In water, bases form alkalis. Examples of bases that form alkalis in water are sodium hydroxide and ammonia. Some bases, such as copper oxide, are not soluble in water.
Alkalis have a pH value that is greater than 7. The more powerful the alkali, the greater the pH value is. When a red litmus paper is dipped into an alkali, it turns the paper blue. A weak alkali such as ammonia turns a universal indicator paper bluish-green and gives a pH reading of 10 to 11. A strong alkali such as sodium hydroxide turns the paper dark purple or blue, and it gives a pH reading of 13 to 14.