Acids are a group of chemicals that taste sour, turn litmus paper red and react with metals to form salts. Acids release hydrogen ions, H+ in solution. Bases are a group of chemicals that feel soapy to touch. They behave in an opposite manner to acids. Alkalis are soluble bases. Alkalis turn red litmus blue. Alkalis release hydroxide ions, OH-, in solution. Neutral substances, such as water, are neither acidic or alkaline. The strength of an acid is measured using a scale called the pH scale. The numbers go from 0 to 14. An acidic solution has a pH number less than 7. An alkaline solution has a pH number greater than 7. A neutral solution has a pH number of exactly 7. To find the pH number of any solution you use universal indicator. Universal indicator is a mixture of dyes that change colour depending on what they have been placed in. A neutralisation reaction occurs when you add an acid to an alkali - they cancel one another out. A salt and water are the two products of neutralisation.