Section b water and its boiling point

OINT OF WATER

fig.(a) the b. p. of Water (approximate)

Fig. (a) is a graph. It shows the boiling point of water at different pressures. The vertical scale gives different temperatures and the horizontal scale gives different pressures. On the horizontal scale, one (1) is normal pressure.

At normal pressure, pure water boils at exactly 100°C. It does not always boil at exactly 100°C. The b. p. changes with pressure. The graph line shows the b. p. at different pressures.

At low pressures, water has a low b. p. For example, at normal pressure, water does not boil below 100°C, but at one tenth normal pressure, it boils at approx. 50°C. At high pressures, water has a high b. p. At normal pressure, it does not have a b. p. above 100°C, but at ten times normal pressure, the b. p. is approx. 185°C.

Note: The freezing point does not change with pressure. Pure water always freezes at exactly 0°C.

Exercise 3 Are these statements true or false? If they are

false, change the statement into the negative form.

1. Pure water normally boils at exactly 100°C.

2. Water always boils at exactly 100°C.

3. Fig. (h) shows the b. p. of water at different pressures.

4. The graph line shows the b. p. of water at different pressures.

5. The horizontal scale gives different temperatures.

6. At normal pressure, water boils below 100°C.

7. At low pressures, water boils at exactly 100°C.

8. At high pressures, water has a b. p. of 100°C.

9. At a hundred times normal pressure, water boils at approx. 300°C.

10. The f. p. of pure water changes with pressure.

Exercise 4 Look at fig. (b) and complete the paragraphs from the wordlist.

Solid, liquid, gas, temperature, pressures, graph, CO2, liquefies, solidify, changes, horizontal, vertical, normal, approximate, times

Fig. (b) the b. p. of CO₂

Fig. (b) is а . . . .It shows the b. p. of .... at different pressures. The .... scale gives different temperatures in °C. The .... scale gives different pressures. On the horizontal scale, one (1) is . . . . pressure.

A gas does not always liquify at the same.... The b. p .... with pressure. The graph line shows the b. p. at different ....

At normal pressure, CO₂ .... at approx. … °С, i.e. it is a . . . . above this temperature and it becomes a .... below this temperature. At ten .... normal pressure, CO₂ liquifies at approx. ……. °C.

Exercise 5 Look at these two examples.

a) Does the b. p. of water change with pressure? Yes, it does.

b) Does the f. p. of water change with pressure? No, it doesn't.

Now look at the diagram about water. Answer the questions.

			Does the		volume density			of water change with temperature ?
Does	ice water steam	have		a volume		of	more than less than exactly		1 ml/cm3	?
				a density					1 g/cm3

LANGUAGE NOTES

1. Volume, density and pressures changes with temperature.

2. ……….. has a volume ( a density, a pressure, a temperature of ) …

3. …… at different volumes (densities, pressures, temperatures).

4. …g/cm² = grams per centimeter squared.

5. …g/cm³ = grams per centimeter cubed.

Exercise 6 Check up whether you know the following words and expressions.

(a) density, show, approximate(ly), (a) volume, burn, exact(ly), a graph, per (/), horizontal, vertical, a scale, times (x), a line, pure, steam, gram(mе) (g), wax, milliliter (ml)