Acids, Alkalis & pH

An ion is a particle with electric charge. Acids increase the concentration of hydrogen ions, written H⁺, in water. Concentration means the amount in a given volume. For dilute solutions at the same temperature, more H⁺ in that volume means lower pH. Alkalis are bases that dissolve in water and increase hydroxide ions, OH⁻. These can react with H⁺ to form water.

This lesson uses water-based solutions at 25°C. A pH below 7 is acidic, 7 is neutral and above 7 is alkaline. Lemon juice is often around pH 2 and vinegar around pH 3. Bicarbonate of soda solution is mildly alkaline, often around pH 8 to 9. Household bleach is also alkaline. These values vary between samples. Low pH alone does not tell you whether an acid is strong or weak.

Universal indicator estimates pH from colour. Use its supplied chart. Acids give red, orange or yellow; neutral is green; alkalis give blue or purple. An acid turns blue litmus red. An alkali turns red litmus blue. A neutral solution changes neither red nor blue litmus. One unchanged strip is not enough to identify a neutral solution.

The words show typical colour ranges as pH increases. Use the chart supplied with the indicator to estimate a reading.

During neutralisation, H⁺ and OH⁻ react to make water, H₂O. An acid reacting with an alkali also forms a salt, which may stay dissolved. Adding alkali can raise an acid's pH. Too little leaves the mixture acidic; excess alkali can make it alkaline. A green indicator reading at pH 7 shows a neutral mixture at 25°C. It does not show that the mixture is pure water.

Use subtraction to compare positions on a pH scale. For example, pH 4 is 7 - 4 = 3 units below 7. A point equally far above 7 is 7 + 3 = 10. Equal distances describe positions on the scale. They do not tell you which amounts of acid and alkali will neutralise each other.