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⚗️ Science · Chemistry · Lesson 5 of 6 · 6 min

Solutions, concentration and dilution

💧 IV fluids, saline rinses and medication doses are all solutions. Knowing how concentration and dilution work is chemistry you will use on the floor.

💡 Key idea

Dilution adds solvent, not solute, so concentration × volume stays the same: C₁V₁ = C₂V₂.

🧠 The idea

A solution is a uniform mixture. The SOLUTE is the substance dissolved (often in smaller amount); the SOLVENT does the dissolving, and water is called the universal solvent because it dissolves so many polar and ionic substances. "Like dissolves like": polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar ones, which is why oil and water separate. Solubility is the most solute that dissolves at a given temperature. An unsaturated solution can dissolve more; a SATURATED solution holds the maximum; a supersaturated one holds more than the maximum and is unstable. Most solids dissolve better in warmer water, while gases dissolve better in colder liquid. CONCENTRATION tells how much solute is in a given amount of solution, such as percent (g per 100 mL) or molarity (moles per liter). When you dilute, the amount of solute stays the same, so C₁V₁ = C₂V₂. OSMOLARITY counts dissolved particles; water moves toward the side with more particles.

✏️ Worked example

How much 20% stock solution makes 100 mL of 5% solution? Step 1: write C₁V₁ = C₂V₂. Step 2: 20% × V₁ = 5% × 100 mL. Step 3: V₁ = 500 ÷ 20 = 25 mL. Step 4: measure 25 mL of stock and add water to a total of 100 mL (75 mL of water).

📌 Takeaways

  • Solute dissolves; solvent does the dissolving; like dissolves like.
  • Unsaturated can take more solute; saturated is full.
  • Dilution: C₁V₁ = C₂V₂, and the solute amount does not change.

📖 Terms in this lesson

Solute and solvent: The solute is the substance dissolved, and the solvent is what it dissolves in; for example, in salt water, salt is the solute and water is the solvent.

✅ Check yourself

Question 1 of 3

In salt water, the salt is the

  1. precipitate
  2. solvent
  3. suspension
  4. solute
Show the answer and rationale

Answer: D

Salt is the substance being dissolved, so it is the solute; water is the solvent. A precipitate is a solid that comes out of solution, and a suspension is a cloudy mixture whose particles settle, not a true solution.

Question 2 of 3

A nurse dilutes 50 mL of a 10% solution to a total volume of 250 mL. What is the new concentration?

  1. 25%
  2. 0.5%
  3. 5%
  4. 2%
Show the answer and rationale

Answer: D

C₁V₁ = C₂V₂: 10% × 50 mL = C₂ × 250 mL, so C₂ = 500 ÷ 250 = 2%. Five percent would require a total of only 100 mL, and 25% is higher than the starting stock, which a dilution can never produce.

Question 3 of 3

More sugar is stirred into a glass of tea, and the extra crystals sit undissolved at the bottom. The tea is

  1. supersaturated
  2. dilute
  3. unsaturated
  4. saturated
Show the answer and rationale

Answer: D

Undissolved solute in contact with the solution means it holds as much as it can at that temperature: saturated. An unsaturated solution would dissolve the extra sugar, and a supersaturated one holds more than the maximum with no crystals until disturbed.

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