Unit 3 · Topic 3.8 Beta

Representations of Solutions

When an ionic compound dissolves, ion-dipole attractions pull it apart into separate ions, and each is hydrated: water's oxygen faces cations and its hydrogens face anions.

Practice 1: Models and RepresentationsPractice 3: Representing Data and Phenomena

Question set for this topic

Part 1 · Hook

Why this matters

Stir table salt into water and it seems to vanish. It is still there, split into ions, each wrapped in a shell of water molecules all turned the same way. Drawing that shell the wrong way round is one of the most common errors on the exam.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which end of a water molecule carries a partial negative charge?

  1. The oxygen end
  2. The hydrogen end
  3. Neither; water is nonpolar
Show the answer

Oxygen is more electronegative and pulls the shared electrons toward itself.

  • Correct: The oxygen end:
  • The hydrogen end:
  • Neither; water is nonpolar:

2. What is an ion-dipole force?

  1. An ion attracting the oppositely charged end of a polar molecule
  2. A covalent bond between an ion and water
  3. The attraction between two ions
Show the answer

An ion attracts the opposite partial charge of a polar molecule.

  • Correct: An ion attracting the oppositely charged end of a polar molecule:
  • A covalent bond between an ion and water:
  • The attraction between two ions:

3. What is the molarity of Cl⁻ in 0.10 M CaCl₂?

  1. 0.20 M
  2. 0.10 M
  3. 0.30 M
Show the answer

Each CaCl₂ gives two Cl⁻ ions.

  • Correct: 0.20 M:
  • 0.10 M:
  • 0.30 M:

Part 4 · See it

See it first

A sodium ion ringed by six water molecules with their oxygen atoms pointing toward it, and a chloride ion ringed by six water molecules with a hydrogen atom of each pointing toward it.
Hydrated ions: water's oxygen faces a cation; a hydrogen faces an anion. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Water molecules are polar, with δ− O and δ+ Hthey attract the ions at the surface of an ionic crystal
  2. Ion-dipole attractions pull ions out of the latticethe solid dissociates into separate ions
  3. Opposite charges attractwater points O toward cations and H toward anions, forming a hydration shell
  4. Each formula unit gives a fixed set of ionsa correct diagram keeps the formula ratio and a total charge of zero

Part 6 · Key ideas

Key ideas

  • Dissolved ionic compounds are separate, hydrated ions; molecular solutes stay as whole molecules.
  • Water orientation: O (δ−) toward cations, H (δ+) toward anions.
  • Keep the formula ratio (CaCl₂: 1 Ca²⁺ to 2 Cl⁻) and keep polyatomic ions whole.
  • Spread solute particles evenly: a solution is homogeneous.

Part 7 · Misconception

A common mistake

The wrong idea: Water always points its oxygen toward a dissolved ion, because oxygen is the part of water that attracts things.

What actually happens: Water turns its δ− oxygen toward a cation but its δ+ hydrogens toward an anion. Like charges facing each other would repel.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Particle view

Four drawings of sodium chloride in water

+−W+−X+−Y++−Z

Key: blue +, Na⁺; green −, Cl⁻; orange circle, oxygen; white circle, hydrogen.

1. Which box correctly represents NaCl dissolved in water?

  1. W
  2. X
  3. Y
  4. Z
Show the answer

W shows separate ions in a 1 : 1 ratio, with water's δ− oxygen toward Na⁺ and a δ+ hydrogen toward Cl⁻.

  • Correct: W: Right: separate ions, 1 : 1, correct orientation.
  • X: X has every water molecule backwards: H toward the cation and O toward the anion, so like charges face each other.
  • Y: Y shows the ions still paired, but dissolving separates them into hydrated ions.
  • Z: Z shows two Na⁺ for one Cl⁻, which has a net charge of +1 and the wrong ratio for NaCl.

2. What is wrong with box X?

  1. Like partial charges face each other: water's δ+ H points at Na⁺ and its δ− O at Cl⁻.
  2. It shows the ions separated, but in water the Na⁺ and Cl⁻ ions stay bonded together.
  3. It has too many water molecules, because each ion attracts one water molecule.
  4. It shows Cl⁻ as larger than Na⁺, but the chloride ion is the smaller of the two.
Show the answer

Ion-dipole attraction lines up opposite charges. X turns every water molecule backwards, which would put repelling charges side by side.

  • Correct: Like partial charges face each other: water's δ+ H points at Na⁺ and its δ− O at Cl⁻.: Right: the orientation is reversed for both ions.
  • It shows the ions separated, but in water the Na⁺ and Cl⁻ ions stay bonded together.: Dissolving does separate the ions; that part of X is correct.
  • It has too many water molecules, because each ion attracts one water molecule.: Each ion is surrounded by several water molecules, so a shell of four is reasonable.
  • It shows Cl⁻ as larger than Na⁺, but the chloride ion is the smaller of the two.: Cl⁻ (181 pm) is larger than Na⁺ (102 pm), as drawn.

3. Why is box Y not a correct picture of the solution?

  1. Dissolving separates the ions, and each ion is hydrated; Y shows an ion pair that has not come apart.
  2. Y has too few water molecules in it to represent water as the solvent for the solution.
  3. Y shows the Na⁺ and Cl⁻ in a 1 : 1 ratio, but the ratio in the solution is 2 : 1.
  4. Y shows covalent bonds inside the water molecules, which break when salt dissolves in water.
Show the answer

Ion-dipole attractions pull the ions out of the lattice and apart. A correct drawing shows each ion separately, with its own water shell.

  • Correct: Dissolving separates the ions, and each ion is hydrated; Y shows an ion pair that has not come apart.: Right: the ions should be separate and hydrated.
  • Y has too few water molecules in it to represent water as the solvent for the solution.: Y shows several water molecules; the number drawn is a sampling choice.
  • Y shows the Na⁺ and Cl⁻ in a 1 : 1 ratio, but the ratio in the solution is 2 : 1.: NaCl gives a 1 : 1 ratio, which Y gets right.
  • Y shows covalent bonds inside the water molecules, which break when salt dissolves in water.: The O–H bonds in water do not break when salt dissolves.

Particle view

A potassium sulfate solution

2−2−2−++++++

Key: blue +, K⁺; yellow center with four orange O, SO₄²⁻ (water molecules not shown).

4. How many solute particles (ions) are shown in the box?

Type a number in ions.

Show the answer

Three SO₄²⁻ ions and six K⁺ ions: 3 + 6 = 9 ions. Each sulfate ion counts as one particle; its atoms stay bonded together.

  • Answer: 9 ions

5. Why does the drawing show twice as many K⁺ ions as SO₄²⁻ ions?

  1. K₂SO₄ gives two K⁺ for each SO₄²⁻, so the charges balance: 2(+1) + (−2) = 0.
  2. K⁺ ions are smaller, so more of them fit into the same volume of the solution.
  3. Half of the sulfate ions break apart into sulfur and oxygen atoms when dissolved.
  4. Potassium ions attract more water, so more of them dissolve than sulfate ions.
Show the answer

The formula K₂SO₄ fixes the ratio: two K⁺ per SO₄²⁻, which makes each formula unit, and the whole box, neutral.

  • Correct: K₂SO₄ gives two K⁺ for each SO₄²⁻, so the charges balance: 2(+1) + (−2) = 0.: Right: the ratio follows the formula and the charges.
  • K⁺ ions are smaller, so more of them fit into the same volume of the solution.: Ion size does not set how many of each ion form.
  • Half of the sulfate ions break apart into sulfur and oxygen atoms when dissolved.: Sulfate stays whole; the S–O bonds are covalent.
  • Potassium ions attract more water, so more of them dissolve than sulfate ions.: Every formula unit gives the same two K⁺ and one SO₄²⁻.

6. Water molecules are added to the drawing around one K⁺ ion and one SO₄²⁻ ion. How should they point?

  1. O toward K⁺; H toward the O atoms of SO₄²⁻.
  2. H toward K⁺; O toward the O atoms of SO₄²⁻.
  3. O toward both ions, because oxygen is the larger atom.
  4. H toward both ions, because hydrogen bonds to both.
Show the answer

Opposite charges face each other: water's δ− oxygen toward the cation, and its δ+ hydrogens toward the negatively charged sulfate ion (whose outer O atoms carry the negative charge).

  • Correct: O toward K⁺; H toward the O atoms of SO₄²⁻.: Right: δ− to the cation, δ+ to the anion.
  • H toward K⁺; O toward the O atoms of SO₄²⁻.: This reverses both: like charges would face each other.
  • O toward both ions, because oxygen is the larger atom.: Atom size does not decide; charge does, and O is δ− so it suits only the cation.
  • H toward both ions, because hydrogen bonds to both.: H is δ+, so it is attracted to the anion but repelled by the cation.

7. A student draws dissolved KBr with each water molecule's oxygen pointing toward BOTH the K⁺ and the Br⁻ ions, "because oxygen is the part of water that bonds." Which response is best?

  1. Water orients by charge: δ− O faces K⁺, but δ+ H must face Br⁻, since O facing Br⁻ would repel.
  2. The drawing is correct, because the oxygen atom forms hydrogen bonds with the ions in solution.
  3. The drawing is correct for K⁺ and Br⁻ alike, because both are ions with a full charge on them.
  4. Water should point H toward both ions, because H is the δ+ end and so it attracts ions.
Show the answer

Ion-dipole attraction is Coulombic. A δ− oxygen is attracted to a cation and repelled by an anion, so around Br⁻ the water molecules turn their δ+ hydrogens inward.

  • Correct: Water orients by charge: δ− O faces K⁺, but δ+ H must face Br⁻, since O facing Br⁻ would repel.: Right: orientation depends on the ion's sign.
  • The drawing is correct, because the oxygen atom forms hydrogen bonds with the ions in solution.: Ion-dipole forces are not hydrogen bonds, and O–to–anion would repel.
  • The drawing is correct for K⁺ and Br⁻ alike, because both are ions with a full charge on them.: A full charge of either sign attracts the opposite end of water, which differs for + and −.
  • Water should point H toward both ions, because H is the δ+ end and so it attracts ions.: δ+ H would be repelled by K⁺.

Part 9 · Summary

Summary

When an ionic compound dissolves, ion-dipole attractions pull it apart into separate ions, and each is hydrated: water's oxygen faces cations and its hydrogens face anions. A correct particle diagram shows separate ions in the formula ratio, whole polyatomic ions, an even spread and zero total charge; molecular solutes stay as whole molecules.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections