By definition, the Boiling Point is the temperature at which the vapour pressure of the liquid equals the surrounding pressure, and the liquid turns into vapour. The phenomenon of boiling is pressure dependent and hence, the Boiling Point of a liquid may change depending upon the surrounding pressure. For example, due to the change in atmospheric pressure at different altitudes, water boils at 100°C (212°F) at sea level, but at 93.4°C (200.1°F), at 1,905 metres (6,250 ft). For a given pressure, different liquids will boil at different temperatures.
Elevation in Boiling Point
We know that pure water boils at 100°C at 1 atm pressure, but an interesting thing happens to the Boiling Point if we add a small amount of salt to that water. It has been experimentally proven that adding any form of non-volatile soluble to a liquid increases its Boiling Point. The amount by which the Boiling Point changes is directly proportional to the amount of soluble added.
Let T0b denote the Boiling Point of a pure liquid, and Tb denote the Boiling Point of a solution (solute + pure liquid). Then,
[ Delta T_{b} alpha {m} ]
[ Delta T_{b} = K_{b} times {m} ]
Where, ΔTb = T0b − Tb is the elevation in Boiling Point,
‘m’ is the molality,
‘Kb’ is the molal elevation, Boiling Point or ebullioscopic constant whose value depends only on the solvent
The equation can further be written as,
ΔTb = [frac{1000 times K_b times w}{M times W}]
where, ‘’ is weight of the solute, ‘’ is the molar mass of the solute and ‘W’ is the weight of the solvent in grams.
Example: What is the Boiling Point elevation when 147g of lactic acid (C6H105) is dissolved in 647g of cyclohexane (C6H12)? The Boiling Point constant for cyclohexane is 2.79°C/m.
Solution: Determine the molality of the lactic acid solution
m = (147g / 162.14 g/mol) / 0.647 kg = 1.40127
ΔTb = Kb m
ΔTb = (2.79°C kg mol-1) (1.40127 mol/kg)
ΔTb = 3.91°C
Impurities and Boiling Point
When an impurity or a solute (non-volatile) is added to a liquid that is being boiled, its Boiling Point will increase. This indicates that solutions tend to have a higher Boiling Point as compared to pure solvents.
For instance, if you add non-volatile impurities like some sugar or a dash of salt to boiling water, its Boiling Point will elevate.
Question: A solution of 10.0 g of a nonvolatile, non-dissociating compound dissolved in 0.200 kg of benzene boils at 81.2 °C. Calculate the molecular weight of the compound. Ebullioscopic constant for benzene: 2.53 °C/m and Boiling Point of pure benzene: 80.1°C
Options:
(a) 115 gm/mol
(b) 145 gm/mol
(c) 120 gm/mol
(d) 100 gm/mol
Answer: (a)
Solution:
ΔTb = [frac{1000 times K_b times w}{M times W}]
1.1 = [frac{1000 times 2.53 times 10}{M times 200}]
M = 115 gm/mol
Interesting Facts
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Did you know that if you climb a mountain, you’re going to find the Boiling Point there reduced? This is mainly because the higher the temperature, the more vapour you’re going to find!
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You can boil water to use it as a disinfectant. Yes! This is because when you boil water, the microorganisms and bacteria get damaged due to its scalding heat, as a result of which it can easily be used as a great way to disinfect and clean your house. Just about a minute of boiling water is enough to kill the microorganisms.
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As it is commonly known, a solvent is a liquid that can be utilised to dilute and/or dissolve a solution. But did you know that water is a great and powerful solvent? A number of compounds can be dissolved with the usage of water. For instance, people often use it as a solvent for their beverages like coffee and/or tea.
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Although adding different types of solutes to water when it is boiling tends to alter its Boiling Point by increasing it, alcohol is a type of volatile solute that when added to it, ends up lowering the Boiling Point of the water instead.
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Water tends to boil at a relatively lower temperature when it is being boiled at a higher altitude.
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