Dilute and concentrated solutions

Concentration is the amount of solute present in a fixed quantity of solution (or solvent). It answers the question "how strong or weak is this mixture?" — not by taste or colour alone, but by thinking about the actual quantity of dissolved substance packed into a given quantity of liquid. Two comparison words describe concentration in everyday and early chemistry usage:

• A dilute solution contains a relatively lesser amount of solute in a fixed quantity of solution.
• A concentrated solution contains a relatively greater amount of solute in the same fixed quantity of solution.

The single most important idea in this whole topic is that "dilute" and "concentrated" are relative terms, not fixed labels glued to a solution forever. A glass of lemonade is not "dilute" in some absolute sense; it is dilute compared to a stronger glass, and it could just as easily be called concentrated if compared to a weaker one. There is no numerical formula at this stage — no molarity, no percentage by mass — the comparison is entirely qualitative, built from reasoning about "more solute here" versus "less solute there," always for the same fixed quantity of solution. It is worth separating concentration from a different idea met earlier in the chapter: saturation. A solution can be strongly concentrated and still not be saturated — it may still be able to take in more solute before reaching its limit. Equally, a solution can be quite dilute yet already saturated, if the solvent's maximum capacity for that particular solute happens to be small at that temperature. Concentration is about how much solute is actually present; saturation is about how much solute the solvent can possibly hold. They move independently of each other.

There is no formula to derive here — concentration at this level is a comparison, not a calculation — so the reasoning that matters is why comparing two solutions correctly requires holding one thing fixed while another changes.

Step 1. Fix the quantity of solution being compared. "More solute" only means something if the two solutions being compared use roughly the same amount of solution (or solvent). Ten spoons of sugar in a bathtub of water is not "more concentrated" than two spoons of sugar in a teacup — the huge difference in total volume swamps the comparison. This is why every valid dilute-versus-concentrated comparison first checks: are we comparing solute amounts for the same fixed quantity of solution?

Step 2. Compare the solute amounts once the quantity is fixed. With the solution quantity matched, the solution with less solute is dilute and the one with more solute is concentrated. This is the entire comparison — no formula, no calculation, just a direct read of which sample has more dissolved substance in an equal amount of liquid.

Step 3. Remember the labels can flip. Because "dilute" and "concentrated" only exist relative to whatever they are being compared against, the very same solution can be called dilute next to a stronger one and concentrated next to a weaker one. This is exactly why the labels are useful in everyday and laboratory language — they let you rank solutions quickly — but also why they can never replace an actual measured concentration when precision is needed (a tool met only in higher classes).

Step 4. Watch for the two-variable trap. Sometimes both the solute amount and the solvent amount change together — more syrup and more water, say. In that case you cannot just look at the solute amount; you must think about how much solute sits in the same fixed quantity of total solution, which may mean the solute-rich sample is actually more dilute if it is also spread through much more solvent. Reasoning through both changes together, rather than eyeballing just one, is what separates a correct comparison from a guess.

Question. A student prepares two glasses of lemon squash, each starting with 200 mL of water. Glass A gets 2 spoons of sugar syrup, and Glass B gets 8 spoons of the same syrup. Later, a third glass, Glass C, is made with 20 spoons of the same syrup in the same 200 mL of water. Identify which glass is dilute and which is concentrated in each pairwise comparison, and explain why Glass B's label can change.

Step 1. Confirm the quantities being compared are fixed. All three glasses use the same 200 mL of water as their base, so any two of them can be compared fairly by solute amount alone.

Step 2. Compare Glass A and Glass B. Glass A has 2 spoons of syrup, Glass B has 8 spoons, for the same 200 mL of water. Glass A has the relatively lesser amount of solute, so Glass A is the dilute solution; Glass B has the relatively greater amount, so Glass B is the concentrated solution.

Step 3. Now compare Glass B and Glass C. Glass B has 8 spoons of syrup, Glass C has 20 spoons, again for the same 200 mL of water. Compared to Glass C, Glass B now has the relatively lesser amount of solute — so next to Glass C, Glass B is the dilute one, even though it was concentrated when compared to Glass A.

Step 4. State the conclusion clearly. Glass A is dilute compared to Glass B; Glass B is concentrated compared to Glass A but dilute compared to Glass C; Glass C is concentrated compared to both. The labels dilute and concentrated depend entirely on which pair is being compared — Glass B carries both labels correctly, just against different partners.

Competitive stretch

A shopkeeper pours 50 mL of concentrated fruit squash into a glass and adds 250 mL of water to make Drink 1 (total 300 mL). His friend pours the same 50 mL of squash but adds only 100 mL of water to make Drink 2 (total 150 mL). Here the solute amount (50 mL squash) is identical in both, but the total quantity of solution differs, so the same amount of solute is packed into a smaller total quantity in Drink 2. Drink 2 is the more concentrated drink, and Drink 1, with the same solute spread through more total solution, is the more dilute drink — a reminder that concentration compares solute against the total solution quantity, not solute amount alone.

• Treating "dilute" or "concentrated" as a permanent, absolute label for one solution rather than a comparison between two solutions — as Glass B above shows, the correct label depends on the partner in the comparison.
• Confusing "concentrated" with "saturated": a concentrated solution simply has a large amount of solute relative to another sample and may still dissolve more solute (unsaturated), while saturation is about hitting the solvent's maximum capacity at a given temperature.
• Assuming that adding more solvent always makes a solution more dilute — true only if the solute amount stays fixed; if solute is being added or removed at the same time, both changes must be reasoned through together.
• Comparing total solute amounts instead of solute amounts for a fixed quantity of solution — a large glass of weak solution is not automatically more concentrated than a small glass of strong solution just because it holds more solute in total.

Ask the tutor: Want to try comparing three or four solutions at once and see how the dilute or concentrated label shifts depending on which pair you pick?