Water’s Remarkable Properties
- Earth & Space
- Ages 14–18 · Grades 9–12
What you’ll find out
- Water is polar; hydrogen bonds give it a high heat capacity, make it a strong solvent and make ice float.
- Water breaks rock mechanically by frost wedging and chemically by dissolving limestone and turning feldspar to clay.
- Water released from sinking plates lowers the melting point of mantle rock and feeds volcanoes.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Why water is unusual
Water covers about seventy per cent of Earth’s surface, yet it is a highly unusual substance. Each molecule has one oxygen atom bonded to two hydrogen atoms. The oxygen end carries a slight negative charge, and the hydrogen ends carry slight positive charges. Opposite charges attract, so neighbouring molecules cling together through hydrogen bonds. These bonds give water a high heat capacity. It absorbs a lot of energy before its temperature rises. So oceans warm and cool slowly, keeping coastal climates mild. Water also dissolves more substances than almost any other liquid. Unusually, it expands by about nine per cent when it freezes. Ice therefore floats, insulating the water beneath.
Water at work on rock
Water’s properties make it the most powerful agent shaping Earth’s surface. Because water expands when it freezes, it can split rock apart. Water seeps into cracks, freezes, widens them and thaws, again and again. This process, frost wedging, is common in mountains. Rainwater absorbs carbon dioxide and becomes a weak acid. Over thousands of years, it dissolves limestone, carving caves and sinkholes. Water also reacts with minerals such as feldspar in granite, slowly turning them into clay. Flowing water then carries the loosened material away. Water even affects rock deep underground. Where one plate sinks beneath another, water released from it lowers the melting point of the rock above, feeding volcanoes.
Curriculum links: NGSS HS-ESS2-5