Week 15 · Term 1 review (Units 1–2) and Darslab's own review tasks · 3 lessons
1
Review: water, solutions and cells
Integrated Science Sec 1 Term 1, Units 1–2: Review
GoalUse the ideas of Unit 1, first half, in mixed tasks: water, density, heat, solutions, osmosis, cells and enzymes.
New words
polar molecule · a molecule with a slightly positive end and a slightly negative end, like waterosmosis · movement of water through a membrane toward the more concentrated solutionenzyme · a protein that speeds up a reaction in a living thing
Explanation
This is a review: no new facts. Water is a polar molecule and its molecules join by hydrogen bonds, which is why it dissolves ionic and polar substances. About 97% of Earth’s water is salt water. A salt that comes from a strong acid and a weak base gives an acidic solution, from a weak acid and a strong base gives a basic one, and from a strong acid and a strong base (such as NaCl) a neutral one. Density = mass ÷ volume; an object floats in water if its density is less than 1 g/cm³ (fresh water is densest at 4 °C). Heat gained or lost: Q = m × c × Δt, with c of water about 4200 J/(kg·°C). Kelvin = Celsius + 273. A dissolved substance raises the boiling point and lowers the freezing point. In osmosis water moves toward the more concentrated solution, so a cell in a stronger solution loses water and shrinks. Enzymes work best at an optimum temperature and are damaged (denatured) when it is too hot.
Topic
Key rule
density
d = m ÷ V
heat
Q = m × c × Δt
kelvin
K = °C + 273
osmosis
water goes toward the stronger solution
Worked examples
A solid has mass 700 g and volume 500 cm³. Its density is 700 ÷ 500 = 1.4 g/cm³. This is more than 1, so it sinks in fresh water.
How much heat warms 0.5 kg of water from 20 °C to 30 °C? Δt = 10 °C, so Q = 0.5 × 4200 × 10 = 21 000 J. A 0.2 kg copper piece (c = 390) warmed by 25 °C needs only 0.2 × 390 × 25 = 1950 J.
A red blood cell is put in a salt solution stronger than its inside. Water moves toward the stronger solution, so water leaves the cell and the cell shrinks. In pure water the opposite happens: water enters and the cell swells.
Which salt is basic? Sodium carbonate comes from a strong base and a weak acid, so it gives a basic solution. NaCl is neutral, and NH₄Cl is acidic.
Class activity
Review cards, in pairs. Write 12 cards, one for each of the ideas above (polar molecule, hydrogen bond, density, floating, Q = mcΔt, kelvin, hydrolysis, pH, boiling point, osmosis, enzyme, adaptation) with a question on one side and the answer on the other. Take turns to draw a card and answer without looking. Put a tick for what you know and a star for what to revise.
Practice
1
Why does a polar molecule like water dissolve salt?
Its charged ends attract the ions and pull them apart.
2
A block has mass 330 g and volume 300 cm³. Find its density. Does it float?
330 ÷ 300 = 1.1 g/cm³, more than 1, so it sinks.
3
How much heat raises 2 kg of water by 5 °C?
Q = 2 × 4200 × 5 = 42 000 J.
4
Convert 310 K to °C.
310 − 273 = 37 °C.
5
Why does a fever above a safe level harm enzymes?
Too much heat changes the shape of the enzyme, so it stops working (denatures).
