The problems are on the atmospheric pressure worksheet. Try them on your own first, then compare with the answers below.
Quick answer: Air is a mixture of gases, mainly 78 % nitrogen and 21 % oxygen, and it has mass: about 1.2 kg per cubic metre. These worked answers correct the 20 problems of the grade 7 science worksheet on atmospheric pressure, from weighing a volleyball to the suction cup: each solution shows the formula, the substitution and the final answer with its unit.
Each answer shows the formula, the substitution and the result with its unit in bold. Check the unit conversions first, because mixing up Pa, kPa and hPa is the most common cause of a wrong pressure answer.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-ESS2-5
Problem 2 Gases in air
1. Nitrogen.2. About 21 %.
3. Carbon dioxide.
Problem 3 Oxygen in a room
1. 0.21 × 500 = 105 L.2. 0.78 × 500 = 390 L.
Problem 4 Pressure units
1. 2.5 × 1000 = 2500 Pa.2. 101 300 ÷ 1000 = 101.3 kPa.
3. 1013 × 100 = 101 300 Pa.
Problem 5 Pressure on a table
1. p = F ÷ A.2. p = 200 ÷ 2.0 = 100 Pa.
Problem 6 Reading a barometer
1. The column is 740 mm high.2. It is 20 mm below 760 mm, so the pressure is lower than standard (low pressure often brings rain).
3. The space above holds no air (a vacuum).
Problem 7 Air in a classroom
1. V = 9.0 × 6.0 × 3.0 = 162 m³.2. m = 1.2 × 162 = 194.4, so m ≈ 194 kg.
Problem 8 The air on your hand
1. F = p × A = 101 000 × 0.015 = 1515, so F ≈ 1500 N.2. Air also pushes on the back of her hand, and the fluids inside the hand push outwards with the same pressure, so the forces balance.
Problem 9 Weather balloon graph
1. 62 kPa.2. 100 – 79 = 21 kPa.
3. 50 kPa lies between 62 kPa at 4 km and 47 kPa at 6 km, closer to 6 km: about 5.5 km.
Problem 10 Crisp packets on a flight
1. The outside pressure decreases.2. The pressure inside the packet stays higher than the pressure outside, so the air inside pushes the packet outwards and it expands.
Problem 11 Drinking through a straw
1. Sucking removes some of the air, so the pressure inside the straw falls below atmospheric pressure.2. The atmosphere pushes on the juice in the glass and forces the juice up the straw into the lower-pressure region.
Problem 12 Millimetres to kilopascals
1. 101.3 ÷ 760 = 0.1333 kPa per mm.2. 745 × 0.1333 = 99.3, 99.3 kPa.
3. 99.3 × 10 = 993 hPa.
Problem 13 Totals for air
1. 78.08 + 20.95 + 0.93 + 0.04 = 100.00 %.2. 0.2095 × 5.0 = 1.0475, so ≈ 1.05 m³.
Problem 14 Spot the mistakes
1. Air is matter. It has mass: a pumped ball is heavier than a flat one, and 1 m³ of air has a mass of about 1.2 kg.2. Air pressure acts in all directions, because air particles collide with surfaces from every side.
Problem 15 High pressure day
1. 1030 – 1013 = 17 hPa above standard.2. 17 × 100 = 1700 Pa.
3. High pressure usually goes with settled, dry weather.
Problem 16 Falling pressure
1. 1015 – 995 = 20 hPa.2. 20 × 100 = 2000 Pa.
3. Falling pressure suggests cloud and rain are likely.
Problem 17 A window in a storm
1. A = 1.2 × 0.80 = 0.96 m².2. F = 101 000 × 0.96 = 96 960 N outwards.
3. Outside force = 90 000 × 0.96 = 86 400 N inwards. Net = 96 960 – 86 400 = 10 560 N outwards.
Problem 18 Breathing on a summit
1. m = 1.2 × 0.50 = 0.60 g.2. 50 ÷ 101 = 0.50 (to 2 decimal places).
3. The density is about half, so m ≈ 0.60 × 0.50 = 0.30 g.
4. Each breath holds only about half as many oxygen particles, so the body receives less oxygen.
Problem 19 A water barometer
1. h = 0.760 × 13.6 = 10.336, so ≈ 10.3 m.2. A water barometer would need a tube more than 10 m tall. Mercury is much denser, so a tube less than 1 m tall is enough.
Problem 20 The suction cup
1. F = p × A = 101 000 × 0.0020 = 202 N.2. m = F ÷ g = 202 ÷ 9.8 = 20.6, ≈ 21 kg.
3. Air leaks in under the edge, so the vacuum is not perfect, and the surface is never perfectly smooth.
