A glass of water is placed on a very sensitive scale and several antacids tablets are dropped into the water. The weight of the glass and its contents is less after the tablets dissolve than before the tablets dissolved. Tell how matter is conserved ? Please help! I couldn't figure it out. THANKS!!

If the antacid table is somethign like Alka Seltzer, or sodium bicarbonate, the chemicals in the tablet react when dissolved to release CO2 gas, first as effervescent bubbles and then into the air.

This decreases the amount of mass in the glass, by an amount equal to the mass of CO2 released.