Properties of Matter Investigation 4

Acids and Bases  An acid is a compound that releases hydrogen ions in water.  The word “acid” comes form the Latin word acidus, which means sour.  When you think of an acid, you might a picture a scary smoking liquid eating away at a piece of metal.  Some acids do react that way, but many acids are much more common and safer.

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

Acids  All acids contain hydrogen atoms.  Hydrogen atoms are neutral, that is they have equal numbers of positive and negative charges.  When some elements combine, atoms of one element can lose negative charges to atoms of the other element.  That leaves the atoms of one element with more negative charges and atoms and the other element with more positive charges.  These charged atoms are called ions.  When an acid compound is added to water, the acid dissolves in water to form an acid solution that contains hydrogen ions.

Acids cont.  The more hydrogen ions the acid releases when it dissolves in water, the stronger the acid is.  Strong acids burn your skin and are poisonous.  They react quickly with many metals, releasing hydrogen gas.  Weak acids give foods the sour, sharp taste that you find in vinegar, soda water, spinach, apples, limes and lemons.

Acids cont.  Your stomach contains an acid that dissolves your food.  If you eat too many pickles or other sour foods, the acid can become too strong, giving you a “sour” stomach.

Bases  A base is a compound that releases hydroxide ions when dissolved in water.  Just as with acids and hydrogen ions, the more hydroxide ions that are released into the water as a base dissolves, the stronger the base is.  Bases taste bitter and feel slippery.

Common Bases  Soap is made from a base.  Weak bases are found in baking soda and some antacids.  Strong bases are often found in cleaning products, such as ammonia.

The pH Scale  Because acids and bases can be very dangerous, you shouldn’t touch them or taste them.  So how do you know if something is an acid or a base?  A simple test using litmus paper can tell you if a substance is an acid or a base.  Litmus paper is a paper that changes color when it contacts acids or bases.

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

pH  Litmus paper shows whether you have an acid or a base but doesn’t show how strong the acid or base is.  The strength of an acid or a base is measured using a set of numbers from  This set of numbers is called the pH scale.  The strongest acids are found at the low end of the pH scale.  For example, if a compound has a pH of 1, it is a very strong acid.

pH continued  As the pH number increases, the strength of the acid becomes weaker.  An acid with a pH of 3 is weaker than an acid with a pH of 1.  A solution with a pH of 7—halfway between 0 and 14– is neutral.  Substances with pHs beyond 7 are bases.  The higher the number, the stronger the base.

pH scale

Indicators  Litmus paper can’t tell you how strong an acid or a base is, but certain other substances called indicators, can.  Indicators are substances that change color at a certain range of the pH scale.  Some indicators change color many times as the pH of the solution changes.  For example, red cabbage juice can change from red to green to greenish yellow as the pH changes.

Effects of pH Changes  The pH of soil, rivers, lakes, and even rain is very important to life on Earth.  Many of the plant foods that you eat grow well in soil with a pH range between,5 and 7 which is slightly acidic.  Some of the acid gets into the soil as plants decay and acid forms.  Rain washes the acid into the soil.  This process keeps the soil slightly acidic.

Effects of pH Changes cont.  Farmers also add materials to the soil to keep its pH in the correct range.  In dry areas, the soil tends to be basic, and very few plants grow.

What can be done to change substances that are too acidic or basic?  Plain water is a neutral substance.  When an acid reacts with a base, water is always one of the products.  The other product depends on which acid and base you combine.

Neutralization  Neutralization is a process in which an acid and a base react to produce a salt and water.  The kind of salt produced depends on the acid and base that reacted with each other.  Sodium chloride is only one of the salts that can be formed during neutralization.

Review  What is pH?  What are acids?  What are bases?  What is neutralization?  What are some effects of pH changes?