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Science answers, explained

Five Physics & Chemistry puzzles from our flyer, solved step by step. Each answer uses a rule from the toolkit. Try each one first, then tap Show answer to watch it solved.

tried them yourself first? good!

01 · Litmus test

EASY

Three beakers hold lemon juice (pH 2), water (pH 7) and soap water (pH 10). Which one turns red litmus blue?

pH 2lemon pH 7water pH 10soap water
ANSWER & EXPLANATION
pH 2 pH 7 pH 10 acid · stays red neutral · no change base · turns blue ✓
  1. Toolkit pH bar: below 7 is acid, 7 is neutral, above 7 is base.
  2. Red litmus does nothing in an acid (lemon, pH 2) or in neutral water (pH 7).
  3. Only a base turns red litmus blue. Soap water has pH 10, so it is the base. Memory trick: Base turns it Blue.
ANSWER Soap water

02 · Read the meter

EASY

A 6 V battery is connected to a 3 Ω resistor and an ammeter. What current does the ammeter show?

A 6 V 3 Ω ?
ANSWER & EXPLANATION
6 V 3 Ω A 2 A V = I × R I = V ÷ R I = 6 ÷ 3 I = 2 A
  1. This is one simple loop, so the same current flows through the resistor and the ammeter.
  2. Ohm's law from the toolkit: V = IR, so I = V ÷ R.
  3. I = 6 V ÷ 3 Ω = 2 A. The ammeter reads 2 amperes.
ANSWER 2 A

03 · Heater bill

EASY

A 2 kW room heater runs for 3 hours. How many units of electricity does it use?

2 kW × 3 hours = ?
ANSWER & EXPLANATION
2 kWh 2 kWh 2 kWh hour 1 hour 2 hour 3 kW × hours 2 × 3 = 6 kWh 6 units
  1. Electrical energy = power × time. Keep power in kW and time in hours.
  2. Each hour the heater uses 2 kWh. For 3 hours: 2 × 3 = 6 kWh.
  3. On your electricity bill, 1 unit = 1 kWh, so the heater uses 6 units. (That is 6 × 3.6 = 21.6 MJ.)
ANSWER 6 units

04 · Balance it

MEDIUM

Iron reacts with steam: __Fe + __H₂O → Fe₃O₄ + __H₂. Fill the blanks so every atom balances on both sides.

FeFe₃O₄
ANSWER & EXPLANATION
3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂ ATOM LEFT RIGHT Fe33✓ H4 × 2 = 84 × 2 = 8✓ O44✓
  1. Start with the biggest formula, Fe₃O₄. It has 3 Fe, so put 3 in front of Fe.
  2. Fe₃O₄ also has 4 O. Each H₂O gives one O, so we need 4 H₂O.
  3. 4 H₂O brings 4 × 2 = 8 H atoms. Each H₂ holds 2, so we need 4 H₂.
  4. Count again: Fe 3 = 3, H 8 = 8, O 4 = 4. Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
ANSWER 3, 4, 4

05 · Find the image

HARD

An object stands 30 cm from a convex lens of focal length 15 cm. Where is the image, and how big is it?

FF 2F2F 30 cm f = 15 cm?
ANSWER & EXPLANATION
F F 2F 2F object image u = 30 cm v = 30 cm real · inverted · same size
  1. Ray 1 travels parallel to the axis, then bends through the focus F on the other side.
  2. Ray 2 goes through the centre of the lens without bending. Where the rays meet, the image forms.
  3. Check with the lens formula: u = −30 cm, f = +15 cm. 1/v = 1/f + 1/u = 1/15 − 1/30 = 1/30, so v = +30 cm.
  4. Magnification m = v/u = 30/(−30) = −1: the image is real, inverted and the same size. Shortcut: object at 2F ⇒ image at 2F.
ANSWER 30 cm behind the lens
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