Water is a neutral liquid, but it must be said that not all chemicals are neutral, and therefore there has to be a definition for these chemicals. These chemicals are called acids and bases:
- An acid is a chemical that donates a proton to the solution.
- A base is a chemical that accepts a proton.
This definition divides the acids into chemicals which possess a proton such as HCl while a base is a chemical that contains a hydroxide group such as NaOH.
A very important aspect is the fact that in order to be an acid or base the chemical must be dissolved and ions must be produced, so Ca(OH)2 is not a base due to the fact that this does not dissolve in water while HCl dissolved in cyclohexane cannot be said to be an acid due to the fact that no protons would be released.
Indicators
In order to see whether a chemical is an acid or a base indicators can be used. As the name implies an indicator is a chemical that indicates the nature of the chemical.
The most important and basic indicator is the litmus paper which is blue when basic and red when acidic.
Apart from litmus paper phenolphthalein can also be used which is pink in basic solutions and colourless in acidic solution.
The most precise indicator is the universal indicator which can distinguish between different levels of acidic or bases solutions. The pH scale is between 0 and 14 with pH 7 being a neutral pH. The lower the pH the more acidic the solution while the higher the pH the more basic the solution would be.
Strong and Weak acids
It must be noted that there are a number of chemicals that can act as a base or as an acid, but not all chemicals would act as an acid or a base, while others are only weakly acidic or basic.
In order to be a strong acid the acid must be able to fully dissociate into ions, and therefore all of its available protons can be found in solution while the same goes for a base. Weak acids and bases are those that can only partially dissociate into protons or partially accept a proton, with H3PO4 and NH3 being a weak acid and base respectively
Salts
A salt can be said to be a product of the reaction between an acid and a base, and the main reactions are the following:
Neutralisation reaction
This is a reaction in a which an acid and a base react to form water and a salt.
NaOH + HCl → NaCl + H2O
Acid + metal
An acid can react with a reactive metal to form a salt and Hydrogen.
2HCl + Mg → MgCl2 + H2
Acid + Carbonates/Hydrogenarbonate
Acids react with carbonates and hydrogen carbonates to produce carbon dioxide and water.
H2SO4 + CaCO3 → CaSO4 + CO2 + H2O
HCl + NaHCO3 → NaCl + H2O + CO2
Direct synthesis
This is when two chemicals are reacted to produce the salt directly, for example, the synthesis of NaCl.
2Na + Cl2 → 2NaCl