Suppose you're attempting to measure the density of the liquid that is highly volatile(it evaporates quickly). if you first measure the mass and then several minutes later measure the volume of the liquid will the measured density be higher then the true value, lower than the true value, or the true density of the liquid.

The density in this case would be higher than the true value of the density of the liquid. This is because you have an erroneous value of the volume of the sample since some of this already evaporated into the atmosphere and is not accounted for when you measured the volume. Then, it will given you a higher value than the true.

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.