ACT Science Ocean Properties: Understand Salinity, Density, and Current Formation

Salinity: Salt concentration in seawater (typically 35 parts per thousand). Salinity increases with evaporation (water leaves, salt stays). Salinity decreases with freshwater input (rain, rivers dilute). Density: Mass per unit volume. Denser water sinks; less dense water rises. Cold water is denser than warm. Salty water is denser than fresh. Differences in density drive ocean currents. Cold, salty water sinks and flows along ocean floor. Warm, less salty water rises and flows at surface. These currents redistribute heat globally, affecting climate. Questions test whether you predict how salinity or temperature changes affect water movement. Process: (1) Note salinity and temperature changes. (2) Predict density change. (3) Predict water movement (dense sinks, light rises). (4) Explain current formation.

Example: In polar regions, water cools and ice forms (increasing salinity of remaining water). Dense, salty water sinks, creating deep currents.

Mistake 1: Forgetting that both cold and salt increase density. A warm, salty solution might be less dense than a cold, fresh one (cold wins). Mistake 2: Assuming currents flow randomly. They don't; density differences drive them in predictable directions. Mistake 3: Confusing local salinity changes with global patterns. A river mouth has low salinity; the open ocean averages 35 ppt. Always consider both temperature and salinity when predicting density and water movement.

During practice, trace how salinity and temperature changes affect density and predict resulting currents.

Scenario 1: Warm, salty tropical water. Less dense; stays at surface. Scenario 2: Cold, salty polar water. Very dense; sinks to ocean floor. Scenario 3: River mouth (fresh water input). Low salinity, less dense; floats atop ocean water. Scenario 4: Evaporation increases water salinity. Density increases; water sinks. Scenario 5: Melting sea ice adds fresh water. Salinity decreases locally; density decreases; water rises or flows less dense. Current patterns may be disrupted. For each scenario, predict density changes and resulting water movement.

Find three ACT Science passages with ocean salinity/density questions. Predict water movement based on property changes. By the third passage, ocean dynamics understanding will be systematic.

Ocean property questions appear on some ACT Science sections. They test understanding of how physical properties drive water movement. Students who accurately predict density and current formation pick up 1 point because the relationships are predictable and based on consistent physics.

On your next practice test, predict water movement based on salinity and temperature changes. By test day, you should explain how ocean properties create currents confidently.