freezing point for water

Summary

The freezing point is the temperature at which a liquid changes its phase to a solid phase. The 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

• The freezing point for pure liquid water is not 0°?
• At 40C water exhibits strange behaviour?
• water can coexists as a liquid, gas and solid phase?
• The freezing point can be changed?
• Which point of the water is considered an internationally accepted point for the Absolute Temperature scale?
• Ice can be evaporated without melting it?

Introduction

The freezing point for water depends on pressure and temperature.

Depending on the 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 the solid and liquid phase.

When the melting is completed it is a liquid phase.

On partial 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 the melting point as at this temperature substance from the solid phase starts to melt and transform into a liquid phase during heating.

What is the freezing point for water?

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

What is the freezing point for pure water?

For pure liquid water, it is incorrect to say that the 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 that 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 the ice will gradually rise on heating till the melting (fusion) temperature is reached. There will be a small increase in the 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 the water till the melting is completed. At atmospheric pressure, the Melting temperature of the 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 the water is 373.15 Kelvin(100°C) and the process is shown by lines 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 is 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 the steam to rise. Now the steam is called 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 substances coexist in solid, liquid and gaseous phases in thermodynamic equilibrium.

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 water ( refer fig 4). This 0.01°C temperature is an internationally accepted point for the 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 sublimation. Ice can be converted to Vapour without melting just by lowering the pressure below 0.06 112 bar at triple point! ( refer fig 4)

How water is different from other liquid?

If you observe 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 a 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 the 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.

Meaning of the terms used in this article

Volume

It is defined as the amount of space and normally measured in terms of cubic metre

Specific volume

It is the amount of space occupied by 1kg of matter or substance

Density

It is the mass of the unit volume of substance or matter

Boiling Point

The boiling point is defined as the temperature at which water starts to evaporate. This temperature is also known as saturated temperature and this temperature depends on pressure.

Thermodynamic Equilibrium

A system is in thermodynamically equilibrium if the temperature and pressure at all points are the same there should be no velocity gradient and also chemical equilibrium also required!

System

A system is a finite quantity of matter or a prescribed region of space

Homogeneous system

A system that consists of a single-phase is termed as ‘homogeneous system’ e.g. a mixture of air and water vapour

Nucleation

Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization.

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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