RockLicker · Grade 4 · Lesson 1 of 5

Hardness Is Not Toughness

Interpret supplied scratch-test results and distinguish hardness from breaking strength.

  • Grade 4
  • 40 minutes
  • Worksheet + teacher guidance
Back to Grade 4

What you will learn

Mohs hardness ranks resistance to scratching. A higher-ranked mineral can scratch a lower-ranked mineral; the scale is not a measure of how hard a sample is to break or a linear strength scale.

Words to use

  • hardness: resistance to scratching
  • rank: relative order
  • toughness: resistance to breaking

Evidence for this lesson

Known-mineral data: gypsum 2, calcite 3, fluorite 4, quartz 7. Ideal supplied test results: fluorite scratches calcite; calcite does not scratch fluorite. Quartz scratches fluorite.

Use the supplied record as written; do not invent missing results.

Work through the activity

  1. 5 min · Define the testRead the distinction between hardness and toughness. Explain why dropping a sample would not measure Mohs hardness.
  2. 15 min · Order the dataArrange gypsum, calcite, fluorite and quartz from easiest to hardest to scratch. Predict which pairs should scratch which using the ranks.
  3. 10 min · Reason both waysUse the supplied paired results to check your ordering. Discuss why a true scratch must be distinguished from a powder mark in real testing.
  4. 10 min · State limitsWrite what the results tell you and what they do not. Do not calculate “twice as hard” from the rank numbers.
RockLicker
GRADE 4 · LESSON 1

Hardness Is Not Toughness

Draw on the printed sheet or write, type or dictate your response.
Draw on the printed sheet or write, type or dictate your response.
Draw on the printed sheet or write, type or dictate your response.
Draw on the printed sheet or write, type or dictate your response.

Responses stay in this open page only. Print before closing or changing pages; no student record is stored online.

Check your understanding

A mineral at Mohs 4 is …

Teacher & family guide · answers and assessment

Grade 4 · Hardness Is Not Toughness

Learning goal: Interpret supplied scratch-test results and distinguish hardness from breaking strength.

Expected responses

  1. Gypsum 2; calcite 3; fluorite 4; quartz 7.
  2. Yes, in the ideal comparison fluorite is higher on the Mohs ranking.
  3. No. Mohs numbers are a relative ranking, not a linear measurement.
  4. No. Resistance to breaking is different from resistance to scratching.

Understanding check: ranked above a mineral at Mohs 2. Mohs values rank scratch resistance; they are not a linear ratio scale.

Watch for this misconception

Hardness and toughness are different, and Mohs numbers do not support simple ratios.

Assess the evidence

Look for the observations, calculation or explanation named in the learning goal. Accept accurate drawings and dictated responses. Use the expected responses to discuss reasoning; a correct guess without explanation is a starting point for another conversation.

Adapt and extend

Read prompts aloud, enlarge print or let a partner record the learner’s explanation. Use Mineral Detective to combine hardness with another property.