Pure substance made up of one type of atom?

Every element is made up of the same type of atom eg.hyrdogen is made of hyroden atoms

A simple substance consists of one type of atoms.  A pure substance is a substance that does not contain impurities.  The concept itself is abstract, since impurities are everywhere.

Tolerance to high levels of salinity

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.