Concentration of Solutions

Solution
- A homogeneous mixture of two (or more) substances.
Components of a Solution
- Solute – present in the smaller amount; the substance being dissolved.
- Ex.: sugar, coffee powder, acetic acid, isopropyl alcohol, gold in jewelry.
- Solvent – present in the larger amount; the dissolving medium.
- Ex.: water (for sugar, coffee, vinegar), isopropyl alcohol’s water component, copper/silver mixture for gold alloys.
Solubility
- Ability of a solute to dissolve in a given solvent at a specified temperature and pressure.
- Determines whether a solution becomes unsaturated (more can dissolve) or saturated (maximum amount already dissolved).

Unsaturated
- Less solute than the solvent can dissolve at that temperature.
- Example: Slightly sweet coffee (little sugar) still able to dissolve more.
Saturated
- Maximum solute for that solvent at given conditions.
- Example: Sugar starts settling at the bottom of iced tea—no more dissolves.

Qualitative Terms
- Dilute – comparatively small amount of solute.
- Concentrated – comparatively large amount of solute.
- Limitation: purely relative; no numerical precision.
Quantitative (Accurate) Terms
- Use percent concentration so everyone interprets the same value.

The word “percent” literally means “parts per 100.”
Two most common laboratory & consumer measures:
- Percent by Mass (% w/w)
- Percent by Volume (% v/v)

Definition: Ratio of the mass of solute to the total mass of the solution.
Mathematical expression:
\%\,\text{by mass}=\frac{\text{mass of solute}}{\text{mass of solution}}\times100
Conceptual model (100-part diagram):
- 2 mass units solute + 98 mass units solvent = 100 mass units solution ⇒ 2 % w/w
Interpretation: In any 100 g of solution, 2 g are solute and 98 g are solvent.

Definition: Ratio of the volume of solute to the total volume of the solution.
Mathematical expression:
\%\,\text{by volume}=\frac{\text{volume of solute}}{\text{volume of solution}}\times100
Conceptual model (100-part diagram):
- 2 volume units solute + 98 volume units solvent = 100 volume units solution ⇒ 2 % v/v
Interpretation: In any 100 mL of solution, 2 mL are solute and 98 mL are solvent.

Coffee Example
- Too much coffee powder & sugar → darker color, stronger aroma, sweeter taste.
- Intensity of sensory properties directly linked to concentration.
Vinegar (Acetic Acid Solution)
- Label: “5 % acidity.”
- Means 5 g acetic acid + 95 g water = 100 g vinegar.
- Expression: \frac{5\,\text{g}}{100\,\text{g}}\times100=5\% w/w.
70 % Isopropyl Alcohol (Disinfectant)
- Contains 70 mL isopropyl alcohol + 30 mL water = 100 mL solution.
- Expression: \frac{70\,\text{mL}}{100\,\text{mL}}\times100=70\% v/v.
Gold Jewelry & the Carat System (Solid Solutions)
- Pure gold = 24 carats (24 k).
- 18 k gold: 18 parts gold + 6 parts other metals (usually Cu/Ag) out of 24 parts total.
- Percent gold:
  \frac{18\,\text{g}}{24\,\text{g}}\times100=75\% gold by mass.
- Provides a familiar example of percentage in solid-solution alloy form.

Builds on Chapter/Lesson about Solutions & Solubility from Science 7.
Emphasizes that percent concentration allows cross-comparison between products, experiments, & recipes.
Essential for:
- Consumer safety (knowing alcohol strength, acidity levels).
- Laboratory accuracy (preparing reagents with exact properties).
- Industrial quality control (pharmaceuticals, food science, metallurgy).

Accurate labeling protects consumers against misuse (e.g., using too-strong alcohol on skin can cause irritation).
Understanding concentration helps avoid waste (too much solute = costly, may harm environment).
In jewelry, knowing true gold content prevents fraud & ensures fair pricing.

Percent by Mass: \%\,w/w = \dfrac{m{solute}}{m{solution}}\times100
Percent by Volume: \%\,v/v = \dfrac{V{solute}}{V{solution}}\times100
Carat–Percent Conversion for Gold: \%\,\text{Au} = \dfrac{\text{carat value}}{24}\times100

“Keep educating yourself because that’s the key to success.” – Teacher Angelica