MODULO CLIL L’acqua e le sue proprietà
Water and its properties
Lesson 5
Autori:
Elisabetta Vagni
Patrizia Segoloni
Giuseppe Novello
1.
1
States of Water
2
Boiling and Freezing
3
Density
4
The Universal Solvent
5
Adhesion and Cohesion
6
Surface tension
7
Capillary action
8
Communicating vessels
2
WATER AND ITS
PROPERTIES
Water's Physical Properties
1 States of Water
Water is unique in that it is the only natural substance that is found in all three
states: liquid, solid (ice), and gas (steam) at the temperatures normally found
on Earth. Earth's water is constantly in movement (Water Cycle).
2 Boiling and Freezing
Water freezes at 32o F (Fahrenheit) or 0o on the Celsius scale is water's
freezing point and boils at 212o F or 100o on the Celsius scale is water's
boiling point.
boiling point
212o F
freezing point
o
32 F
3
3 Density
Density is defined as the mass of an object per unit volume of the object.
The density of water is 1.00 g/cm³.
The density of water is an important physical constant. It is used as a standard
of reference to which the densities of other substances are compared.
Unlike most substances, which are most dense in their solid form, ice (solid
water) is actually lighter (less dense) than liquid water. As a result, ice floats
on water.
4 The Universal Solvent
Scientists often call water the “Universal Solvent” because water can
dissolve a lot of substances like salt for example. In fact, water, in a “pure”
state, is not found in nature. As the universal solvent, water dissolves almost
any substance to form solutions.
Some substances dissolve more easily in water than do others. Common table
salt (sodium chloride) dissolves in water very easily.
4
distilled water
+
salt
=
solution
5 Adhesion and Cohesion
Water is attracted to other materials. This is called adhesion. Water can also
be attracted to other water. This is called cohesion.
The oxygen end of water has a negative charge and the hydrogen end has a
positive charge. The hydrogen of one water molecule is attracted to the oxygen
from other water molecules. This attractive force is what gives water its
cohesive and adhesive properties.
•
ADHESION: water molecules attach not only to other water molecules,
but also to other types of molecules. Attraction between two unlike
substances such as water and glass is called adhesion. The forces of
adhesion can pull water to the surface, where it can be used by plants.
•
COHESION is a force which holds a solid or liquid together, because of
the attractions between molecules.
5
6 Surface tension
Surface tension is the name we give to the cohesion of water molecules at
the surface of a body of water.
Water has a very high surface tension. In other words, water is
sticky (a
substance that easily becomes joined to the things that it touches) and elastic.
Surface tension allows many aquatic insects to ‘walk’ across rivers and
streams. Surface tension is responsible for capillary action, which allows
water to move through the roots of plants.
Surface tension is related to the cohesive properties of water. Capillary action
however, is related to the adhesive properties of water.
7 Capillary action
Capillary action occurs because water is sticky -- water
molecules stick to each other and to other substances, such as
glass, cloth, organic tissues, and soil. Put a paper towel into a
glass of water and the water will "climb" into the paper towel.
In fact, it will keep going up the towel until the pull of gravity is
too much for it to overcome.
This is more important than you think. Consider:
plants and trees couldn't live without capillary action. Plants put down roots
into the soil which are capable of carrying water from the soil up into the plant.
6
Water, which contains dissolved nutrients, gets inside the roots and starts
climbing up the plant tissue. As water molecule #1 starts climbing, it pulls
along water molecule #2, which, of course, is dragging water molecule #3,
and so on.
Communicating Vessels
Another property of water is the Communicating Vessels.There are a number
of vertical tubes of different shapes which communicate at the bottom by
another tube. We fill it with water to a certain height.
If we add water to any vessel, there is a movement of water until the water
has levelled out. An increase or decrease of water in any tube affects the water
in all other tubes.
7