BIOLOGY  -TEXT
BIOLOGY -TEXT
14th Edition
ISBN: 9781260710878
Author: Mader
Publisher: RENT MCG
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Chapter 2, Problem 1TC
Summary Introduction

To explain:

Why oceans do not freeze.

Introduction:

Water is the universal solvent. It is a polar molecule so it dissolves only polar molecules. Substances (solute) dissociate due to the polarity of both solute and solvent. Solutes change the characteristics of the solvent like lowering the freezing point and elevating the boiling point.

Expert Solution & Answer
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Explanation of Solution

Ocean water is saline which means it has salt in it, mostly sodium chloride (common salt). Sodium chloride (NaCl) is formed by an ionic bond and so are polar. Sodium (Na+) has a positive charge and chloride (Cl-) has a negative charge.

Similar is the case with water (H2O). Oxygen has a negative charge (O-) while hydrogen atoms (H+) have a positive charge. NaCl dissociates in water. The positive sodium is attracted by the negative oxygen of water. And the negative chloride ion attracted to the positive hydrogen of water. Thus, a sodium ion is surrounded by water with oxygen facing the sodium ion and a chloride ion is surrounded by water with hydrogen facing the chloride ion.

When water experiences cold temperatures it initially shrinks and becomes dense till 4°C (water is densest here). Further decrease in temperature does not allow water molecules to move and the hydrogen bonds keep the water molecules apart so its density decreases again and water freezes. That is why ice has a lower density than water and floats.

However, due to the sodium chloride mixed in water, the hydrogen bonds are not allowed to open and water molecules’ movement does not stop. The vapor pressure is also decreased by the solute (salt). Hence, the freezing point of salt water is lowered to -2°C from 0°C.

Also at times when water does freeze on reaching this temperature, only the top layer freezes. The water below it is insulated from cold by the ice and life continues below it.

Conclusion

The salt in water lowers the freezing point of water due to which ocean water does not freeze.

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