I think the correct terms to fill in the blanks are increase and increase or it can also be decrease and decrease. If volume remains the same while the mass of a substance increase, the density of the substance will increase or the opposite would also be true. Density is the ratio of the mass of a substance and its volume. So, we can see that density and mass are directly related so that when one increases, the other would also increase or if one decreases, the other one would also decrease.