Answer:
<h2>Ionic</h2><h3>
Evidence: It can be proved by a simple experiment, sand doesn't dissolve in water but NaCl (table salt) does. (excluding cases like BaSO4 and sugar and amino acids)</h3>
Cause: The electrostatic bonds among ions in a lattice only hold them together like two magnets with unlike poles facing each other. The water molecules can split them through the similar process with several molecules ganging up on them, the H+ parts of the molecules attracts the anion and O2- part; the cations. If this electrostatic attraction is big enough, the bonds between the solid's ions break (into separate ions). And so the solid dissolves. This is usually the case with most.
Why not covalent structures?: Sure, covalent solids have a low m.p and b.p for low inter<em>molecular </em>forces, the covalent bonds here are strong and cannot easily be 'pulled apart' by water molecules. They usually have stronger inter<em>molecular </em>forces than that of water.
But... if their inter<em>molecular </em>forces are weaker or similar, sure they can dissolve easily. Like any often-used alcohol (glacial).
Answer:
By definition, there are 6.022×1023 such molecules, or NA such molecules in ONE mole of water. And thus in such a quantity there are NA oxygen atoms, and 2×NA hydrogen atoms...and the mass associated with this numerical quantity of water molecules is approx. 18⋅g ...
There are many ways to test and identify metal. The easiest way is observing its color. Also how reflective it is. Other ways would be boiling point, melting point, density, or conductivity of the metal.
Hope This Helps and God Bless!