Which arrow represents the change of state described above?
The diagram shows changes of state between solid,
liquid, and gas. The atoms of a substance gain energy
during a change of state. Before the change, the atoms
are close together and cannot slide past one another.
After the change, the substance can fill its container.
OL
OM
gas
ON
o
M
Z
Р
solid
liquid

when [oh-] is 10-5  m, and [h3o+] is 10-9  m, the poh is 5 and ph is 9. again in both cases the sum of ph and poh is 14. however, the hydrogen ion concentration is not always going to be equal to exactly 1 x 10 raised to a negative number. for example, we skipped over the value of 2.0 x 10-7.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.