REED COLLEGE SCIENCE OUTREACH PROPERTIES OF MATTER

Lesson Five- What is pH? 1
Objectives
o Students will learn about acids and bases, indicators, and the pH scale.
Lesson Background- Acids, Bases, and Indicators
So far, most of the properties of matter that students have learned about (magnetism &
physical states) are all physical properties. Many of the other properties that students have
thought of are most likely also physical properties (e.g. color, weight, density, etc.). Now that
students also have a basic understanding of atoms and molecules, we will begin to explore
matter’s chemical properties.
Today students will learn about an important property that chemists use to categorize
chemicals. Chemists divide substances into three categories: acidic, basic, or neutral. Many
chemicals can be classified as either acids or bases. These two types of chemicals are opposites
in chemistry and will react when they are mixed. Many other substances do not act as either
acids or bases, and these substances are called neutral.
• Examples of acids are vinegar, battery acid, and stomach acid.
• Examples of bases are ammonia, detergents, drain cleaners, and baking soda.
• Water, table salt, and plastics (etc.) are considered neutral.
The strength of acids and bases ranges on a scale. Very weak acids and bases are not harmful
to humans and can be detected according to taste. Acids taste sour while bases taste bitter.
However, strong acids and bases are highly corrosive (and hazardous) so the safest way to
determine whether a chemical is acidic or basic is to measure its acidity with an indicator.
Indicators are chemicals that turn different colors depending on whether they are exposed to
an acid or a base. (There are also indicators that will turn color in the presence of other
substances. These indicators are very useful for many other purposes, e.g. blood sugar tests,
lead paint tests, pregnancy tests…)
Today students will use an indicator made from cabbage juice. Cabbage contains a chemical
called anthocyanins that react to acid and bases. Cabbage juices can change to yellow, green,
or blue when exposed to bases and red/pink when exposed to acids. (Many other plants also
have indicator properties.)
Scientists have devised a number scale (called the pH scale) to specify how acidic or basic a
solution is. The scale ranges from 0-14, with 0 indicating a strong acid, 7 indicating a neutral
substance, and 14 indicating a strong base.
1 Lesson Adapted from:
“Of Cabbages and Kings”, Chemistry in the K-8 Classroom, OMSI, 2007.