How acidic or alkaline a substance is (the pH of the substance) can be measured using the pH scale, a continuous range that stretches from below 0 to above 14. Most common pH values occur between 0 and 14.
Acids have a pH of less than 7.
Alkalis have a pH more than 7.
Water and neutral solutions have a pH of exactly 7.
The pH is a measure of the concentration of hydrogen ions in a solution.
Alkalis are soluble bases. An alkaline solution can be formed when a metal oxide is dissolved in water. An acidic solution can be formed when a non-metal oxide is dissolved in water.
For example, magnesium oxide dissolves to form alkaline solutions.
Sulfur dioxide and nitrogen dioxide however, will dissolve to form acidic solutions.
Soluble metal oxides produce alkalis when dissolved in water.
Soluble non-metal oxides produce acids when dissolved in water.