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Acidic, alkaline and neutral solutions
Which of the following properties are related to the number of "free hydrogen ions" in an aqueous solution?
Some substances are acidic. Such as lemon juice. Other substances are the opposite of acidic. Like this powder, that can be used as drain cleaner: sodium hydroxide. The opposite of an acid is called a base.
Aqueous solutions of bases are called basic solutions, or alkaline solutions. When we say that something is acidic or alkaline, it is a measurement of the number of hydrogen ions in the solution. Acidic solutions have many hydrogen ions per liter. In alkaline solutions, there are few hydrogen ions per liter. Hang on, what does that mean?
Many, or few hydrogen ions compared to what? Where’s the line between “many” and “few”? We compare the solution with pure water. If there are more hydrogen ions than in the same amount of pure water, the solution is acidic. If there are fewer hydrogen ions compared to pure water, the solution is alkaline.
What do you mean, “hydrogen ions in pure water"? If it’s pure, it’ll only contain H2O. Then there’ll be no ions at all, right? Yes there will, look: Almost all molecules in the water look like this. One oxygen atom and two hydrogen atoms.
But a small number of the water molecules will split, like this. It will only happen with one water molecule out of six hundred million. But that means there is always a tiny number of hydrogen ions, even in pure water. That is what we compare a solution with, when we want to decide whether it is acidic, or alkaline. A solution with more hydrogen ions than pure water has, is acidic.
A solution with fewer hydrogen ions than pure water has, is alkaline. If we’re exactly in the middle - the same number of hydrogen ions as in pure water - we say that the solution is neutral. There’s another way to describe a neutral solution. Let’s look at the water molecule again. Each water molecule that is divided not only forms a hydrogen ion The other part of the molecule also becomes an ion, made of oxygen and hydrogen.
This ion is called an hydroxide ion, OH minus. This gives us another way to determine whether a solution is acidic, alkaline, or neutral. For each hydrogen atom that is released in pure water, the exact same number of hydroxide ions are formed. This means that pure water has the same number of hydrogen ions as hydroxide ions. An acidic solution has more hydrogen ions than the number of hydroxide ions.
An alkaline solution has more OH minus, than H-plus. What will happen if I mix the acidic and the alkaline solution? Will that make it both acidic and alkaline at the same time? We’ll see. If you use exactly the right amount of both liquids the mixture will have the same number of both types of ion.
And by our definition when the number of hydrogen and hydroxide ions are the same - the solution is neutral. We say that the acidic and the alkaline solutions have neutralised each other. So the neutral solution will contain lots of hydrogen ions from the acid, and lots of hydroxide ions from the base? It won’t, actually. As soon as the acidic and the alkaline solutions mix, almost all hydrogen ions and hydroxide ions will merge, forming water molecules.
Only a few hydrogen ions will remain and the same number of hydroxide ions. Just like in pure water. So, In an acidic solution, there are more hydrogen ions than hydroxide ions. In an alkaline solution it’s the other way around, there are more hydroxide ions than hydrogen ions. When an acidic and an alkaline solution are mixed, a neutralisation occurs.
If the right amounts of the acid and the base have been used, you get a mixture with the same number of hydrogen ions as hydroxide ions - a neutral solution. Almost all ions will merge to form water molecules.