MadSci Network: Chemistry

Re: How does sodium bicarbonate (baking soda) affect water's boiling point?

Date: Sat May 23 22:32:26 1998
Posted By: Matthew Barchok, High School Senior
Area of science: Chemistry
ID: 894832600.Ch

I am sorry I cannot represent this visually.  I would, but my 
computer can't seem to find my scanner.  What happens is, when sodium 
chloride or sodium bicarbonate dissolve in water, they dissociate.
Dissociation is when one ion separates from another.  With sodium chloride,
you get a positively charged sodium ion and a negatively charged chloride 
ion.  Water molecules have two hydrogens bonded to an oxygen.  The oxygen 
holds electrons more strongly than hydrogen.  Although they actually 
share electrons, the electron spends more time around the oxygen.  This 
makes the oxygen very slightly negatively charged and the hydrogen slightly
positive.  Since opposite charges attract, the negative ends of the water 
stick to the positive sodium ion.  The positive ends of other water 
molecules stick to the chloride ion.  It is this process that actually 
causes the salt to dissolve.  Hundreds of water molecules stick to each.
For the water to boil, each water molecule must become free to move.
When they are stuck to an ion, they have to break free before they can 
boil away.  This takes extra energy, which increases the boiling point.
The exact same process occurs with baking soda.  The hydrogen carbonate ion
behaves very much like the chlorine ion.  The problem with this is that 
not much baking soda can dissolve in water.  Also, a different amount of 
water sticks to the bicarbonate ion.  

NaCl ---> Na+  ,  Cl-

NaHCO3 ---> Na+  ,  HCO3-

Hydrogen in water sticks to the Cl- and the HCO3-.
Oxygen in water sticks to Na+ ion.

This also affects the freezing point.  For water to freeze, it 
must form a crystaline structure.  For this to happen, the positive
and negative ends of the water molecules must stick to each other.
It must be much colder for the water to prefer the crystaline 
structure over sticking to the ions.

An interesting side note- it takes energy to separate the sodium 
and chloride ions in salt.  Therefore, dissolving salt in water 
actually cools the water.  When wishing to keep something very 
cold, chemists often pour a lot of salt over ice.  It takes energy
to break down the crystaline structure of ice, and it takes energy
to dissociate the sodium and chloride ions.  Therefore, the final
solution has a lot less energy in it.  Energy is heat, so it is a
lot colder.

Matthew Barchok,
     Mad Scientist

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