Questions: Physical and Chemical Properties of Seawater

Question 1

Two water parcels have the same salinity. Parcel A is at 5°C and Parcel B is at 20°C. Which is denser, and why?

• A. Parcel B, because warmer water expands and contains more dissolved gas
• B. Parcel A, because lower temperature causes molecules to move more slowly and pack together more tightly
• C. They are equal, because salinity is the only variable that affects density
• D. Parcel B, because warm water evaporates less and retains more mass

Answer: (B)

Temperature and density are inversely related in seawater: as temperature rises, water expands (molecules move faster and occupy more volume), reducing density. Parcel A at 5°C is denser than Parcel B at 20°C. Salinity also affects density, but with both parcels at the same salinity, temperature is the deciding variable.

Question 2

The ocean is uniformly saline at about 35 ppt everywhere at the surface.

• T. True
• F. False

Answer: False

Surface salinity varies significantly by region. In tropical regions, high solar radiation drives intense evaporation, concentrating salts and raising salinity above 36 ppt. In polar regions, sea ice melt and high precipitation dilute surface waters, lowering salinity to around 30–33 ppt or less. River outflows near coasts also produce locally fresher water. The 35 ppt figure is an approximate global average, not a uniform value.

Question 3

Seawater freezes at approximately −1.8°C rather than 0°C. What property of seawater explains this, and what happens to the salt when seawater does freeze?

Model answer: Dissolved salts lower the freezing point of seawater via freezing point depression (a colligative property). When seawater freezes, the ice crystal lattice excludes most dissolved ions, so sea ice is nearly fresh water and the remaining liquid becomes saltier and denser.

Freezing point depression is a colligative property: solutes disrupt the formation of the ordered ice crystal lattice, requiring a lower temperature to freeze. Because ions are excluded from the solid lattice as seawater freezes, the rejected brine sinks into surrounding seawater, increasing its salinity and density — a key driver of deep water formation in polar regions.