Recommended for: Grades 3-8
Resource: Acids and Bases: Cabbage Juice Indicator
Media Type:
QuickTime Video
Length: 1m 30s
Size: 2.1 MB
The substances that chemists collectively call acids and bases come in many different forms. Despite this variety, all acids tend to react with other substances in much the same way. Reactions involving bases also share many common characteristics with one another. In this video segment adapted from ZOOM, cast members demonstrate how to use cabbage juice to determine whether a solution is acidic or basic.
Teachers' Domain, Acids and Bases: Cabbage Juice Indicator, published February 20, 2004, retrieved on ,
http://www.teachersdomain.org/resource/phy03.sci.phys.matter.zcabbage/
- Background Essay
- Questions for Discussion
- Standards
Different types of substances can combine in many different ways. Some substances react chemically with one another when they come into contact, recombining to form other substances. Two classes of substances that combine readily, and sometimes violently, are acids and bases.
To understand the chemical behavior of acids and bases, we have to begin with water. Water molecules consist of two hydrogen atoms and one oxygen atom (HOH). In liquid water, some of these molecules come apart to form two kinds of ions: positive hydrogen ions (H+) and negative hydroxide ions (OH-). In water, there are equal numbers of hydrogen and hydroxide ions, because every molecule that comes apart produces one of each type.
An acid is a solution of a chemical in water that results in more H+ ions than OH- ions. A base is a solution of a chemical in water that results in the opposite: more OH- ions than H+ ions.
When acidic and basic solutions are combined, the H+ ions from the acid and the OH- ions from the base combine to form water molecules. At the same time, other ions, which formed when the H+ and OH- ions were first produced, combine to form new molecules, such as salts and/or gases. The type of molecules formed depends on the makeup of the chemicals initially combined.
Many natural substances are good acid-base indicators. For example, a solution made from the pigment of red cabbage varies in color from deep red when exposed to a strong acid to yellowish green when exposed to a strong base. The details of why various chemicals produce molecules that absorb different wavelengths of light and are therefore of different colors are complex and varied.
To understand the chemical behavior of acids and bases, we have to begin with water. Water molecules consist of two hydrogen atoms and one oxygen atom (HOH). In liquid water, some of these molecules come apart to form two kinds of ions: positive hydrogen ions (H+) and negative hydroxide ions (OH-). In water, there are equal numbers of hydrogen and hydroxide ions, because every molecule that comes apart produces one of each type.
An acid is a solution of a chemical in water that results in more H+ ions than OH- ions. A base is a solution of a chemical in water that results in the opposite: more OH- ions than H+ ions.
When acidic and basic solutions are combined, the H+ ions from the acid and the OH- ions from the base combine to form water molecules. At the same time, other ions, which formed when the H+ and OH- ions were first produced, combine to form new molecules, such as salts and/or gases. The type of molecules formed depends on the makeup of the chemicals initially combined.
Many natural substances are good acid-base indicators. For example, a solution made from the pigment of red cabbage varies in color from deep red when exposed to a strong acid to yellowish green when exposed to a strong base. The details of why various chemicals produce molecules that absorb different wavelengths of light and are therefore of different colors are complex and varied.
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