4.4.2.4 - The pH scale and neutralisation

What do acids produce in aqueous solutions?

Acids produce hydrogen ions (H+) in aqueous solutions

What do alkalis produce in aqueous solutions?

Alkalis produce hydroxide ions (OH-) in aqueous solutions

What is the pH scale?

From 0-14 it is a measure of the acidity or alkalinity of a solution

What are two ways you can measure the pH of a solution?

• Universal indicator

• pH probe

What is universal indicator?

A dye that gradually changes colour over a broad range of pHs

(red = alkali , green = neutral , alkali = purple )

What colour are acids?

Red, orange , yellow

What colour are neutral solutions?

Green

What colour are alkalis?

Turquoise, blue, purple

What is a pH probe?

Device used to measure the pH electronically.

The pH is given on a digital display as a numerical value

What is more accurate, a universal indicator or the pH probe?

pH probe

What are the values of different solutions?

A solution with pH 7 is neutral.

Aqueous solutions of acids have pH values of less than 7

Aqueous solutions of alkalis have pH values greater than 7.

What occurs in neutralisation reactions between an acid and an alkali in terms of ions?

Hydrogen ions react with hydroxide ions to produce water.

Give the ionic equation for neutralisation.

H+ (aq) + OH-  (aq)  =   H20 (l)