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Thermal Expansion

 

Effects of Anomalous Expansion of Water

Icebergs

Due to anomalous expansion, water is least dense at 0 degrees Celcius, the temperature at which ice forms. This partly explains why ice is less dense than water which is above the freezing temperature; so it floats on water. In cold regions, therefore, large masses of ice, called icebergs, float in the sea and are a serious hazard to sea transport.

Effects of Anomalous Expansion of Water

Icebergs

Due to anomalous expansion, water is least dense at 0 degrees Celcius, the temperature at which ice forms. This partly explains why ice is less dense than water which is above the freezing temperature; so it floats on water. In cold regions, therefore, large masses of ice, called icebergs, float in the sea and are a serious hazard to sea transport.

Iceberg

 

 

 

Frozen Lake

In cold climates, temperatures sometimes fall below 4 degrees Celcius. Due to anomalous expansion, cold water in rivers and lakes is expanded and less dense; so it floats at the top. As the temperature falls further, ice forms at the top, thereby insulating the water underneath. This prevents the rest of the water mass from freezing.  The liquid water below the ice continues to support the life of fish and other living things found in the sea.

 

Frozen lake

Bursting of water pipes in cold weather: The expansion of water as it freezes in a pipe forces the pipe to burst open.

Breaking of rocks: When water held in small cracks in a rock freezes, it expands and widens the cracks, causing the rock to break.

 

 

Objectives

By the end of the lesson, you should be able to:

  1. Define temperature
  2. Describe the functioning of various thermometers
  3. Describe thermal expansion in solids, liquids and gases
  4. Explain expansion in terms of particle behaviour

Quiz

 

 

 

 

 

Temperature

When water is heated it becomes warm or hot, depending on how long it is heated. On cooling, it becomes cold. In what way then do hot water, warm water and cold water differ?

Hot, warm and cold samples of water differ in terms of temperature. Temperature is the degree of hotness or coldness of a substance.  It is measured in kelvins (K), using an instrument called a thermometer. 

Quizzes

Ordinary Thermometer

The diagram below show the main parts of an ordinary liquid-in-glass thermometer, the type commonly used in the school laboratory.

The thermometer consists of mercury held in a glass tube with a narrow bore (hole). Mercury expands when temperature rises and contracts when temperature falls. This makes the level of mercury thread to move up or down the narrow bore; so temperature can be read directly from the scale.

Clinical Thermometer

Clinical thermometer
 

Clinical thermometer is another type of liquid-in-glass thermometer. It also uses mercury as the thermometric liquid; but its use is limited to measuring body temperatures of patients. For this reason, it has a short temperature range, usually 35-42 degrees celcius. It also has a larger mercury bulb, a feature that makes it sensitive to changes in body temperature.

 

Liquid-in-Glass Thermometer

The diagram represents an ordinary liquid-in-glass thermometer.  As the name suggests, the thermometer consists of a suitable liquid held in a glass tube, with a narrow bore. The liquid expands when temperature rises and contracts when temperature falls.  This makes the liquid column move up or down the scale; so the temperature can be read.

Using a Thermometer

The animation below shows how an ordinary thermometer can be used to measure the temperature of water. What is the boiling point of water? Suggest a place where this experiment was probably carried out.

NB: The thermometer should not touch the bottom of the beaker.

 

 

 

 

 

 

 

 

Maximum and Minimum Thermometer

This thermometer indicates both maximum and minimum temperatures reached in the course of the day.


Bulb A contains alcohol, while the U-tube contains mercury column and two steel indices. One of the steel indices (X) shows the maximum temperature and the other (Y) indicates the minimum temperature reached.

Contraction and expansion of alcohol causes movement of mercury thread which pushes the mercury and steel indices to give either maximum or minimum temperatures.  The thermometer is reset by pulling the indices back using a magnet. The animation shows the movement of X as temperature of the surroundings increases and of Y as temperature decreases.

 

 








 

Exercises

Exercises

Expansion of Solids, Liquids and Gases

What happens to a substance when it is heated? There are many possible answers to this question. However, one of the possible effects of heating a substance is expansion. In this section, we learn about the expansion of solids, liquids and gases.

Expansion of Solids


 

 

Bimetallic Strip

The animation on the next page demonstrates the expansion of a bimetal strip consisting of metals X and Y. Click on the play button and observe what happens when the bimetallic strip is heated. Which of the metals expands faster? 

Did you get it right? It is observed that the bimetallic strip bends towards Y. This observation indicates that X, which now occupies the outer edge of the curve has become longer than Y. Therefore X expands faster than Y. This behaviour of bimetallic strip is applied in electric iron box and in the circuit of a car indicator lamp.

Expansion of liquids

The following activity demonstrates expansion in liquids. Click on the play button and observe what happens. 

When the flask is heated, the level of water drops slightly then rises steadily. This is because the glass container initially expands faster than water but after some time, water expands faster. On cooling, the level of water rises sharply then drops steadily. Why is this?

Expansion of Gases

The following animation demonstrates the effect of heating a gas. In the set-up, air is trapped in the flask, with a liquid index acting as the cover. Take observations and explain the behaviour observed.

When air is heated, the liquid index moves steadily outwards. This shows that air expands to occupy a larger space. All gases expand when heated.

 

 

Applications of Expansion and Contraction

Expansion and contraction have desirable and destructive effects. The following are examples of precautions usually taken to avoid or minimize the damaging effects.

(a) Steel bridges: Some bridges are supported on rollers to enable them move freely as they expand or contract. In other designs, expansion gaps are provided to absorb the increase in length during hot seasons.

(b) Railway lines: Expansion gaps are provided between rails. Without these gaps, bending easily occurs during expansion and this has been a major cause of train accidents in the past. The modern ways of preventing bending of rails due to expansion, however, involve overlapping and welding of rails.

(b) Steam pipe: A steam pipe is looped at some points along its length to allow for expansion and contraction. This prevents the pipe from bursting due to expansion.

Expansion Gap in a Bridge

Expansion Gaps in a Bridge

Expansion Gap in a Bridge

Precautions Against Expansion and Contraction

 

 

 

Exercise

 

 

 

 

 

LESSON OBJECTIVES


By the end of the lesson you should be able to

  • Describe the unusual expansion of water
  • Describe the effects of unusual expansion of water

Anomalous Expansion of Water


 

 

Meaning of Anomalous Expansion of Water

When water is cooled from 4 to 0 degrees Celcius, its volume increases.  However, other substances decrease in volume when cooled.  Water therefore shows an unusual behaviour.  This unusual behavior of water is referred to as anomalous expansion. The graph below shows how volume varies with temperature in the anomalous behaviour of water.

Explaining the Anomalous' Expansion of Water

The water molecule is shaped as shown

Note 'the angle 0 between the hydrogen-oxygen bonds.  (Less than 180oC) at 4oC the value of 0 is smallest.  At other temperature value of 0 is larger

 

Consequences  

When 0 is smaller, more molecules can be packed in a given space.  When 0 gets bigger without reaching 180oC, molecules occupy more space

Icebergs

Due to anomalous expansion, water is least dense at 0 degrees Celcius, the temperature at which ice forms. This partly explains why ice is less dense than water which is above the freezing temperature; so it floats on water. In cold regions, therefore, large masses of ice, called icebergs, float in the sea and are a serious hazard to sea transport.

Iceberg

Frozen Lake

In cold climates, temperatures sometimes fall below 4 degrees celcius. Due to anomalous expansion, cold water in rivers and lakes is expanded and less dense; so it does not sink but floats at the top. As the temperature falls further, ice forms at the top, thereby insulating the water underneath; so it does not freeze. The liquid water below the ice continues to support the life of fish and other living things found in the sea.
 

Frozen lake

Exercise

Exercise

 

 



 Thermal Expansion 

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 Thermal Expansion  

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 Thermal Expansion 




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