Adecrease in the density of a sample of a gas inside a balloon could be caused by a)putting the balloon in the refrigerator. b)taking the balloon outside on a hot day. c)shaping the balloon into the shape of a dog. d)taking the balloon down to the bottom of the ocean.

Ideal gas law: PV = nRT
Density: ρ = m/V (density = mass/volume)

Rewrite the ideal gas law by substituting m/ρ for V
Pm/ρ = nRT

Solve for ρ
ρ = Pm/(nRT)

So now you can see that density is proportional to pressure (P) and mass (m) and inversely proportional to amount (n) and temperature (T)

a)putting the balloon in the refrigerator.
This lower temperature so increases density

b)taking the balloon outside on a hot day.
This increases temperature so decreases density

c)shaping the balloon into the shape of a dog.
This has no effect on density

d)taking the balloon down to the bottom of the ocean.
This increases pressure so increases density

This is what a calorimeter is used for simpler ex, to measure calories in food.