How does a mixture not retain its chemical identity?

Question

My book says that matter can be classified as an element, compound, or mixture. It further says that a pure substance (or substance) is matter that has distinct properties and a composition that doesn't very from sample to sample. So far, so good. Then it says that

all substances are either elements or compounds

An element is one type of atom (something that "cannot be decomposed into simpler substances") and a compound is composed of two or more elements; these both clearly are substances. Then, it says that

Mixtures are combinations of two or more substances in which each substance retains its chemical identity.

This is where I'm confused. How are mixtures not substances? They have distinct properties, and their composition doesn't vary from sample to sample, if you set a standard for the sample. You can create any arbitrary mixture, and you can create any arbitrary compound, and if you follow the "recipe" for either you will always get the same thing, as far as I know.

Where is my logic flawed?

Answer 1

As your book uses the terms:

• Elements: Single atoms (e.g. O).
• Compound: Consists of only one type of molecule, where that molecule consists of more than one element (e.g. H₂O).
• Mixture: Consists of more than one type of molecule (e.g. Na⁺ + Cl^- + H₂O, which is saltwater, or for example the air you breathe, which is mostly N₂ with some O₂ floating around and a bunch of other stuff).

I'm not entirely sure where your book classifies materials that consist of a single type of molecule where that molecule consists of only one element (e.g. O₂) but whatever, you should be able to fit that in somewhere. It probably includes those as "elements".

To use a loose macro analogy (don't deconstruct this too deeply or you will soon run into trouble), let's say "blue paint" and "yellow paint" are elements. When combined they form the compound "green paint". On the other hand say "blue marbles" and "yellow marbles" are elements. When combined, they don't form "green marbles", they just form a mixture of blue and yellow marbles.

Note also that samples of your pile of blue and yellow marbles may vary: If you pick up a handful of them you very well could end up with slightly different proportions each time. On the other hand, every sample you take of the green paint will be identical green paint.

Answer 2

A good example of the difference might be to look at water, $\ce{H2}$, and $\ce{O2}$.

If you have gaseous $\ce{H2}$, and $\ce{O2}$ in a container you have a mixture; you can change it to have $99\%$ oxygen and $1\%$ hydrogen or vice versa or any ratio in between, but it is still a mixture of the two gases. These mixtures will have different properties depending on the ratio of the two gases. Each gas still has its own properties and the mixture exhibits some combination of them depending on the ratio.

If they are allowed to react to form water, now we have a substance. Regardless of how we make water, it will always have the same composition (two hydrogens for each oxygen) and the same properties. Importantly, these properties are completely different from hydrogen, oxygen, or any mixture of the two.

Answer 3

The phrase about retaining the chemical identity refers to the reactivity of the compounds in question. A compound reacts in certain ways that cannot be explained simply by looking at the elements a compound is made up of. For example, sodium bromide $\ce{NaBr}$ will react with chlorine gas to liberate sodium chloride $\ce{NaCl}$ and bromine. However, mixing just the elemental components with chlorine gas will either create a mixture that practically does not react at all (bromine and chlorine) or a mixture that will react strongly in a wildly different way (sodium and chlorine).

Having established that certain compounds react specifically we can look at how mixtures react. For example, you might create a mixture (i.e. solution) of sodium bromide in water. You can then bubble chlorine gas through this solution. The chlorine gas will not react to any noticeable extent with water but it will react with the dissolved sodium bromide — in the exact same way as above, i.e. it will also liberate bromine.

$$\ce{2 NaBr + Cl2 -> 2 NaCl + Br2}$$

Thus, even in the mixture with water sodium bromide has retained its chemical identity, i.e. its reaction with chlorine to give bromine and sodium chloride.

This principle can be extended to any mixture whose components have distinct reactivities. For example, nitrogen monoxide reacts with oxygen to give nitrogen dioxide — and these two will also react in this way (with no other reactions taking place) if, for example, the nitrogen monoxide is mixed with methane and the oxygen with carbon dioxide.