EXPANSION IN EVERYDAY LIFE
Expansion of solids has both advantages and disadvantages. It has disadvantages to construction of railways lines, bridges, in wrist watches, transmission wires e.t.c.
- Railway lines
These are constructed with gaps between the rails. This is done so that on hot days when the railways expand they have room for expansion. If no gaps are left between, accidents may be caused.
These are constructed with one end foresting on rollers and the other fixed. These allow the metal structure of the bridge to expand and contract freely without weakening or damaging the bridge
- Transmission wires
E.g. electric and telephone wires and these wires are left sagging in order to allow for contraction during cold days which may cause them to break.
- House roofs
Some house roofs are made with steel girders that expand on hot days; rollers are fixed at one end of roof allowing for free expansion and contraction.
Liquids expand when heated but different liquids expand at different rates. This can be show as bellow.
When equal amounts of water and ethanol are heated with the same amount of heat as shown above, the level of ethanol rises than that of water. This is because the ethanol expands more than water
Also rate that liquids expand more than solids when equally heated. This is because molecules in liquids will therefore expand more easily since less energy is needed to separate them.
THIS VIDEO EXPLAINS MORE ABOUT EXPANSION IN LIQUIDS
EXPANSION IN WATER
The flask is put in a hot water bath s the water level in the glass is then observed.
Initially, there is a momentary drop in level, eventually rises and water may even pour out through the tubing, depending on the temperature of the water bath.
When the flask is first dipped in the hot water. It expands, causing the level of the water to fall slightly. When the water is heated however, its expansion is much more.
This compensates the volume change in the flask and the level of water rises beyond the original level.
ANOMALOUS EXPANSION OF WATER
Taking an example of rice at -100c when the ice is heated it will expand like any other solid up to 00c.
At 00c it melts as it contracts after all the ice has melted to liquid form. The contraction continues up to 4oc as one continues to heat.
All this point, the volume is at its minimum.
Further heating the liquid will continue expanding normally.
VARIATION OF VOLUME WITH TEMPERATURE
GRAPH OF VOLUME AGAINST TEMPERATURE
Between AB: ice is expanding as temperature increase.
BC: Ice melts as it contracts at constraint temperature of 0o c
CD: water contracts as temperature increases
DE: water expands as temperature increases
All point D, water has its minimum volume. This is at 40c.
Between AB, the density of ice reduced as I temperature increases. Between BC; the density increase as the ice melts at constant temperature of 00c.
At CD the density of water decreases as temperature rises [abnormal]. Beyond D, the density reduces as temperature increases.
The density of a substance reduced on heating
– When water is cooled it contracts but between 00c and 40c. It expands instead of contracting.
BIOLOGICAL IMPORTANCE OF THE ANOMALOUS EXPANSION OF WATER
It helps preserve aquatic life during very cold seasons [winter].
Aquatic animals e.g. fish can exist in water below the ice level. This is because as water at the top of the pound cools, its density increases so it sinks.
This happens until the water get to a temperature of 40c further cooling of water on top will instead cause expansion thereby reducing the density.
The water does not sink then. Since water is a poor conductor of heat not all heat from the bottom will.
The water at the bottom therefore remains at 40c while that on top cools to ice below 40c.
The diagram below shows the variation of the water temperature in a frozen pond.
EXPANSION IN GASES
Gases much than liquids or solids and to show that gas expand, the following experiment is used.
A round bottomed flask is filled with a rubber bang [cork] in a glass tube.
A beaker of water is placed on a flat surface such that the glass tube from the flask leads into it.
Rub your hands toughly together hold the flask
Bubbles of air seen coming out of the glass into the water. Then the hands are removed and the flask left to cool, water is seen to rise into the tube.
The heat produced by the hands is enough to excess air escapes in form of bubbles into the water though the tube.
On cooling the air in the flask contracts causing the pressure inside the flask to reduce. The water is therefore forced to rise in the tube.