I am taking an Earth Science course now and currently reviewing some basic chemistry knowledge needed for further study on earh mineral and rocks. But I haven't touched chemistry for almost 2 years that I start to forget things, can someone help and clarify some basic chemistry stuff? I would appreciate! :)

1) The smallest unit of a covalent compound is molecule. But what is a formula unit? Is it the smallest unit of an ionic compound that still retains its chemical and physical properties?

2) Why is a solution considered to be a mixture instead of a compound? Is dissolving a chemical reaction?

3) For examle, predict the chemical formula of carbon chemically combining with oxygen, C2O4 is obtained which must be reduced to its lowest ratio, CO2, but why something such as C6H12O6 cannot be reduced to CH2O? How can I know when a chemical formula is to reduce and when not to?

1 please rephrase

2 because until there is a chemical bond between the two molecules you mix, they can still be separated by nonchemical means and are therefore a mixture not a compound.

3 the only way you get C₂O₄ instead of CO₂ is because you balanced your equation wrong. you cant simplify it to CO₂, because whatever C₂O₄ is, needs to have all of those atoms to be what it is.

1 please rephrase

2 because until there is a chemical bond between the two molecules you mix, they can still be separated by nonchemical means and are therefore a mixture not a compound.

3 the only way you get C₂O₄ instead of CO₂ is because you balanced your equation wrong. you cant simplify it to CO₂, because whatever C₂O₄ is, needs to have all of those atoms to be what it is.
1) An atom is the smallest unit of an element that still retains its chemical and physical properties.
A molecule is the smallest unit of a covalent compound that still retains its chemical and physical properties.
A ________ is the smallest unit of an ionic compound that still retains its chemical and physical properties.
Would the answer to the blank be "formula unit", since ionic bonding doesn't form a molecule...?!

2) Say you dissolve MgCl2:
MgCl2 -> Mg(2+) + 2Cl(-)
New substance is formed, why still consider the solution as a mixture in which no chemical reaction occurs?

3) Say, when you are asked to draw the Lewis dot diagram for the compound that would form and write its chemical formula at the end:
a) C and O
b) Mg and O

The chemical formulas are CO2, MgO, not C2O4 and Mg2O2, right? You always reduce to its lowest ratios when writing chemical formulas?

I would argue that dissolving is a chemical reaction by almost any definition of "chemical reaction".

Not true, Dissolving is not a chemical reaction. A chemical reaction is a exchange of interaction of valence electrons between the substrate atoms/molecules (i.e. redox reaction)

Since dissolving does not involve this, it cannot possibly be a chemical reaction.

Not true, Dissolving is not a chemical reaction. A chemical reaction is a exchange of interaction of valence electrons between the substrate atoms/molecules (i.e. redox reaction)

Since dissolving does not involve this, it cannot possibly be a chemical reaction.Dissolving does involve changes in valence electron interaction. You are breaking apart the interacting atoms in the dissolving complex and creating new interactions between the dissolved atoms and the water molecules. Chemical reactions don't have to involve oxidation or reduction.

Edit: Obviously I was talking about things like salts dissolving. With something like a diatomic gas dissolving into water, I agree that it wouldn't count as a chemical reaction.

I just googled this myself as I wasn't sure what a "formula unit" was either except as a unit of measurement used in a formula:

"The smallest unit of an ionically bonded substance. For example, the formula unit of sodium chloride is NaCl."

"The simplest whole-number ratio which will describe an ionic crystal lattice. [NaCl is a ionic crystal bond]"

also "the smallest repeating unit of a substance" or atoms per formula unit (APFU).