Chemistry: Atoms First 2e · Solutions and Colloids

The Dissolution Process

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

When you stir sugar into tea, the sugar seems to disappear — but it has not vanished; it has dispersed into individual molecules spread evenly through the water. That process is : the mixing of a into a to form a , a homogeneous mixture with uniform composition. Dissolution is not a chemical reaction — the solute's identity is unchanged, and the process is reversible. This topic covers what happens at the particle level when a solid dissolves: the three energy steps, why some dissolutions feel hot or cold, why "like dissolves like," and how saturation limits how much solute can dissolve.

Why this matters

Medicine: Tablets, powders, and syrups must dissolve before their active ingredients enter the bloodstream; dissolution rate is a major factor in how fast a drug acts.
Cold packs and heat packs: Instant cold packs rely on the endothermic dissolution of ammonium nitrate, and some heat packs on exothermic dissolution — the enthalpy of solution is the whole mechanism.
Industry: Desalination, wastewater treatment, and manufacturing all depend on controlling what dissolves in what.
Biology: Nutrients, gases, and waste products move through the body dissolved in water; cell function depends on the concentrations of dissolved species.
Exams: Predicting whether a substance dissolves (like dissolves like), identifying saturated vs. unsaturated solutions, and molarity calculations all trace back to this topic.

The college version

Core Concepts

Solutions are homogeneous mixtures

A solution is a homogeneous mixture: uniform composition throughout, with the solute dispersed at the molecular or ionic level. Solutions can be solid (brass — copper dissolved in zinc), liquid (salt water), or gaseous (air — oxygen and other gases dissolved in nitrogen). The solvent is the component present in the largest amount, or the one that keeps its phase; the solute is everything else.

The three steps of dissolution

Dissolving an ionic solid in water involves three conceptual steps:

Separate solute particles from each other (break ionic or intermolecular attractions) — requires energy, so it is endothermic, ΔH₁ > 0.
Separate solvent molecules to make room (break hydrogen bonds between water molecules) — also endothermic, ΔH₂ > 0.
Mix solute particles with solvent molecules (form new ion–dipole or dipole–dipole attractions) — releases energy, ΔH₃ < 0.

The overall enthalpy change is the sum:

$Δ H soln = Δ H 1 + Δ H 2 + Δ H 3$

If step 3 releases more energy than steps 1 and 2 consume, dissolution is exothermic — the solution warms up (NaOH or CaCl₂ in water). If steps 1 and 2 dominate, dissolution is endothermic — the solution cools (NH₄NO₃, the basis of instant cold packs).

Why "like dissolves like"

Polar solvents (like water) dissolve polar solutes and ionic compounds, because the new solute–solvent attractions in step 3 are strong enough to compensate for separating the solute. Nonpolar solvents (like hexane, C₆H₁₄) dissolve nonpolar solutes (like wax or cooking oil), because dispersion forces match. A nonpolar solute in water fails step 3: the weak solute–water attractions cannot repay the energy cost of breaking water's hydrogen-bond network, so the solute stays undissolved. This rule of thumb — "like dissolves like" — predicts solubility with no calculations.

Saturation: how much can actually dissolve

A holds the maximum amount of solute that can dissolve at a given temperature; extra solute simply settles at the bottom. An unsaturated solution can still dissolve more. A — usually made by cooling a saturated solution very slowly — holds more solute than the equilibrium limit; it is unstable and crystallizes dramatically if disturbed (the classic sodium acetate "hot ice" demonstration).

Dissolution is driven by more than energy

Some endothermic dissolutions still happen — the driving force is entropy: a solid is highly ordered, and dispersing its particles through the solvent creates a much more disordered mixture. The disorder gain can outweigh an unfavorable enthalpy, which is why ammonium nitrate dissolves readily even though it cools the water.

Common Confusions

Do Not Confuse	With	Difference
Dissolution	Chemical reaction	Dissolution is physical and reversible; the solute's identity is unchanged
Exothermic dissolution	Endothermic dissolution	Exothermic releases heat and warms the solution (NaOH, CaCl₂); endothermic absorbs heat and cools it (NH₄NO₃ cold packs)
Saturated	Supersaturated	Saturated is at the equilibrium limit (stable); supersaturated is beyond the limit (unstable, crystallizes on disturbance)
Saturated	Concentrated	"Saturated" is a specific equilibrium state; "concentrated" is a vague description — a saturated solution of a sparingly soluble salt can be quite dilute
Solvent	Solute	Solvent is the majority/phase-keeping component (water in salt water); solute is what dissolves
Dissolves	Disappears	The solute is still present, distributed as molecules or ions; you can recover it by evaporating the solvent
Volume of water added	Volume of solution	Molarity uses total solution volume, not the water you started with

Eli explains

The same idea, in plain words

Explain it like I’m 10

Dissolving is like a dance where the sugar molecules let go of each other, the water molecules make space, and then the sugar and water hold hands in a new pairing. Sometimes letting go takes more energy than the new hand-holding gives back — then the water feels cold, like a cold pack. If the new pairing gives back more energy than it cost, the water warms up. And a solution can only hold so much sugar; once it is full (saturated), the extra sugar just sits at the bottom.

Worked example

Example 1: Molarity of a solution from mass and volume

A student dissolves 2.00 g of NaOH (molar mass 40.00 g/mol) in water and dilutes to 250.0 mL. What is the molarity?

Step 1 — Convert mass to moles (formula before substitution):

$n = m M = 2.00 g 40.00 g mol -1 = 0.0500 mol$

Step 2 — Convert volume to liters:

$V = 250.0 mL \times 1 L 1000 mL = 0.2500 L$

Step 3 — Molarity:

$M = n V = 0.0500 mol 0.2500 L = 0.200 mol L -1 = 0.200 M$

Dimensional check: g cancels in step 1, mL cancels in step 2, leaving mol/L. Note that molarity uses liters of solution, not liters of water added.

Example 2: Preparing a solution by mass

How many grams of glucose (C₆H₁₂O₆, molar mass 180.16 g/mol) are needed to make 500.0 mL of 0.100 M solution?

Step 1 — Moles of solute needed (formula before substitution):

$n = M \times V = 0.100 mol L \times 0.5000 L = 0.0500 mol$

Step 2 — Convert to mass:

$m = n \times M = 0.0500 mol \times 180.16 g mol -1 = 9.01 g$

Dimensional check: L cancels in step 1, mol cancels in step 2, leaving grams. In the lab you would dissolve this 9.01 g in a portion of water, then add water to the 500 mL mark — not simply add 500 mL of water.

Example 3: Predicting solubility with "like dissolves like"

Predict whether each pair will form a solution: (a) table salt (NaCl) in water; (b) cooking oil (nonpolar triglycerides) in water; (c) iodine (I₂, nonpolar) in hexane (C₆H₁₄, nonpolar).

(a) Yes. NaCl is ionic; water is polar. The ion–dipole attractions formed in step 3 (hydration) repay the energy needed to break the crystal lattice and water's hydrogen bonds.
(b) No. Oil is nonpolar; water is polar. Weak dispersion interactions between oil and water cannot repay the cost of breaking water's hydrogen-bond network, so the oil separates into a distinct layer.
(c) Yes. Both are nonpolar; dispersion forces match well, and the entropy gain of mixing makes dissolution favorable.

That is the chemistry behind salad dressing separating (b) and the reason nonpolar grease stains need nonpolar solvents, not water.

Key takeaways

A solution is a homogeneous mixture; the solvent is the majority component, the solute is what dissolves in it.
Dissolution has three steps: separate solute (ΔH₁ > 0), separate solvent (ΔH₂ > 0), mix (ΔH₃ < 0); ΔH_soln = ΔH₁ + ΔH₂ + ΔH₃.
Exothermic dissolution (NaOH, CaCl₂) warms the solution; endothermic dissolution (NH₄NO₃) cools it — cold packs exploit this.
"Like dissolves like": polar/ionic solutes dissolve in polar solvents; nonpolar solutes dissolve in nonpolar solvents.
Saturated = at the solubility limit; unsaturated = can dissolve more; supersaturated = temporarily holds more, crystallizes on disturbance.
Entropy (disorder) can drive endothermic dissolution — energy alone does not decide.
Dissolution is physical, not chemical: the solute's identity is preserved and the process is reversible.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

List the three energy steps of dissolution and state the sign of ΔH for each.
Show answer
(1) Separate solute particles — endothermic, ΔH₁ > 0; (2) separate solvent molecules — endothermic, ΔH₂ > 0; (3) mix solute with solvent — exothermic, ΔH₃ < 0.
Write the expression for ΔH_soln and explain what a positive vs. negative value means physically.
Show answer
ΔH_soln = ΔH₁ + ΔH₂ + ΔH₃. Negative = exothermic (solution warms); positive = endothermic (solution cools).
Why does an endothermic dissolution like NH₄NO₃ in water still occur spontaneously?
Show answer
The large increase in entropy (disorder) when the ordered crystal disperses through the water overcomes the unfavorable enthalpy — spontaneity depends on both energy and disorder.
A solution is prepared by dissolving 5.85 g of NaCl (molar mass 58.44 g/mol) in water to make 500.0 mL. What is its molarity?
Show answer
n = 5.85/58.44 = 0.100 mol; M = 0.100/0.5000 = 0.200 M.
How many grams of sucrose (C₁₂H₂₂O₁₁, molar mass 342.30 g/mol) are needed for 250.0 mL of 0.200 M solution?
Show answer
n = 0.200 × 0.2500 = 0.0500 mol; m = 0.0500 × 342.30 = 17.1 g.
Explain using "like dissolves like" why gasoline (nonpolar) and water do not mix, but gasoline and motor oil do.
Show answer
Water is polar and gasoline is nonpolar; the weak water–gasoline attractions cannot repay breaking water's hydrogen bonds. Gasoline and motor oil are both nonpolar, so their dispersion forces match and they mix.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Solution: A homogeneous mixture of two or more substances
Solute: The substance that gets dissolved
Solvent: The substance that does the dissolving (usually the majority)
Dissolution: The process of forming a solution
Enthalpy of solution Δ Hsoln: Net heat change when solute dissolves
Hydration: Surrounding of solute particles by water molecules
Saturated solution: Holds the maximum solute at a given temperature
Supersaturated solution: Holds more solute than equilibrium, unstably

Sources & references

openstax.org — Chemistry Atoms First 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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