Positive ions usually form from and are called cations.

Positive ions usually form from losing electrons and are called cations.

Explanation

An atom can lose one or more valence electrons and it may also gain one or more valence electrons leading to the formation of ions.

They are electrically charged particles unlike atoms which are neutrally charged.

The atoms can be converted to positive ion on removal of electrons from the valence shell.

Similarly the atoms can also be converted to negative ions when extra electrons get added in the valence shell.

So the losing of electrons leads to the formation of positive ions which are termed as cation while the gaining of electrons will form negative ions which are termed as anions.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.