GCSE Extension Science - Acids, Alkalis and Titrations

Acids can be strong or weak.

• Strong acids (e.g. Hydrochloric and sulfuric) ionise almost completely in water. This means almost every hydrogen atom is released to become a H+ ion.
• Weak acids ionise only very slightly and reach dynamic equalibrium: acid molecules split up but at the same time, the H+ ions and the negative ions that are made join up again. They form the original acid molecules.

Bases are substances that:

• Turn litmus blue
• Turn Universal indicator dark green, blue or purple
• React with the H+ ions in acids
• Are called alkalis when they react with water.

Name of Alkali..................Formula...............Strong or weak?

Sodium Hydroxide.............NaOH..................... Strong

Potassium Hydroxide........KOH....................... Strong

Calcium Hydroxide........... CA(OH)2 ................ Strong

Ammonium Hydroxide....... NH4OH................... Weak

Type of compound............ Contain............ React with acid to give...

Metal Hydroxides................. OH- .................... Water + a salt

Metal oxides........................ O2- .................... Water and a salt

Metal carbonates.............. O3 2- ................. Water and a salt and CO2

A titration is an experimental technique to measure the amount of substance in a solution. The end point of a titration is the point at which it is exactly neutralise, you can tell when you have reached the end point because the indicator changes colour. You should repeat a titration several times so you can make the experiment more reliable.

Indicators are chemicals that are one colour in acid and a different colour in alkali.

When performing a titration between a strong acid and strong alkali, you can use any indicator.

When performing a titration between a strong acid and weak alkali, you can use only some indicators, eg. Methyl Orange.

When performing a titration between a weak acid and strong alkali, you can only use some indicators, eg. Phenolphthalein.