Point 8 of 8 — The Molecular Structure of Water, Its Unique Properties and Water SustainabilityQuestion bank: 0 questions (0 with pictures)
08Worked examples
How much heat raises 2 kg of water from 20 °C to 100 °C? — Q = 2 × 4184 × 80 =669,440 J ≈ 669 kJ.
How much steam is needed to cool a 70 kg athlete by 2 °C by sweating alone (assume no other loss)? — Q = 70 × 3500 × 2 ≈ 490 kJ (body tissue c ≈ 3500 J/kg·°C), mass of water = 490,000 / 2,400,000 ≈0.2 kg = 200 mL.
A water sample gives an effervescence with dilute HCl and a white precipitate on boiling. Identify the hardness. — Temporary hardnessdue to calcium bicarbonate.
A capillary of radius 0.2 mm is dipped in water (γ = 0.073 N/m, θ = 0°). Find the rise. — h = 2γ/(ρgr) = 2(0.073)/(1000 × 9.8 × 2×10⁻⁴) ≈7.4 cm.
A family uses 0.25 m³ of water a day. Its annual volume, and how it compares with the water footprint of 1 kg of beef (about 15,000 L)? — 0.25 × 365 = 91 m³/yr, so one kilogram of beef costs about 5 monthsof the family's domestic water.
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