Law of conservation and mass

This week we did a lab that involved using many different chemical and masses. We used a mass measuring box to measure the mass of the chemicals in the tubes. We used the tubes to mix the chemicals inside. First we would weigh the substances individually and then after we mix, then we measure the mass of the combined chemicals. Sometimes it would make quite a diffidence but sometime it wont. One of the chemical we tried was a water bottle and these tablets which make a gas. First we measure the tablet in the cap of the water bottle. Then we put the tablet in, and sealed the bottle. The gas that was being made, was making the water bottle full of gas. I was afraid it would pop but it didn't.