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Why can a diamond resist scratches but still break?

A diamond can resist a scratch and still chip from an impact. Those are different tests of a material. Hardness concerns scratching; toughness concerns resistance to fracture. A diamond’s crystal structure also makes some breaking directions easier than others, so one impressive score cannot describe every way a gemstone might fail.

3–7 min

Questions you could explore

  • What would a scratch test miss about an object’s durability?
  • How does resistance to scratching differ from resistance to cracking under an impact?

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Chemistry and materials

From dissolved minerals to kettle limescale

Rainwater starts out soft, but as it percolates through limestone and chalk, it dissolves minerals that travel invisibly to your kitchen tap. That dissolved calcium and magnesium is what makes water 'hard', and when a kettle heats the water, the chemical balance shifts and leaves solid calcium carbonate behind as crusty limescale.

3–5 min

From dissolved minerals to kettle limescale

Chemistry and materials3–5 minRainwater starts out soft, but as it percolates through limestone and chalk, it dissolves minerals that travel invisibly to your kitchen tap. That dissolved calcium and magnesium is what makes water 'hard', and when a kettle heats the water, the chemical balance shifts and leaves solid calcium carbonate behind as crusty limescale.

Chemistry and materials

Why does ice take up more room than water?

An ice cube floats in the water it came from, and freezing water can push against its container. Ordinary ice arranges water molecules differently from the liquid, connecting a microscopic pattern with a familiar change in volume.

3–5 min

Why does ice take up more room than water?

Chemistry and materials3–5 minAn ice cube floats in the water it came from, and freezing water can push against its container. Ordinary ice arranges water molecules differently from the liquid, connecting a microscopic pattern with a familiar change in volume.

Chemistry and materials

Identifying elements through patterns of light

A glowing gas can produce a pattern of spectral lines characteristic of its elements. Comparing several lines gives more evidence than spotting one color or line alone, while temperature, motion, and other conditions also affect the spectrum.

5–7 min

Identifying elements through patterns of light

Chemistry and materials5–7 minA glowing gas can produce a pattern of spectral lines characteristic of its elements. Comparing several lines gives more evidence than spotting one color or line alone, while temperature, motion, and other conditions also affect the spectrum.

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