Properties of Acids & Bases (Cambridge (CIE) IGCSE Chemistry)

Properties of acids

• 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)

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

 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

What are 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

What are synthetic indicators?

• 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

Properties of bases & alkalis

• 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)

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

  • For 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