6. Materials

Question 1

A material is dense if it

Answer 1

has a large mass per unit volume. Solid materials vary in density mainly because elements have different atomic masses. Lead is much more dense than aluminium, mainly because lead atoms are much heavier than aluminium atoms

Question 2

A material is tough if

Answer 2

the material does not break by snapping cleanly. A tough material is resistant to the propagation of cracks. Toughness is the opposite of brittleness. Metals are tough, and break by plastic flow. There is no one simple measure of toughness, but a tough material will dissipate a large amount of energy per unit area of new fracture surface.

Question 3

A material is brittle if

Answer 3

it breaks by snapping cleanly. The brittleness of glass is a consequence of defects such as fine surface cracks, which propagate easily through the material.

Question 4

A material is stuff if it is

Answer 4

difficult to stretch or bend the material (e.g. a metal sheet is stiffer than a polythene sheet of the same dimensions). The stiffness is indicated by the Young modulus.

Question 5

A material is hard if it is

Answer 5

difficult to dent the surface of the material (e.g. a steel knife is much harder than a plastic knife). Hardness is tested by machines that indent the surface. Many ceramics are very hard.

Question 6

A material is malleable if it is

Answer 6

easy to hammer or press a sheet of material into a required shape (e.g. a lead sheet is easier to fit on a roof than a steel sheet).

Question 7

A material is ductile if it is

Answer 7

easy to draw a material into a wire (e.g. copper is easier to draw into a wire than tungsten). Metals are ductile because the non-directional metallic bonds allow ions to slide past one another.

Question 8

A material is elastic if it

Answer 8

regains its shape after stretching (e.g. a rubber band regains its original length when released). When a metal or ceramic stretches elastically, the bonds between neighbouring atoms extend very slightly. In a polymer the atoms rotate about their bonds

Question 9

A material is plastic if it

Answer 9

undergoes large permanent stretching or distortion before it breaks (e.g. a polythene strip stretches permanently if pulled).