When water combines with mineral grains do the grains become smaller or larger?
When water combines with mineral grains, the grains become larger. The increase in size of mineral grains strains the rock material and weakens it.
This is due to the rate of cooling - a slower rate of cooling results in large mineral grains because the molecules had time to maneuver into a crystalline lattice. A faster rate of cooling results in smaller mineral grains because the molecules were locked into place faster and therefore couldn't form the crystalline lattice structures before getting stuck.
The rate at which magma cools may affect the size of the mineral grains that formAs magma cools its atoms tend to move away from each other?
No, but the size of mineral grains is due to the growth of crystals. Once the rock has cooled down and become solid the growth of crystals within it can no longer continue. So rocks that cool down quickly have smaller crystals (or grain) than those that cool down more slowly, which gives the crystals a longer period in which to grow.