The most common concept of hard and soft substance is: (A) Relative resistance to indentation, (B) Relative resistance to abrasion, (C) Relative resistance to fracture, (D) All of the above
Answer is A. The most common concept of hard and soft substances is their relative resistance to indentation. Hardness is therefore a measure of the resistance to plastic deformation and is measured as a force per unit area of indentation. Hardness may be broadly defined as the resistance to permanent surface indentation or penetration.