The term 'ACID OPPOSITE' refers to two substances that have opposite properties. The first is ALKALI, which is a type of BASE with six letters. It is a substance that has a pH greater than 7 and is capable of neutralizing acids. The second is BASE, which is an acid with four letters. It is a substance that has a pH less than 7 and can react with alkalis to form salts. Both alkalis and bases are important components of many chemical reactions.