Deep Drift

Deep Drift: density and convection currents for Cambridge IGCSE Marine Science

This free game is part of our Cambridge iGCSE Marine Science (0697) course for exams in EXAM YEARS 2027-2029.

Make the water move

In Deep Drift you are a float pilot at the Ocean Lab. Your drift probe has no engine: it only goes where the water takes it. To steer it you change the density of the water around it with a HEAT, COOL, SALT or FRESH probe, and a small fluid simulation shows the water rising, sinking and flowing. The game covers the density part of the Cambridge IGCSE Marine Science (0697) course: density = mass ÷ volume, the two density practicals, convection currents, particle theory and why colder water generally lies beneath warmer water.

• Water that becomes less dense than the water around it rises. Water that becomes denser sinks.
• Coloured specks in the tank show every movement, and warm water shows up orange and cold water blue.
• A particle view shows what the particles are doing under the probe while you hold it in the water.

Temperature, density and particle theory

Heating water gives its particles more energy, so they move faster and spread further apart. The particles do not get bigger and none are lost, so the mass stays the same, but it takes up a bigger volume. Because density = mass ÷ volume, the density falls and the warm water rises. Cooling does the opposite: the particles slow down and move closer together, the same mass fits into a smaller volume, and the denser water sinks.

In the Weigh-In mission you do the practical. Pour exactly 100 cm³ of tap water into a measuring cylinder (read the bottom of the meniscus at eye level), stand it on a balance zeroed with the empty cylinder, and work out the density:

• 5 °C: 100.0 g, so 1.000 g/cm³
• 20 °C: 99.8 g, so 0.998 g/cm³
• 50 °C: 98.8 g, so 0.988 g/cm³

Each sample is then released into a tank of 20 °C water with a drop of dye: the cold sample sinks and spreads along the bottom, the 20 °C sample stays where it is, and the warm sample rises to the top. Colder water is denser, so it lies beneath warmer water. We say generally because fresh water is densest at about 4 °C and ice floats, while sea water keeps getting denser as it cools, down to its freezing point of about −2 °C.

Salinity and density

In the Salt mission you weigh 100 cm³ of water from three seas, all at 20 °C: the Baltic Sea (low salinity, about 7 ppt), the Pacific (typical, about 35 ppt) and the Red Sea (high salinity, about 40 ppt). Dissolved salt adds mass to the water but hardly changes its volume, so the saltier the water, the denser it is: about 1.004, 1.025 and 1.029 g/cm³. The sums use the formula the other way round too: mass = density × volume and volume = mass ÷ density.

Released into a tank of 35 ppt sea water, the Baltic water rises, the Pacific water stays and the Red Sea water sinks. Then you sink the probe using salt alone and raise it again with fresh water.

Convection currents and the ocean conveyor

A convection current forms when water is heated in one place and cooled in another. Warm, less dense water rises; cold, denser water sinks; and the two flows join up along the top and the bottom into a continuous loop. In the game you heat the bottom of the tank on one side and cool the top on the other to keep the loop turning and carry the probe round. Heating from the top or cooling from the bottom makes no current: the layers just stay still.

The final mission is a long model of the Atlantic, with Greenland and its sea ice at one end and the tropics at the other. The sun warms the tropical surface, but that warm, less dense water stays on top. Deep water forms at the pole, where sea water is cooled and becomes saltier as sea ice forms (most of the salt is left behind in the water). The cold, salty, dense water sinks, spreads along the sea floor, rises far away and returns along the surface: the global conveyor belt, or thermohaline circulation. Fresh meltwater makes the polar water less dense and slows the sinking.

The four missions

Rise and Sink: lift the probe with heat, drop it with cold, then build a convection current round four gates. The Weigh-In: measure the density of water at three temperatures and watch where each sample goes. Salt: measure three seas, use the rearranged formula, then a 60-second Ring Rush with all four tools. The Conveyor: keep the model ocean's deep current running and take the probe round all four stations. Every mistake shows a short explanation, and quick checks between missions test the key ideas.