Page 76 - Chemistry ICSE Class IX
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64 ICSE Chemistry – 9
When expressed in SI units, the latent heat of vaporisation of water
–1
5
is 22.6 × 10 J kg . It is due to its high latent heat of vaporisation that
steam causes more severe burns than water at the same temperature.
Dielectric constant. Water has high dielectric constant value (80). It is
due to its high dielectric constant that water dissolves large number
of polar and ionic substances.
Fig. 3.6 1 kg of steam at 100°C has
5
22.6 × 10 joules more energy than 1 kg The Anomalous Behaviour of Water
of water at the same temperature (100°C)
In general, all forms of matter expand on heating and contract on cooling.
/ĐĞ ŽĐĐƵƉŝĞƐ ŵŽƌĞ ǀŽůƵŵĞ ƚŚĂŶ ĞƋƵĂů But water (and few other substances) shows an unusual behaviour between
ŵĂƐƐ ŽĨ ǁĂƚĞƌ͘ 0°C and 4°C.
Water when cooled, contracts up to 4°C.
When cooled below 4°C, it expands until 0°C.
When ice at 0°C is heated, it contracts until 4°C and expands thereafter.
These observations indicate that,
at 0°C, water has higher volume and lower density than that at 4°C.
at 4°C, water has minimum volume and maximum density.
The density of water at 0°C is 0.9998 g/mL, whereas that of ice at 0°C
is 0.9168 g/mL. These values show that ice is less denser than water. In
Ice floats on water
other words, ice is lighter than water. This is because water expands on
freezing.
In ice, water molecules form a three-dimensional structure
with large open space in between. So, ice occupies more volume
than an equal mass of water. That is why ice is lighter than water
and oats over it. For the same reason, density of ice is lower than
that of the liquid water.
This property of water plays a very important role in nature.
In cold countries during winters, the upper layer of lakes and
ponds gets frozen. The ice being lighter than water, oats over
it and the lower layers of water remain in the liquid form. This
enables the organisms living there to survive even during severe
Fig. 3.7 Temperature of different layers of water in a
frozen lake/pond during severe winter winter.
ASSIGNMENT 2
Physical properties of water, The anomalous behaviour of water
ϭ͘ ,Žǁ ĚŽĞƐ ƉƵƌĞ ǁĂƚĞƌ ďĞŚĂǀĞ ƚŽǁĂƌĚƐ ůŝƚŵƵƐ͍
Ϯ͘ tŚĂƚ ĂƌĞ ƚŚĞ ĨƌĞĞnjŝŶŐ ĂŶĚ ďŽŝůŝŶŐ ƉŽŝŶƚƐ ŽĨ ǁĂƚĞƌ ƵŶĚĞƌ ŶŽƌŵĂů ƉƌĞƐƐƵƌĞ͍
ϯ͘ ,Žǁ ĚŽĞƐ ƚŚĞ ďŽŝůŝŶŐ ƉŽŝŶƚ ŽĨ ǁĂƚĞƌ ĐŚĂŶŐĞ ǁŚĞŶ ĐŽŵŵŽŶ ƐĂůƚ ŝƐ ĚŝƐƐŽůǀĞĚ ŝŶ ŝƚ͍
ϰ͘ ĞĮŶĞ ƐƉĞĐŝĮĐ ŚĞĂƚ ĐĂƉĂĐŝƚLJ ŽĨ ǁĂƚĞƌ͘ 'ŝǀĞ ŝƚƐ ǀĂůƵĞ͘
ϱ͘ tŚLJ ĚŽĞƐ ŝĐĞ ŇŽĂƚ ŽŶ ƚŚĞ ƐƵƌĨĂĐĞ ŽĨ ǁĂƚĞƌ͍
Solutions
^ŽůƵƟŽŶƐ ĂƌĞ ŚŽŵŽŐĞŶĞŽƵƐ Ăƚ
ŵŝĐƌŽƐĐŽƉŝĐ Žƌ ŵŽůĞĐƵůĂƌ ůĞǀĞů͘ A homogeneous mixture of two or more substances is called a solution.
ͻ
,ŽŵŽŐĞŶĞŽƵƐ ƐŽůƵƟŽŶ ŽĨ Ă ƐŽůŝĚ ŝŶƚŽ Homogeneous means physically and chemically uniform throughout. In
ĂŶŽƚŚĞƌ ƐŽůŝĚ ŝƐ ĐĂůůĞĚ solid soluƟon͘ common use, the term ‘solution’ actually means true solution.
ŽŵŵŽŶ ŵĞƚĂů ĂůůŽLJƐ ĂƌĞ ƐŽůŝĚ A solution in which size of the solute particles is about 10 –10 m
ƐŽůƵƟŽŶƐ͘ dŚĞ ŐŽůĚ ƵƐĞĚ ĨŽƌ ŵĂŬŝŶŐ is called a true solution. In a true solution, the solute particles and the
ũĞǁĞůůĞƌLJ ŝƐ Ă ƐŽůŝĚ ƐŽůƵƟŽŶ ĐŽŶƚĂŝŶŝŶŐ solvent molecules cannot be distinguished even under microscope. In a
ŐŽůĚ ĂŶĚ ƐŵĂůů ƋƵĂŶƟƟĞƐ ŽĨ ĐŽƉƉĞƌ true solution, the solute particles are trapped into the space between the
Žƌ ƐŝůǀĞƌ͘
solvent molecules.
