Natural sciences Year 5: Water: properties, drinking water and labels
Natural sciences, Year 5 (Portugal): Water is a solvent that living things use to move substances and keep their temperature; some waters are fit to drink and others are not, and a bottle label tells you what is in it.
Properties of water and what they do for life
Pure water is colourless, odourless and tasteless. It is a good solvent: it dissolves many substances. It melts at 0 °C and boils at 100 °C at sea level, and it heats and cools slowly. These properties explain its jobs in living things: it dissolves and carries substances (in blood and in the sap of plants), it is where the body's reactions take place, and it helps control temperature (sweat). More than half of the human body is water.
Four kinds of water
Not all water is fit to drink. Drinking (potable) water is treated and controlled, so it is safe: it is the water from your tap. Mineral water comes naturally from a protected underground spring and has a stable set of dissolved substances. Brackish water has some salt, more than fresh water and less than the sea, as in estuaries, and is not for drinking. Polluted water carries waste, chemicals or germs and is unfit for drinking.
Reading a water label
Example label (made up): «Natural mineral water. Total mineralisation 160 mg/L. Bicarbonate 95, calcium 22, sodium 9, magnesium 5 (mg/L). pH 7,4.» Step 1: «natural» and «spring»: it comes from a protected underground source. Step 2: 160 mg/L: the dissolved substances in each litre. Step 3: bicarbonate is the main one (95 mg/L). Step 4: drinking 1,5 L gives 9 × 1,5 = 13,5 mg of sodium. Step 5: pH 7,4 is close to neutral (7).
Clear does not mean drinkable
It is tempting to trust water that looks clear and has no smell. But germs and many chemicals cannot be seen or smelt, and sea water is clear and undrinkable because of its salt. Only tests and treatment tell you if water is fit to drink. That is why water from an unchecked stream, well or borehole should not be drunk just because it looks clean, and why polluted water is a problem even when it seems fine.
Water and your health
The body loses water when it breathes, sweats and urinates, and has to replace it. With too little water you get thirsty, tired and may have headaches (dehydration). How much you need changes with age, heat and exercise. On a bottle, the label tells you where the water comes from and what it contains. A more expensive water is not automatically better. Choosing water instead of sugary drinks is good for your teeth and weight.
Test what you learned
Which property of water explains how it carries substances in blood and plant sap?
- a) It is colourless
- b) It is a good solvent
- c) It boils at 100 °C
Correct answer: b) It is a good solvent — Right: dissolved substances travel with the water in blood and in sap.
A water label says sodium 9 mg per litre. How much sodium is in 1,5 L of that water?
- a) 6 mg
- b) 9 mg
- c) 10,5 mg
- d) 13,5 mg
Correct answer: d) 13,5 mg — Right: 9 × 1,5 = 13,5 mg.
Water looks clear and has no smell. What can you say about it?
- a) It is drinking water, so it is safe
- b) It is mineral water, because it is clear
- c) You cannot know without tests: it could still be unsafe
Correct answer: c) You cannot know without tests: it could still be unsafe — Right: only tests and treatment show if water is fit to drink; sea water is clear too.
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