|MadSci Network: Chemistry|
Your research has been spot on, the sodium carbonate will turn the stain into soap. This is in fact the reason that bases such as sodium carbonate feel slimy to touch (they are turning the oils in your hand into soap)
To answer you question fully I need to explain how a soap works. Soaps have two components or parts. They have a long-chain organic part (sometimes called the hydrophobic tail) which is fat soluble, and an ionic water soluble part at one end. When the soap comes in contact with an oily stain, the fat soluble organic tail binds with the oils in the stain, while the water soluble (hydrophyllic) end pulls it out into the water. When millions of soap molecules do this together, it disperses the stain into the water where it can be washed away.
Now how the soap forms from your oil stain with the sodium carbonate is this:
You mght be familiar with the term acids and bases. When acids react with bases, they always form a salt and water. Depending on the structure of an organic acid (carbon-containing acid) they sometimes release carbon dioxide as well in the process.
In your case this is exactly what happens.
The oil has an acidic group on the end called a carboxylic acid. The molecule looks a little like this:
/\/\/\/\/\/\/\/\/\/\/\/-COOH long chain fatty bit acidic bitWhen it reacts with Sodium carbonate it forms:
/\/\/\/\/\/\/\/\/\/\/\/-COO- and Na+ long chain fatty bit water soluble endIf you remember I said above that your average soap has a long-chain organic part (sometimes called the hydrophobic tail) which is fat soluble, and an ionic water soluble part at one end.
So the molecule that is formed fits the description of a soap pretty well.
The actual chemical reaction looks like this:
(I'll use the letter 'R' to represent the fatty chain)
2(RCOOH) + Na2CO3 ---> CO2 + H2O + 2(RCOO- Na+)
Hope this helps.
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