Which of the following statements about physical models is not true?
a. physical models are often three-dimensional representations.
b. physical models are used to represent an object or event.
c. physical models are usually in the form of a graph.
d. physical models can be used to depict microscopic objects.

Liquid ammonia is produced at high temperatures and under great pressure in a tank by passing a mixture of nitrogen gas and hydrogen gas over an iron catalyst. the reaction is represented by the following equation. n2(g) + 3h2(g) → 2nh3(g) changing all but one experimental condition will affect the amount of ammonia produced. that condition is a) increasing the concentration of both reactants b) changing the temperature within the tank c) decreasing the pressure within the tank. d) increasing only the amount of nitrogen present.