Top chemistry mistakes that people make

These drive me crazy. Even chemistry students can't get this stuff straight sometimes because of culture conditioning.

1. If you can't read it, it's bad. Why? What's dihydrogen monoxide? Maltodextrin? The first is just a technical name for water and the second is a relatively safe food additive. Yet there are always people falling for the "dihydrogen monoxide hoax" and some of the top searches on google for maltodextrin include "maltodextrin side effects" and "maltodextrin dangers" - people search for the bad effects before they know what it actually is. Technical names say nothing about the substance itself. We are too developed a society to still have "chemophobia".

2. Atoms are round, hard objects. This was sort of like what the Greeks thought of, but we are not in 500 BC. Atoms actually have no boundaries; they are basically composed of nuclei (containing positively charged protons and neutral neutrons) surrounded by negatively-charged electrons that move around really fast. A very close (but incorrect) simplification is that the atom is a "solar system" - the nucleus is the "sun" and the electrons are the "planets". If this works for you, believe this, but just don't buy into the hard sphere concept.

3. Reactions: mix chemicals, wait, isolate product. If everything were this easy then people could be chemists in 30 minutes. Lab work is MUCH harder than it seems. Sometimes the compounds can't be exposed to air. Other times, you finish your reaction and can't find your products. Maybe they will decompose if they are not purified so you have to purify them immediately (which usually is a difficult process in itself, requiring hours of manual work). Often you will use other substances to speed up the reaction; how do you remove them? How do you know if your reaction is done (it's not always a color change!)? How do you know that you actually make the right thing? There are extensive methods to answer each question that take years of experience to master. Even more interesting is when things go wrong - then you really have to know what's going on to fix it.

4. Water conducts electricity. It doesn't. It is a molecular substance, and doesn't exist as charged species (aka ions), so an electrical charge cannot travel through. Tap water, however, does conduct electricity because it contains dissolved ions, like F^- and Ca²⁺, which creates paths for charges.

5. Assuming that the chemical definitions of "metals", "sugar", and "salt" are the same as the common ones. This is a more obscure difference that most people are not expected to know. Metals are not necessarily shiny solids. Mercury is not a solid, lead is not shiny. The concept of metals has more to do with the bonding, i.e. how the atoms are held together. Sugar is not just the white powder you put in cake. Sugar is a rather vague term that refers to a compound with a formula that is a multiple of CH₂O or any related compound. Salt is more than the thing you put as pranks in pixie sticks. When you combine a cation with an anion, you get a salt (cation=positively charged ion, anion=negatively charged ion). NaCl is our familiar salt, but MgCl₂ and Al₂O₃ are all salts. Basically, you combine a metal and a nonmetal to get a salt.

6. Boiling, sublimation, and dissolving are chemical changes. In a chemical change, the composition of the substance must change. When water boils, it is still water. When table salt dissolves, it is still NaCl. Phase changes are called physical changes, which don't involve any new substances formed.
Warning: not all color changes are chemical reactions: heating glass until it is red is a physical change (it's still the glass), and iodine sublimination gives off a purple gas that is still iodine.

7. Atoms of solids don't move. Particles are always in motion. In solids, atoms are actually vibrating rather quickly. Temperature is related to the particle velocities, so if nothing moved in the solid, it would be very, very cold (in fact, it would be "absolute zero", the absolute lowest temperature possible). Remember that the visible properties of the solid itself doesn't tell anything about the atoms contained.