Properties of Acids and Bases

• Physics
• Secondary Education
• Age 12,13

31 March 2023

Properties of Acids

Content objectives

• I can describe the characteristic properties of acids in terms of their reactions with (a) metals; (b) bases; (c) carbonates.
• I can describe acids in terms of their effect on (a) litmus; (b);  thymolphthalein; (c) methyl orange
• I can describe the characteristic properties of bases in terms of their reactions with (a) acids; (b) ammonium salts.

What do I know about acids and bases?

What do I know about acids and bases?

Properties of Acids

• pH values of below 7
• sour taste (when edible) 
• corrosive
• can neutralise a base
• form a salt and water
• acids + water = forms hydrogen ions (H⁺)
• presence of H⁺ ions = acidic solution

HCl (aq) → H⁺ (aq) + Cl^- (aq)

Example of acid

Typical reactions of acids

Acids + metals

• Only metals above hydrogen in the reactivity series will react with dilute acids.
• Acid + Metal → Salt + Hydrogen
• The name of the salt is related to the name of the acid used, as it depends on the anion within the acid

Typical reactions of acids

Acids + metals

• Only metals above hydrogen in the reactivity series will react with dilute acids.
• Acid + Metal → Salt + Hydrogen
• The name of the salt is related to the name of the acid used, as it depends on the anion within the acid

Typical reactions of acids

Acids + bases

• Metal oxides and metal hydroxides (alkalis) can act as bases
  
• Acid + Base → Salt + Water (neutralisation)

Typical reactions of acids

 Acids + metal carbonates

• Acid + Metal Carbonate → Salt + Carbon Dioxide + Water

Indicators

• Two colour indicators are used to distinguish between acids and alkalis
• Many plants contain substances that can act as indicators and the most common one is litmus which is extracted from lichens
• Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear in different colours in acids and alkalis

Synthetic Indicators

• thymolphthalein 
• methyl orange 
• these two are frequently used in acid-alkali titrations

Synthetic Indicators

• used to show the endpoint in titrations as they have a very sharp
• very sharp change of colour when an acid has been neutralised by alkali and vice-versa
• Litmus is not suitable for titrations as the colour change is not sharp and it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint.

Synthetic Indicators

• Litmus is very useful as an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases