Raven spreads her rock collection across a cloth. Each rock has a number but no name.
"Geologists identify minerals with a scratch test," she says.
Rocket pulls out a steel nail. "So the harder one wins the scratch?"
"What do you notice when one material leaves a line on another?" Raven asks.
Nova makes a glowing ladder in the air with ten steps. "Here is a clue," she says. "Softest at the bottom. Hardest at the top."
Hardness is how well a material resists being scratched.
A harder material can scratch a softer one. The softer one cannot scratch the harder one.
Geologists use the Mohs Hardness Scale, which goes from 1 to 10. Talc is the softest at 1. Diamond is the hardest at 10.
You can test with common objects. A fingernail is about 2.5. Glass is about 5.5. A steel nail is about 6.5.
Reflectivity is how much light bounces off a surface. Shiny materials reflect a lot of light.
Some materials look shiny and some look dull. Shine is one more clue, not a full answer.
To find a rock's hardness, test it with things you know.
If glass scratches it, the rock is softer than 5.5.
If it scratches glass, the rock is harder than 5.5.
| Statement | True or false? |
|---|---|
| A softer material can scratch a harder one. | ? |
| The Mohs scale goes from 1 to 10. | ? |
| Shiny materials reflect a lot of light. | ? |
| Shine alone is enough to identify a material. | ? |
Great work using a scale to reason. Tomorrow we will see how properties help sort materials in the real world.