2HCl + Zn—>ZnCl2 + H2 When you use 25mL of 4.0M HCl to produce H2 gas, how many grams of zinc does it react with? What volume of H2 gas is produced at STP?

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant. 

That depends. cause the owl finished it in one lick

Use a magnet to separate iron, evaporate water, i'm not sure how to separate the other two i'm sorry but good luck