What Makes Something an Acid: teacher key

Mixing and Reactions, grade 9 edition, written for 9th grade. Chemists have defined acids and bases in more than one way, from Arrhenius in 1884 to a broader idea in 1923. The pH scale, indicators and blood show how those words are used.

Read the book. Print this page for the key.

Questions and answers

1. How did Arrhenius define a base in 1884?
   • A compound that accepts a proton
   • A compound that dissolves in water to give hydroxide ions (answer)
   • A solution with a pH of 7
2. What does the p in pH stand for?
   • The power of ten of the hydrogen ion concentration (answer)
   • Protons
   • The word potential
3. Why can vegetable oil not be given a pH?
   • It is too thick to measure
   • Only a water-based solution has a pH (answer)
   • It is always neutral
4. Water has a pH of 5. Another sample has a pH of 6. How do they compare?
   • The pH 6 water is ten times more acidic
   • The two are equally acidic
   • The pH 5 water is ten times more acidic (answer)
5. What does a buffer do in blood?
   • It mops up added acid or base so the blood stays steady (answer)
   • It turns the blood purple
   • It makes carbon dioxide bubbles
6. What does the word 'indicator' mean for red cabbage juice?
   • A liquid that shows a different color in an acid than in a base (answer)
   • A gas given off in a reaction
   • A metal that wears away

Words

• acid: gives away protons
• proton: hydrogen ion
• buffer: resists pH change
• precipitate: solid from liquids
• indicator: color clue