freezing point for water

Summary

Freezing point is a temperature at which liquid changes its phase to solid phase. Freezing point for water depends on pressure and at atmospheric pressure freezing point of water is 0°C or 273.15 Kelvin

Do you know

Freezing point for a pure liquid water is not 0°C

At one point water coexists as a liquid, gas and solid phase?

The behaviour of water, slightly different from other liquids during phase change?

Which point is considered an internationally accepted point for the Absolute Temperature scale?

Can ice evaporate without melting it?

If you want to know more about these interesting facts of water then read this article

Table of Contents

      Introduction 

    Freezing point for water depends on pressure and temperature.

    Depending on pressure and temperature water or any substance can exist in a solid, liquid or gaseous phase or in a mixture of three phases.

    Solid phase of water on partially melting it becomes a combination of solid and liquid phase.

    When the melting is completed it is a liquid phase.

    On partially evaporation it is a combination of liquid and gaseous phase or wet steam.

    When the evaporation is completed it is a gaseous phase.

    Definition of a freezing point 

    A freezing point is a temperature at which a substance starts to change its phase from liquid to solid during cooling.

    This point is also known as melting point as at this temperature substance from the solid phase starts to melt and transform in to liquid phase during heating.

    What is the freezing point for water?

    Freezing point for water is 0°C or 273.15 Kelvin at atmospheric pressure. This means at 0°C water starts to transform into Ice from liquid phase on cooling.

    What is the freezing point for pure water?

    For pure liquid water it is incorrect to say that freezing point is 0°C. Because pure liquid water can be supercooled well below that temperature without freezing if the liquid is not mechanically disturbed!

    That means, it can remain in a fluid state down to its homogeneous nucleation point of about 231 K (−42 °C or −44 °F).

    Change of phase of water From Ice to steam during constant pressure process 

    Fig 1 Temperature – Volume relation for heating of water

    Imagine a unit mass of ice which is well below freezing point temperature at atmospheric pressure. Supply the heat to this Ice at constant atmospheric pressure. You will experience the following observation. For your convenience to understand it properly I have drawn a graph of observation (fig 1).

    The temperature of ice will gradually rise on heating till the melting (fusion) temperature is reached. There will be small increase in volume of ice, this is indicated by the line 1-2 in the temperature volume graph

    After this addition of heat does not cause the temperature of water to rise but the ice begins to melt into water till the melting is completed. At atmospheric pressure the Melting temperature of water is 273.15 Kelvin(0°C). This is indicated by the line 2-3 in the temperature volume graph.

    Once the melting is completed further heating causes the temperature of water to rise till the water reaches the vaporization or boiling point.

    At atmospheric pressure the boiling temperature of water is 373.15 Kelvin(100°C) and the process is shown by the line 3-4-5 in the graph.

    In this process there is a reduction in specific volume until the temperature is about 277 Kelvin (4°C) and subsequently there is an increase in the specific volume until the boiling temperature reached.

    Further heating does not cause temperature rise but water begins to boil and get changed to steam. This 2nd phase change also occurs at a constant temperature and there is a large increase in volume until the liquid is completely vaporized. This is indicated by the line 5-6 in the TV diagram.

    Further heating causes the temperature of steam to rise. Now the steam is called as superheated steam.

    Effect of Pressure on Freezing point for water

    Fig 2 Temperature – volume relation for heating of water at different pressure

    If you conduct the experiment of heating ice at different pressures you will notice many interesting observations. The graph ( fig 2 ) shows the change of phase of water during heating at a different pressure.

    Observation

    From the graph ( fig 2 ) you can notice the following observation

    If pressure is decreased its freezing point rises slightly whereas the boiling point decreases considerably.

    when the pressure is reduced to 0.006 112 bar the freezing point (or melting point) and boiling temperature becomes equal.

    What is triple point?  

    It is the temperature and pressure at which substance coexist in solid, liquid and gaseous phase in thermodynamic equilibrium.

    Fig 3 Triple point for substance other than water

    What is the triple point for water? 

    0.006 112 bar pressure and the temperature 273.16 Kelvin (or 0.01°C) is the triple point of a water ( refer fig 4). This 0.01°C temperature is internationally accepted point for Absolute Temperature scale.

    Fig 4 Triple point and critical point for water

    Only at triple point ice water and steam coexist in Thermodynamics equilibrium in a closed vessel.

    what is sublimation?

    If ice is instead of melting directly evaporated to Vapour then this process is called as sublimation. At triple point Ice can be converted to Vapour without melting just by lowering the pressure below 0.06 112 bar!( refer fig 4)

    How water is different from other liquid?

    If you observe the Fig 1 you can see that the specific volume varies with temperature but not linearly. When the temperature increases to 3.98°C its specific volume decreases to low value (or density rises to a peak) and then increases. This is unusual as this phenomenon only happens in the case of water (Compare Fig 1 with Fig 5) or (Compare Fig 3 with Fig 4).

    Fig 1 Temperature – Volume relation for heating of water

    Fig 5 Temperature Volume relation for heating of other liquid

    These effects are due to the reduction of thermal motion with cooling, which allows water molecules to form more hydrogen bonds that prevent the molecules from coming close to each other.

    While at below 4 °C, the breakage of hydrogen bonds due to heating allows water molecules to pack closer despite the increase in the thermal motion (which tends to expand a liquid).

    Above 4 °C water expands as the temperature increases. Water near the boiling point is about 4% less dense than water at 4 °C (39 °F).

    The density of water is about 1 gram per cubic centimetre (62 lb/cu ft) and is a reciprocal of specific volume of water.

    Regular, hexagonal ice is also less dense than liquid water upon freezing, the density of water decreases by about 9%.

    Difference between gas and vapour 

    Unlike a gas, vapour can be liquefied by applying pressure at constant temperature and also laws of perfect gas do not apply to vapour.

    References

    1. Thermal Science & Engineering by M.L. Mathur, F.S. Mehta
    2. Thermal Engineering by R K Rajput
    3. Properties of water – Wikipedia
    4. Nucleation – Wikipedia

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