Properties of Acids & Bases (Cambridge O Level Chemistry)

• Acids have pH values of below 7, have a sour taste (when edible) and are corrosive
• Acids are substances that can neutralise a base, forming a salt and water
• When acids are added to water, they form positively charged hydrogen ions (H⁺)
• The presence of H⁺ ions is what makes a solution acidic

 Example: Hydrochloric Acid

HCl (aq)   →    H⁺ (aq)    +    Cl^- (aq)

Typical reactions of acids

Acids and metals

• Only metals above hydrogen in the reactivity series will react with dilute acids.
• When acids react with metals they form a salt and hydrogen gas:

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.
• Examples of the names of salts from specific acids and metals are:

Acids with bases

• Metal oxides and metal hydroxides (alkalis) can act as bases
• When they react with acid, a neutralisation reaction occurs
• In all acid-base neutralisation reactions, salt and water are produced

Acid    +    Base   →    Salt    +    Water

• Examples of reactions between acids and bases:

 Acids with metal carbonates

• Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water:

Acid  +  Metal Carbonate → Salt  +  Carbon Dioxide  +  Water

• Examples of reactions between acids and carbonates:

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 different colours in acids and alkalis
• Thymolphthalein and methyl orange are synthetic indicators frequently used in acid-alkali titrations

Two Colour Indicators Table

• Synthetic indicators are used to show the endpoint in titrations as they have a 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
• Litmus is very useful as an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases

• Bases have pH values of above 7
• A base which is water-soluble is referred to as an alkali
• In basic (alkaline) conditions red litmus paper turns blue, methyl orange indicator turns yellow and thymolphthalein indicator turns blue
• Bases are substances which can neutralise an acid, forming a salt and water
• Bases are usually oxides or hydroxides of metals
• When alkalis are added to water, they form negative hydroxide ions (OH^–)
• The presence of the OH^- ions is what makes the aqueous solution an alkali

Example: Sodium Hydroxide

NaOH (s)   →    Na⁺ (aq)    +    OH^- (aq)

Typical reactions of bases

Bases and acids

• When bases react with an acid, a neutralisation reaction occurs
• Acids and bases react together in a neutralisation reaction and produce a salt and water:

Acid    +    Base   →    Salt    +    Water

Examples of reaction between bases and acids:

 Alkalis and ammonium salts

• Ammonium salts undergo decomposition when warmed with an alkali
• Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali
• A salt, water and ammonia are produced

Example:

NH₄Cl + NaOH →NaCl + H₂O + NH₃

• This reaction is used as a chemical test to confirm the presence of the ammonium ion (NH₄⁺)
• Alkali is added to the substance with gentle warming followed by the test for ammonia gas using damp red litmus paper
• The damp litmus paper will turn from red to blue if ammonia is present