Mistake Master
Representations of solutions
A bottle labeled 1.0 M NaCl(aq), a drawing of scattered ions, and the words 'salt dissolved in water' all describe the same thing. Fluency in chemistry is being able to move between those representations without dropping any information.
§1
Three ways to show a solution.
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A solution can be represented symbolically (a formula and concentration, like NaCl(aq), 1.0 M), particulately (a drawing of the actual particles present), or in words. Each shows the same reality from a different angle.
The particulate view is the one students misread. A dissolved ionic compound is drawn as its separated ions — Na⁺ and Cl⁻ dispersed among water molecules — not as intact NaCl units, because it dissociates in water.
A faithful representation keeps the numbers consistent: the particle drawing must show the right ions in the right ratio and a particle count that reflects the stated concentration.
§2
Translating between representations.
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Move from symbols to particles by asking what is actually in the beaker.
- Read the symbolic form. Identify the solute, its formula, and the concentration.
- Decide what dissociates. Soluble ionic compounds separate into ions; molecular solutes (like sugar) stay as whole molecules.
- Draw the right particles in the right ratio. For NaCl, show equal numbers of Na⁺ and Cl⁻; for CaCl₂, twice as many Cl⁻ as Ca²⁺.
- Match count to concentration. A more concentrated solution shows more solute particles per volume of water.
§3
The pieces you'll meet.
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Keep the representations consistent with one another.
§4
Worked example: draw 1.0 M CaCl₂(aq).
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Question. How should a particulate drawing of calcium chloride solution look?
Dissociation. CaCl₂ is a soluble ionic compound, so it separates into ions: one Ca²⁺ and two Cl⁻ per formula unit.
Ratio. The drawing must show twice as many Cl⁻ ions as Ca²⁺ ions, all dispersed among water molecules — never intact 'CaCl₂ units.'
Concentration. A 2.0 M drawing would show twice as many of these ions per unit volume as a 1.0 M drawing. The particle picture, the formula, and the concentration all have to agree.
§5
Mistakes that cost real points.
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"Dissolved NaCl is drawn as intact NaCl units floating in water."
When an ionic compound dissolves, it dissociates into separate ions. A correct particulate drawing of NaCl(aq) shows individual Na⁺ and Cl⁻ ions dispersed among water molecules, not joined NaCl pairs.
Fix. Draw soluble ionic solutes as their separated ions, in the formula's ratio, surrounded by water.
"Every solute breaks into ions when it dissolves."
Only ionic (and some acidic) solutes dissociate into ions. Molecular solutes like sugar or ethanol dissolve as whole, intact molecules. Drawing sugar as ions is as wrong as drawing NaCl as intact pairs.
Fix. Ask whether the solute is ionic or molecular. Ionic → ions; molecular → whole molecules in solution.
"The number of particles in the drawing does not have to match the concentration."
A faithful particulate representation shows more solute particles per volume for a more concentrated solution. Mismatching the count to the stated molarity misrepresents the solution.
Fix. Scale the particle count with concentration: a 2 M drawing shows twice the solute particles per volume as a 1 M drawing.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.