Solutions and Concentration
Identify true solutions, distinguish rate from solubility, calculate concentration, and predict dilution, evaporation, and crystallization.
Solutions and Concentration
Not yet verified on this browser.
A solution is homogeneous; concentration tells how much solute is present relative to the mixture
A solution contains solute particles uniformly dispersed in a solvent. The label (aq) means a substance is dissolved in water; it does not mean the pure substance is a liquid.
An unsaturated solution can dissolve more solute. A saturated solution has reached its equilibrium solubility under the stated conditions. Undissolved excess does not increase the dissolved concentration.
Rate and solubility are not the same
Stirring and crushing commonly increase how fast a solid dissolves. They do not necessarily change the maximum amount that can dissolve at equilibrium.
Track solute amount during dilution or evaporation
Adding solvent lowers concentration if solute is unchanged. Removing solvent raises concentration until saturation may cause crystallization.
Read the phase label before using chemical intuition
(aq): dissolved in water—a homogeneous aqueous solution.
(s): solid.
(l): pure liquid phase.
(g): gas.
For dilution, the number of moles of solute stays constant, so use M₁V₁ = M₂V₂.
Why it works
The phase label resolves official-style identification questions immediately and prevents a pure liquid or solid from being mistaken for an aqueous solution.
Five forms you should recognize
Problem: Which sample is a substance dissolved in water: KOH(aq), NaCl(s), HCl(g), or H₂O(l)?
Recognize: The phase label tells the physical condition. Only (aq) explicitly means “dissolved in water.”
Answer: KOH(aq) is the aqueous solution. NaCl(s) is a solid, HCl(g) is a gas, and H₂O(l) is pure liquid water—not a solute dissolved in water.
Problem: A student dissolves 10 g of salt in 90 g of water. What percent by mass of the final solution is salt?
Recognize: The denominator is the total solution mass, not the water mass alone.
mass solution = 10 g + 90 g = 100 gmass percent = 10/100 × 100% = 10%Answer: The solution is 10% salt by mass.
Problem: A laboratory solution contains 0.50 mol of glucose in a final solution volume of 2.0 L. What is its molarity?
Recognize: Molarity means moles of solute per liter of solution.
M = 0.50 mol / 2.0 L = 0.25 MAnswer: The glucose concentration is 0.25 M.
Problem: A student needs 100 mL of 0.50 M solution using a 2.0 M stock solution. What volume of stock should be measured before adding water to a final volume of 100 mL?
Recognize: Dilution changes volume and concentration but not the moles of solute taken from the stock.
M₁V₁ = M₂V₂2.0(V₁) = 0.50(100 mL)V₁ = 25 mLAnswer: Measure 25 mL of stock, then add water until the total volume reaches 100 mL.
Problem: At 80°C, 50 g of a salt can dissolve in 100 g of water. At 20°C, only 30 g can remain dissolved. A saturated solution containing 50 g of dissolved salt is cooled from 80°C to 20°C. What happens?
Recognize: Cooling lowers this salt’s solubility, so the cooler water cannot keep all 50 g dissolved.
mass crystallized = 50 g − 30 g = 20 gAnswer: About 20 g crystallizes, while 30 g remains dissolved.
Check before you commit
- Treating every liquid sample as a solution
- Reading (l) as dissolved in water
- Using solvent volume instead of final solution volume for molarity
- Saying stirring always increases final solubility
- Forgetting to convert milliliters to liters
- Assuming gas solubility increases with temperature
Do you need the lesson-or just practice?
One original question in each form recommends your next step. It does not yet verify mastery.
Work at the level you need.
Foundations
Build the core procedure with immediate explanations.
Core Practice
Use mixed forms with less scaffolding.
UPCAT-Style Transfer
Apply the competency in unfamiliar representations.
Ready to verify this competency?
A score of 5/5 verifies mastery. An unsuccessful attempt loads a different five-form bank.
Solutions and Concentration FAQ
Can a solution contain a gas or solid solvent?
Yes. Solutions may occur in different phases; air and metal alloys are common examples.
Does a saturated solution always have visible solid?
Not necessarily. It is at the solubility limit; visible solid appears when excess solute is present.
Why does opening a soft drink cause fizzing?
Lower pressure above the liquid reduces carbon-dioxide solubility, so dissolved gas escapes.
Continue your science review.
Your progress stays on this browser.
Mastery results save to your Teacher Abi study profile.
Return to Student Hub View UPCAT Coverage
No comments:
Post a Comment