25° C is the temperature required for the cell potential to be the standard cell potential
Controlled variables: do not change as they must be held constant
Independent variables: are controlled and manipulated
Dependent variables: effected by changes to the independent variable
Answer: The force of gravity decreases.
Explanation: There are a lot of explanations for it. Look at Einstein's Theory of General Relativity & Newton's law of universal gravitation.
Answer:
a) 
b) NR
c) NR
d) 
e) NR
Explanation:
a) Nickel is more reactive than copper so the first one will displace the other:

b) Due to the higher reactivity the magnesium has compared to the zinc, the first will remain in solution and the second will reamin solid. So, NR.
c) No reaction occurs, gold is resistant to acid in normal conditions due to its characteristic as noble metal.
d) Chromium is more reactive than cobalt so the last one will be displaced:

e) Nothing happens, the reduction potencial of the hydrogen gas bubbled ins't enough to reduce the silver.