NH3, also known as ammonia, is a weak base. It is basic because the molecule reacts with water to form negatively charged ions of OH. Strong bases completely dissociate in reaction to water, but NH3 retains its original form.
A base or alkali is any substance that forms negative OH ions in reaction to water. Acids, by contrast, create positively charged hydrogen ions in reaction to water. Acids and bases also are defined in terms of whether the substance is likely to take or donate electrons when reacting with other substances. Acids take electrons, while bases donate. Rarely, NH3 may act as a weak acid. For example, ammonia has an acidic reaction with lithium, which forms lithium amide.