralization is a is a process in which an acid process in which an acid and a base react to and a base react to produce a salt and water. produce a salt and water.

The kind of salt produced The kind of salt produced depends on the acid and depends on the acid and base that reacted with base that reacted with each other. each other.

Sodium chloride is only Sodium chloride is only one of the salts that can one of the salts that can be formed during be formed during neutralization. neutralization.

ReviewReview

What is pH?What is pH? What are acids?What are acids? What are bases?What are bases? What is What is

neutralization?neutralization? What are some What are some

effects of pH effects of pH changes?changes?