Although Armstrong was active in many areas of scientific research, such as the chemistry of naphthalene derivatives, he is remembered today largely for his ideas and work on the teaching of science.

Chemical training in those days was not lengthy, and at the age of 18 he was selected by Edward Frankland to assist in devising methods of determining organic impurities in sewage.

W. P. Wynne was his most important collaborator; their 263 naphthalene samples, accrued over several decades, are now preserved at Imperial College as the Armstrong-Wynne Collection.

[citation needed] Armstrong's later researches dealt with terpenes, particularly camphor, with water purification, helping to eradicate typhoid fever, and with crystallography.

[citation needed] In 1887, Armstrong became interested in classifying substituents of benzene in terms of their meta- and ortho-para directing influences.