Question

why diamond is the hardest substance but a graphite is soft substance

Answer 1

It is because of the chemical and physical property

Answer 2

Diamond is currently thought to be thehardest natural material on Earth, having a hardness of ten out of ten on the Mohs scale of mineral hardness. Diamond is made up of carbon atoms which share strong covalent bonds (where electrons are shared between atoms) and are equally spaced in a lattice arrangement.The bonds between the layers are much weaker than covalent bonds. This enables the layers to slide across each other, making graphite soft.