An ex-housemate of mine claimed to be a former chemist, and once told me not to use boiling water (or recently boiled water) when leaving dishes to soak, as the temperature would chemically alter the dishwashing liquid, rendering it useless or at least significantly less effective. However I've never found anything in internet searches to support this assertion. Can you lovely peeps help give me a definitive answer?
It depends rather strongly on your brand.
Soap, by itself, is unlikely to be affected by the boiling water. They're relatively stable, especially for an organic molecule. However, many of us use detergents that include enzymes that may denature in the boiling water, which is what your friend may have been referring to: A denatured enzyme is not very effective at cleaning at all.
So, look on your bottle! If it mentions enzymes, it probably won't go over well with boiling water. If it's just soap, and, say, bleach, boil away!