Like dissolves like
This topic is intended to follow Solutions
and Talking solutions
and looks at how liquid solvents and different solutes form liquid solutions.
It would also help if you were familiar with the topic From
atoms to molecules.
In the FAQ section of Talking
solutions the following question and answer was put forward:
Will
all solvents dissolve all solutes?
No. Generally both solvents and solutes are separated
into two types: polar/ionic and non-polar.
The rule of thumb is that "like dissolves like". Polar/ionic
solvents dissolve polar/ionic solutes and non-polar solvents dissolve
non-polar solutes. For example, water is a polar solvent and it will
dissolve salts and other polar molecules, but not non-polar molecules
like oil. Petrol is a non-polar solvent and will dissolve oil, but will
not mix with water.
The rule mentioned above, "like dissolves like", is a very
handy way of determining which substances can be dissolved in which
solvents.
Types of substances that form solutes
and solvents
Let's start with a review of the types of substances that can make up
solutes and solvents. Generally speaking, matter is divided into elements
(metal and non-metal) and compunds (ionic (salts) and molecular). Molecular
compounds are further divided into polar and non-polar.
Matter
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Elements
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Compounds
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Metals
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Non-metals
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Molecular
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Ionic
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Polar molecular
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Non-polar molecular
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Metals are generally not very soluble in the common solvents
but non-metals, molecular compounds and ionic compounds are all soluble
in at least one common solvent. Liquid solvents are normally either
polar or non-polar but, at high temperatures, liquid salts and metals
can act as solvents. We will limit ourselves to polar or non-polar solvents,
marked in red in the above table, as they are the most common solvents.
Dipoles
Molecules are made of atoms that have bonded together in set patterns.
Depending on how the atoms are arranged in the molecule, the molecule
itself can sometimes have different charges on each end of the molecule
thus forming a dipole. Such molecules are said to be "polar",
while molecules with no or little charge separation are called "non-polar"
molecules.
Polar molecules are aggressively attracted to other polar
molecules, or even free ions, and form strong bonds between themselves.
They feel little attraction to non-polar molecules and tend to ignore
them in their rush to find other polar molecules or ions.
Non-polar molecules tend to group together, because they
are pushed out of the way by polar molecules, forming weak bonds between
themselves. In this way, polar molecules aggressively seek each other
out, excluding non-polar molecules from mixing with them and forming
a solution. While non-polar molecules form much weaker attractions for
each other, they will mix and form solutions. Like dissolves like!
Polar solvents
Water is the most common polar solvent on Earth. It will dissolve acids,
ionic salts, alcohol, ammonia, sugars and even silicates from rock if
under high pressure and temperature.
Non-polar solvents
The most common non-polar solvents and solutes are the hydrocarbons.
Mainly found in crude oils and tars, hydrocarbons, like petrol and mineral
turpentine, will dissolve oils, grease, wax, tar, methane gas and most
organic molecules.
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