how can something be very hard but also very brittle?

Answer 1

Hard and brittle are two separate concepts. Hard v. soft refers to an object's ability to resist deformation. Malleable v. brittle refers to an object's ability to resist fracture. Glass, for instance is pretty hard, yet it is also brittle. A glass cutting board is hard enough to dull a knife's blade easily, yet it shatters when thrown on the ground. A malleable object, on the other hand, will tend to stretch as tension is applied.

Hard and Brittle- Glass
Hard and Malleable- Gold
Soft and Brittle- Silly Putty
Soft and Malleable- Bubble Gum

Answer 2

Most very hard substances are brittle. Both properties arise from the interatomic bonds being particularly strong, which means that they can't stretch much without fracturing.

Answer 3

Very hard substances are also very brittle because they resist deformation and the crack that initiates in a material under tensile deformation needs very little energy to propagate further unlike ductile materials where the crack tip gets blunted.

Answer 4

On a molecular level, imagine something with strong bonds that are very dependent on nearby other bonds. It's hard to start bonds breaking, but break a few and the whole thing falls apart.

Diamond would be a good example.

Answer 5

THINK OF AN EGG SHELL.