Like dissolves like

Will all solvents dissolve all solutes? Types of substances that form solutes and solvents
Dipoles Polar solvents Non-polar solvents

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:

Click for larger imageWill 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.

Water dissolving salt
Non-polar dissolution.
Oil in 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.

Polar molecular
Non-polar molecular

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.

Click for larger image 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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