Many cleaning products act best in somewhat alkaline solution.
This is because alkalis
- emulsify grease by reacting with insoluble fatty acids to form ionic salts which are soluble
- protect the metal of washing machines and dishwashers from acid corrosion
- help to reduce re-deposition of dirt that has been removed, by coating particles of it with negatively charged hydroxide ions - this means the dirt particles repel each other and remain in suspension rather than clumping together to form large aggregates which would tend to precipitate out onto dishes
- percarbonate-based bleaches work best in somewhat alkaline solution
The dishwasher water is kept at about pH 10 - significantly alkaline. At the high temperature of the wash, hydroxide ions react with the molecules of grease and break them up into salts of fatty acids and glycerol. This is the main mechanism for removing grease.
The reaction is shown in Figure 14 and is the same as that for making soap from fats and oils - saponification.
Sodium carbonate, Na2CO3, is used in the dishwasher product to make the solution alkaline. It is the salt of a strong alkali (sodium hydroxide) and a weak acid (carbonic acid) and is therefore alkaline. It dissociates in solution to form carbonate ions, CO32-, which help to maintain the pH of the washing water at around 10.
- Explain why the products of the reaction in Figure 14 are more soluble than the original fat molecules.
- Give the systematic name of glycerol.
- One product is a negative ion which can interact with water molecules better than a neutral molecule. The other is a small molecule with a high proportion of -OH groups that can hydrogen bond with water.
Explain how the carbonate ion can make an aqueous solution alkaline.
It can accept protons (H+ ions) from the solution to form HCO3-, leaving an excess of OH- ions.