Water
Senior 1 · Chemistry
Water
Lesson 1 of 5
25 min
Drop an ice cube into a glass of water: it floats. That should surprise you. The solid form of almost every other substance is denser than its liquid, so solid wax sinks in melted wax and a lump of solid iron sinks in molten iron. Water breaks the rule. The same odd behaviour explains why a full glass bottle of water left in a freezer cracks. In this lesson you find out where the water you use comes from, and which measurements tell you that a sample of water is pure.
Almost all fresh water starts as rain. Rwanda has two rainy seasons, and the rain that falls either runs off the hills into streams, rivers and lakes (surface water), or soaks into the soil and rock and collects there (underground water, or groundwater).
Table 1.1: Sources of water
| Source | Examples in Rwanda | What to know |
|---|---|---|
| Rain | rain collected from roofs in tanks and jerrycans | cleanest source as it falls, but picks up dust, bird droppings and dirt from roofs and gutters |
| Rivers and streams | Nyabarongo, Akanyaru, Akagera, Sebeya | carry soil washed off the hills, especially in the rainy season; the Nyabarongo supplies much of Kigali's tap water (Lesson 4) |
| Lakes | Kivu, Muhazi, Burera, Ruhondo, Ihema | large stores of water; Lake Kivu also holds dissolved gases deep down |
| Springs | protected springs on hillsides across the country | groundwater that flows out at the surface; filtered by the rock it passed through |
| Wells and boreholes | hand-pump wells and drilled boreholes | groundwater lifted by a pump; often contains more dissolved minerals |
Definition
Pure water: water that contains nothing but water molecules, . No natural source is pure: rain, rivers, lakes and groundwater all contain dissolved gases and minerals, and often suspended soil and germs. Pure water is made in the laboratory by distillation (Unit 4) and is called distilled water.
Set up the apparatus in Figure 1.1 with about 100 cm³ of distilled water and a few anti-bumping granules. Heat until the water boils steadily, wait one minute, and read the thermometer. Let the flask cool, add one level spatula of common salt, stir until it dissolves, and repeat. Finally, put crushed ice made from each liquid in a test tube and record the temperature at which each one freezes.
Figure 1.1: Finding the boiling point of water.
Warning
Safety. Boiling water and steam scald. Never seal the flask: the steam must have a way out, or the pressure builds up until the glass bursts. Point the steam tube away from people, wear goggles, and let the apparatus cool before you touch it. Stir with a glass rod, never with the thermometer.
Table 1.2: Results of a class in Kigali
| Liquid | Boiling point | Freezing point |
|---|---|---|
| distilled water | ||
| salt solution (about 30 g of salt per kilogram of water) |
Two things stand out. First, the pure water boiled at , not : Kigali is about 1,500 m above sea level, where the air pressure is lower (Unit 3, Lesson 3). Second, the dissolved salt raised the boiling point and lowered the freezing point. A pure substance has fixed melting and boiling points; an impurity changes them. That is the physical test for pure water.
Definition
Boiling point and freezing point of pure water: pure water freezes (and ice melts) at , and boils at at sea-level pressure. Example: a sample that boils at at sea level, or freezes at , contains dissolved substances and is not pure.
Table 1.3: Physical properties of pure water
| Property | Value or description |
|---|---|
| Appearance | colourless, odourless and tasteless liquid at room temperature |
| Freezing (melting) point | |
| Boiling point | at sea level; about in Kigali |
| Density | , highest at ; ice is |
| pH | 7 (neutral) |
| Electrical conductivity | very poor: pure water hardly conducts; dissolved salts make it conduct |
| As a solvent | dissolves more substances than any other common liquid, which is why natural water is never pure |
| Heat capacity | high: water warms up and cools down slowly, so lakes keep a steady temperature |
When most liquids cool, their particles slow down and pack closer, so the liquid becomes denser all the way to its freezing point. Water does this only down to (more precisely ). Below that, the water molecules start to line up into the open, cage-like arrangement they have in ice, which takes up more room. So water expands as it cools from to , and expands by about 9% more when it freezes (Figure 1.2).
Figure 1.2: The density of liquid water between 0 °C and 10 °C.
In a cold country in winter, this saves the life in a lake (Figure 1.3). As the surface cools, water at sinks to the bottom and stays there. Colder water stays on top and finally freezes, and the floating ice insulates the water below, so the lake does not freeze solid.
Figure 1.3: Why a lake freezes from the top down.
Definition
Anomalous expansion of water: the unusual expansion of water when it cools from to , and again when it freezes. Because of it, water has its maximum density at and ice floats.
Worked example
Problem. Uwase fills a 500 cm³ glass bottle with 480 g of water, closes it and puts it in a freezer. Will the bottle break? (Density of water ; density of ice .)
Solution.
Answer: the ice needs about 522 cm³ but the bottle holds only 500 cm³, so the ice pushes on the glass and the bottle is likely to crack.
Worked example
Problem. A student in Kigali tests three clear, colourless liquids, A, B and C. Which one could be pure water?
| Liquid | Boiling point in Kigali | Freezing point |
|---|---|---|
| A | ||
| B | ||
| C |
Solution.
Answer: A. Being colourless is not enough: the fixed boiling and freezing points are the test.
Tip
Remember 0, 100, 4, 1: pure water freezes at 0 °C, boils at 100 °C at sea level, is densest at 4 °C, and has a density of 1 g per cm³.
Warning
Common mistake: saying that water is densest at 0 °C, or that ice is "heavier" than water. Water is densest at 4 °C; ice is less dense than water, which is why it floats.
TED-Ed: Why does ice float in water? (4 min). Watch for the hydrogen bonds that hold water molecules in an open pattern in ice.
Practice
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
