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
HCl (aq) → H+ (aq) + Cl- (aq)
Acid with Metal
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
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
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
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
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
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
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
How do acids and bases react with each other in a neutralization reaction?
What is the significance of the acid dissociation constant (Ka) and the base dissociation constant (Kb)?
How does the concept of conjugate acid-base pairs help in understanding acid-base reactions?
What are the environmental impacts of acid rain, and how is it formed?