Question

When sodium hydroxide dissolves in water is it a chemical or physical change?

Answer 1

My own feeling is that this is a chemical change.....but your teacher may think differently.

Explanation:

Chemical changes are characterized by the formation of new substances and the making and breaking of strong chemical bonds.

When caustic soda is dissolved in water, strong ionic bonds between sodium and hydroxide ions are disrupted, are broken, and strong bonds are formed between the sodium ions and the water molecules. The balance of bonds formed versus bonds broken lies in favour of bond formation in that the dissolution is quite exothermic:

`NaOH(s) + "excess "H_2O(l)rarr Na^(+)(aq) + HO^(-)(aq)`

We write, `Na^(+)(aq)`, to represent the aquated ion, `[Na(OH_2)_6]^+`, and likewise the hydroxide ion, represented as `HO^-`, is probably a cluster of 3 or 4 water molecules, LESS a proton, to give `[H_5O_3]^-`. Given this chemical reality, in which bonds ARE BROKEN, and BONDS ARE FORMED along with NEW SUBSTANCES, the dissolution is probably best regarded as an example of chemical change.

Now this is an undergraduate treatment, not an A level treatment. If you are in high school, canvas your teacher's opinion, and adopt it.