One situation indicates a physical change rather than a chemical change. which situation indicates a physical change?
a) solid copper melts to form liquid copper.
b) a firecracker explodes on the fourth of july.
c) water reacts with the iron on your bicycle to form rust.
d) magnesium metal is mixed with hydrochloric acid and hydrogen gas is formed. eliminate

Solid copper melts to form liquid copper,

Explanation:

In a chemical change, something new is being formed. When copper melts, or any phase change occurs, nothing new is formed chemically, so this would be a physical change.

d. platinum would have the greatest temperature increase because it has the lowest specific heat.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.