To calculate the change in heat energy of water, we need to use the formula:
Q = mcΔT
where Q is the change in heat energy, m is the mass of the water, c is the specific heat capacity of water, and ΔT is the change in temperature.
First, we need to calculate the mass of the water. We know that the mass of 1 mL of water is 1 gram, so we can convert the volume of water to mass using the density of water, which is 1 g/mL:
mass = volume x density = 12 mL x 1 g/mL = 12 g
Next, we need to calculate the change in temperature:
ΔT = final temperature - initial temperature = 28°C - 31°C = -3°C
Note that the change in temperature is negative because the water is cooling down.
Finally, we can calculate the change in heat energy:
Q = mcΔT = 12 g x 4.184 J/g°C x (-3°C) = -150.192 J
The negative sign indicates that the water is losing heat energy as it cools down. Therefore, the change in heat energy of 12 g of water cooling from 31°C to 28°C is -150.192 J.