What is the arrow pointing to?
The figure shows a solution of different molecules. Most of these molecules consist of two small atoms attached to a bigger one. Some of the molecules are negatively charged, and consist of one big atom connected to the small one. These molecules are pointed by an arrow. There are also small positively charged atoms.
A. hydronium ion
B. hydroxide ion
C. water
D. proton
E. hydrogen ion

The most abundant molecules, consisting in two small atoms attached to a bigger one, are water molecules. Water molecules have two hydrogen atoms, which are the small atoms, and one oxygen atom, which is the bigger one. Water can dissociate, forming hydroxide ([tex]OH^{-}[/tex]) and hydronium ([tex]H^{+}[/tex]). If the arrow is pointing at a negatively charged molecule, that consists of one big atom attached to a smaller one, what this arrow is pointing to is an oxygen atom attached to an hydrogen atom, with a negative charge that was "stolen" from the hydrogen that was separated from the water molecule. So, the arrow is pointing to the hydroxide ion.