Some materials are hard but easy to break. Some, however, are soft but difficult to break. Discuss from a physical science point of view.
In our daily lives, the words "soft" and "hard" are used to describe objects. A "hard" object like a stone is extremely difficult (or impossible) to break by human strength. Beancurd, on the other hand, is "soft" and easily breakable. There are objects, however, that are hard but easy to break, such as glass and ice, while some are soft but difficult to break such as nylon string and plastic bottles. Why is that so?.
The concept of "toughness".
To help us understand the mentioned phenomenon, we need to know the meaning of "toughness". When we say that a material is hard or soft, we are actually referring to the hardness: a measure of how much force is required to deform the material. It is different from toughness. Toughness is a measure of the energy a sample can absorb, undergoing plastic deformation, before it breaks. A material with good toughness may bend and deform as a result of an impact or overloading situation, but it will resist rupture (breakage) to a high degree as it absorbs a high amount of energy. Such a material is described as ductile. On the other hand, a brittle material is one that fractures and breaks easily. Picture a rubber ball being thrown with a great force through a window pane. The glass pane, being a hard but brittle material, shatters, due to its low toughness. The rubber ball, on the other hand, doesn't break although it may be softer than glass, because it is tougher and hence, more ductile. In greater detail, toughness also refers to the ability of a material to resist the growth of a crack. .
The formation and propagation of cracks.
We know that a solid is a collection of atoms (and molecules) held together by chemical bonds. A crack forms when a material body is subjected to external forces or stresses, and the atoms and molecules inside the material move around to absorb the effects of the stress, resulting in a strained state, its energy higher than the energy of its normal (unstrained) state.
