Question

SEP Construct an Explanation Why would sea water trapped in a lake become saltier as glaciers form on top of it?

Answer 1

Claim
- The trapped seawater becomes saltier as glaciers/ice form on top of it.

Evidence (observations and facts)
- Ice that forms from seawater is almost pure water; most dissolved salts do not enter the solid ice lattice.
- When seawater freezes, the rejected salts concentrate in the remaining liquid as brine pockets and channels.
- If the body of water is isolated (a trapped lake) the concentrated brine cannot escape easily.

Reasoning (how the evidence supports the claim)
- Freezing is a phase change that removes water as solid ice. The crystal structure of ice accommodates water molecules but excludes most dissolved ions (Na+, Cl−, etc.). Those ions are therefore left behind in the unfrozen liquid.
- Because the total mass of dissolved salt in the lake stays essentially the same while the amount of liquid water decreases (some water becomes ice), the concentration of dissolved salts in the remaining liquid rises (conservation of mass: same salt mass / less water → higher salinity).
- As salinity rises the liquid’s freezing point is depressed, so further cooling concentrates the remaining liquid even more rather than freezing it immediately. Dense, very salty water may sink and remain trapped in the lake, producing an overall saltier lake.
- In short: ice formation removes almost-pure water and rejects salts into the remaining liquid; in an enclosed lake that means the liquid gets progressively saltier as more ice forms.