Dilute 25mm of Hydrochloric acid with 75mm of distilled water. Shake the test tube to ensure the solution is properly mixed.
Put 7g of Borax into a beaker. Pour in 20mm of boiling water and allow the Borax to dissolve.
Put some ice into the large beaker. Pour the test tube of hydrochloride solution into the smaller beaker containing the Borax solution. Put the small beaker into the ice bath to speed the reaction. Wait for the Boric acid solution to chill completely.
Put filter paper over the top of a clean test tube and pour the Boric acid solution through the filter paper. The solid Boric acid substrate will catch in the filter paper.
Scrape the Boric acid substrate onto a petri dish and allow to dry.