Final answer:
Dr. Ignaz Semmelweis used a chlorine solution, specifically chlorinated lime water, to wash hands and medical instruments, substantially reducing puerperal fever mortality rates.
Step-by-step explanation:
The chemical that Dr. Ignaz Semmelweis used to improve hygiene practices in the clinic where he worked was a chlorine solution, specifically chlorinated lime water. Semmelweis observed a high mortality rate due to puerperal fever among women attended by physicians and medical students who came directly from performing autopsies. He inferred that the cause was some form of contamination that was being transferred from the autopsied bodies to the mothers. To combat this, he urged the medical staff to wash their hands with the chlorine solution before examining pregnant women, which remarkably reduced the mortality rate from around 18% to 2%.
This discovery by Semmelweis came years before Joseph Lister's work with carbolic acid, which similarly aimed to reduce infections in surgical practices. Semmelweis' work is particularly important as it refuted the miasma theory of disease and helped establish the vital role of handwashing in preventing the spread of infections, leading to improved public health practices.