ChemistryOnline is another engaging and interactive subject site from the ScholarNET Online Education stable of online learning resources. It covers the major Chemistry topics that are fundamental to every senior Chemistry course - Atomic Structure and Bonding, Aqueous Chemistry, Organic Chemistry, Redox Chemistry as well as Thermochemistry (Energy).
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In this document:
You should already know:
What we mean when we say particle, ionic compound or ionic species
Be familiar with ions
Solubility is the concept that describes whether something will dissolve in a liquid or not. Most of the time, when we talk about solubility, the liquid in question is water, but it may not always be, so be aware of that.
Solubility key words
- Solvent is the liquid that something is dissolving in or not. If it doesn't say what the liquid is, then it's probably water.
- Solute is the stuff being dissolved in the liquid. It can be another liquid, or a gas, but is more often a solid.
- Solution is a liquid with something else dissolved in it.
- Soluble is the word to describe something that dissolves in the solvent.
- Insoluble is the word that describes something that won't dissolve in the solvent.
- Sparingly soluble is the term used to describe something that only dissolves a little bit in the solvent.
Dissolving is when something appears to "disappear" into a liquid with little or no trace. This is a physical change. What actually happens is that the particles of the solute get separated and completely surrounded by the solvent particles.
Before a solute is put into the solvent, for example a teaspoon of sugar crystals into a cup of hot water, the sugar particles are all packed tightly together and held together by attractive forces. There are so many million sugar particles being held together in the same crystal that it's easy to see the overall crystals with your eye. But once the sugar crystals are added to the solvent, water, it breaks the particles holding the sugar together and separates them into individual particles. Then the water molecules surround each sugar particle and encloses it in its own, personal sphere of water molecules, sort of like a protective circle of bubble wrap that goes around all 360°. This keeps the sugar particles individualised, which is why we can no longer see them - an individual sugar particle is way too small to see with the naked eye, even with its tiny sphere of water.
Like dissolves like
The next thing to note is that only certain solvents and solutes go together. You'll be familiar with the idea that oil and water don't mix, that the oil just floats on the top of the water (unless it's a dense oil which will sink to the bottom). The guideline for what dissolves in what is the phrase "like dissolves like". What does that mean?
The word "like" refers to the way you can divide molecules into two categories - polar and non-polar. (Note: if you haven't studied what these words mean yet, it will be enough to know that water is polar and oil, petrol and cyclohexane are non-polar.) A solute will dissolve in a solvent if they are both of the same kind - either both polar or both non-polar. This is why water and oil won't mix, they are not "like". Oil is non-polar and water is polar, so they won't mix together, they won't dissolve.
Next note: ionic species are with the polar group when we talk about solubility. Ionic salts like copper sulfate, sodium bicarbonate and sodium chloride are made up of ions. When these solids hit the water (a polar solvent), they are broken up into individual ions and again surrounded by a sphere of water molecules. Water has a sort of positive end and a sort of negative end (called δ+ and δ-, don't worry if you haven't learnt about this yet). The water molecules in the sphere surrounding a negative ion line themselves up delta positive side in, to balance the charge by creating a countercharge atmosphere. By the same token, the positive ions are surrounded by their sphere of water molecules, all lined up delta negative end in, again to counter the charge. This allows the ions to float freely and separately, without needing to counter each other's charges. This also means that the ions are free to react if they encounter any appropriate reactive species as they float around in their ocean of solution.
When you want to react two ionic compounds, most of the time, it's only one of the two ions in each compound that you are interested in. The other half of each ionic compound is often just there for convenience.
What do I mean? Ions are charged species, either positively or negatively. But they can't exist on their own, charged like that. All ions need to be balanced and neutralised by ions of the opposite charge. So where you get a positive ion, there must also be a negative ion of some sort. Where you have a negative ion, there has to also be some kind of positive ion around too. There's always two ions present, you can't get one without the other.
But that's often not a problem, because each positive ion can go with a range of negative ones and vice versa. So you just need to match up the two ion half you want with two ion half that won't bother your reaction, and you don't have any problems. Sodium ions are a popular positive ion that doesn't interfere with your reaction, and chloride ions are an excellent choice for an unobtrusive negative ion. For example, in film photography silver ions are used as an ingredient in the coating on photographic paper because they are very sensitive to the presence of light, and they help capture the scene from the negative. But we can't get silver ions on their own, so we have to find them paired up in an ionic compound with something that won't do anything to our film processing reactions. A halide like bromide is often used to pair up with the silver ions.
The name for the other half of an ionic compound that isn't required by the reaction is a spectator ion. For some equations when the spectator ions aren't important, they are no included. The equation is now called an ionic equation. It only shows the ions of interest.
A precipitate is the insoluble solid that forms when two sets of ions react together. Basically, if you mix two solutions together and hey, presto! a solid forms out of nowhere, that little bit of chemical magic (the solid) is called a precipitate.
Some ions are always soluble. Some are never soluble. For some ions it depends on what their counter-ion is. But the good news is that these details follow some pretty hard and fast patterns that aren't too hard to remember. You can use them to predict whether you'll get a precipitate or not, and what it is, when you mix certain ionic solutions together.
||Na+, K+, NH4+, NO3-
||SO42-, Cl- (and the other halides)
||Exceptions: BaSO4, PbSO4, CaSO4, AgCl, PbCl2.
||OH-, O2-, CO32-
||Exceptions: When these ions are with an "always soluble" ion such as NaOH, K2CO3.
This is how to put this information to good use:
Talking about colours
It is very important to describe the colours of your solutions or precipitates. These are actually pretty easy to remember, too, because they follow trends as well. Things with iron(II) are always some sort of green colour. Iron(III) stuff is some variation of orange, ranging from yellow to reddy brown, depending on what the other ion is. Colours are generally given straightforward names, not fancy names like is often found in the fashion world - no chartreuse, magenta or aubergine please, it confuses those people who are not colour-literate!
Most importantly, what do you call a solution that looks like water? White? Transparent? See-through? No. White would describe milk. Transparent and see through mean that light can get through it, but it can still be coloured. A solution of copper(II) ions is a lovely blue colour, and you can still see what's on the other side of the beaker it's in, such as in the photo below (copper sulfate solution on the left, water on the right). It is transparent, but also blue. So transparent's not the right word. The word you are looking for to describe solutions like water, that have no colour, is colourless.
Summary of this page
- Solubility is about whether something will dissolve in a liquid or not.
- Dissolving is when a substance "disappears" into a liquid. The particles are separated and surrounded by the solvent so that it is no longer possible to see the original substance.
- Solvent is the liquid that the substance is dissolved in.
- Solute is the substance being dissolved in a liquid. Usually solid, occasionally liquid or gas.
- Solution is the resulting liquid with the substance dissolved in it.
- Soluble describes something that dissolves in that solvent.
- Insoluble describes something that doesn't dissolve in that solvent.
- "Like dissolves like" - polar substances only dissolve in polar solvents (like water) and nonpolar substances only dissolve in nonpolar substances (like cyclohexane). Ions are classed as polar substances in this situation.
- Spectator ions are the other half of the ionic compounds that aren't taking part in the reaction.
- A precipitate is an ionic solid, that forms when two ionic solutions are mixed together.
- There are patterns for which ionic compounds dissolve in water or not. These patterns form the solubility rules.
- It is important to be able to recognise ions by their colours where possible. Solutions that show no colour (like water) are called colourless.