Murph's Blog of Chem Joy: SCH 4C

Clean water is essential for life on Earth. Without water, a human can survive for a maximum of ten days. However, billions of people do not have access to clean water or proper sanitation, which results in thousands dying each day from water related diseases. This seems unthinkable since 70% of the Earth is covered in water.

97% of the Earth’s water is in oceans and is undrinkable and another 2% is locked in ice caps. Therefore, 1% of the Earth’s water is fresh liquid water.

In Canada, we have a ready supply of fresh, clean water. However, what would happen if there was prolonged drought or the water system became contaminated? You would have to conserve water or boil it before using it.

If asked to restrict water usage, which uses would you give up first?

Water Cycle

Early settlers would drink water from the closest source – a lake or stream. There was little concern about water contamination – it was naturally purified in processes like the water cycle.

Recall the water cycle:

The Sun warms the water, which evaporates (leaving behind any dissolved contaminants) and rises into the atmosphere. As the vapour rises, it cools and condenses into mist, fog and clouds or returns to Earth as as precipitation (rain, snow, sleet or hail). This sequence repeats itself indefinitely.

Most precipitation soaks into the soil and becomes ground water. Ground water collects in porous rock structures called aquifers (as the water collects, it dissolves bits of soil and rock, which add impurities like iron, magnesium, zinc and calcium).

Precipitation that finds its way into lakes, ponds, rivers and streams becomes surface water.

Although ground water is used, surface water is the primary source of Canadian drinking water. As the population grows, the natural purification process cannot keep up with demand.

Physical Properties of Water

Water is the universal solvent, because it can dissolve such a large number of substances. Pure water is clear, colourless, odourless and tasteless. Tap water usually has a taste and smell as a result of dissolved solutes – some due to natural processes, some not.

The density of pure water is 1.0 g/mL; sea water is slightly higher.

Crude oil spilled from a tanker, which has a lower density than seawater, will float on the water’s surface, where it is subjected to currents and wind. Some of the heavier components sink to the ocean floor causing ecological damage.

Water is unique in how its density changes with temperature (solid is less dense than liquid). As water cools, it expands and the density is 0.9 g/mL. This is important since floating ice on a lake acts as an insulating blanket, keeping the water below from freezing, so the fish can live through the winter.

Heat capacity is a measure of how much energy is needed to change the temp of 1 g of a substance by 1ºC. Water has a high heat capacity, so it absorbs a lot of heat with only a small temp increase. Thus, large bodies of water, like the Great Lakes, can act as heat reservoirs. They absorb a lot of heat in the summer before water temp increases significantly Wind blows over the cool lake water to moderate warmer temps over the land. In the winter, the heat stored in lakes helps warm the adjacent land.

Hydrogen Bonding

Recall that water can hydrogen bond (H-bond)– this intermolecular force accounts for many of water’s unique physical properties.

The δ+ H in one molecule is attracted to the δ- O in an adjacent molecule.

This force can be used to explain many of water’s properties:

Water has a high boiling point – it takes a lot of energy to break the H-bonds between the water molecules. Compare H₂O’s boiling point (100 C) to another similar substance that cannot H-bond, H₂S (-61 C).

Water dissolves polar molecules, since “like dissolves like.”

Electrolytes (substances that break into ions – ionic compounds) also dissolve in water – each ion from the salt is surrounded by a sphere of hydration.