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Module – Grade 10-12 - Exploring Acids and Bases

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Frequently, chemists refer to materials as being either an acid or a base.

In order to determine if a material is acidic or basic (alkaline), chemists use a pH Indicator -- pH stands for “potential of hydrogen”.  A pH Indicator expresses how acidic (like an acid) or basic (like a base) a substance is. pH is indicated by a numbered scale:  A pH of 7 is neutral. A pH below 7 denotes acidity while one above 7 denotes alkalinity.

In these experiments, students will make a pH indicator using red cabbage and investigate the properties of several materials found around the home.

Materials:

  • one red cabbage
  • water
  • white vinegar (acetic acid)
  • window cleaner (ammonia)
  • baking soda (sodium bicarbonate)
  • washing soda (sodium carbonate)
  • lemon juice (citric acid)
  • antacids (calcium carbonate, calcium hydroxide, magnesium hydroxide)
  • seltzer water (carbonic acid)
  • soft drink

Preparation of the Red Cabbage Indicator:

  1. Place about half a red cabbage cut into 2.5-centimeter (1-inch) cubes into a pan and add about 750 millilitres (3 cups) of water
  2. Boil on high heat for about 10 minutes.
  3. After the water has cooled, strain the mixture through a sieve.
  4. The resulting strained liquid, from the red-cabbage extract, is our pH Indicator that we will used to explore the world of acids and bases.

Establish the pH range:

  1. Pour 50ml (1/4 cup) of vinegar (acetic acid) into a colourless drinking glass. Add 1/2 teaspoon of red cabbage extract, stir the mixture, and note the colour.
  2. Pour 50ml (1/4 cup) of window cleaner (ammonia) into a colourless drinking glass. Add 1/2 teaspoon of red cabbage extract, stir the mixture, and note the colour.
approximate pH: 2 4 6 8 10 12
colour of extract: red purple violet blue blue-green green

Procedure:
Add 1/2 teaspoon of red cabbage to the following and record your observations:

  1. baking soda (sodium bicarbonate, NaHCO3)
  2. washing soda (sodium carbonate, Na2CO3)
  3. lemon juice (citric acid, C6H8O7)
  4. antacids (calcium carbonate, calcium hydroxide, magnesium hydroxide)
  5. seltzer water (carbonic acid, H2CO3)
  6. soft drink

Scientific Note:
The pH number is the negative exponent of 10 representing hydrogen ion concentration in grams per litter. For instance a pH of 7 represent 10-7 grams per litter. i.e. pH =  (log10{1/[H+]}).

Consequently each whole pH value below 7 is ten times more acidic than the next higher value. For example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times (10 times 10) more acidic than a pH of 6. The same holds true for pH values above 7, each of which is ten times more alkaline—another way to say basic—than the next lower whole value. For example, a pH of 10 is ten times more alkaline
than a pH of 9.

Note: Activity adapted from multiple sources by NRC scientist Dr. Mike Day.