The answer lies in the fact that different substances have different capacities for storing heat. Put a litre of water in a saucepan and heat, it may take a few minutes to boil. Put a metal knife on the same hotplate and it will reach the same temperature much more quickly.
If we were given 1g of both iron and water, we would have a different number of molecules of different type and mass in each sample. Water uses energy to increase the rotation of molecules, internal vibration and bond stretching. Iron atoms use the energy to increase the translational kinetic energy. This means it takes 8 times the amount of heat to raise 1g of water by 1 o C than it does for iron.
Mechanical systems are difficult to achieve accurate results, as there is a loss of energy due to friction that cannot easily be accounted for. The most accurate measurements of the specific heat of water have been made with electrical systems.
SHC of Liquids Thermometer Calorimeter Heating coil Liquid Insulation Stirrer To joulemeter or voltmeter and ammeter
Ten silver spoons, each with a mass of 30g, are removed from a pan of boiling water, quickly dried, and then placed in a pan of water at room temperature (20 o C). The pan contains 500g of water. The temperature rises to 23 o C. What is the specific heat of silver?
Determine the final temperature of a 0.2 kg mass of hot coffee at 90 o C contained in a foam-insulating cup if 0.1 kg of cold water at 10 o C is poured into it? Assume the specific heat capacity of coffee is 4000 J/kg o C.
What will be the final temperature reached when a 250 g rod of copper ( c copper = 385 J/kg o C) is taken from a beaker of boiling water and plunged into 100g of water at 20 o C contained in another beaker?
Add energy to ice and it turns to water. Add energy to water and it turns to steam. The state of matter depends upon its temperature and the pressure that is exerted upon it. To change state, a transfer of energy is required.
A substance can undergo changes of state or phase changes at different temperatures
A change of state from liquid to gas that takes place at the surface of the liquid.
The temperature of any body is related to the average kinetic energy of its molecules. As the molecules move in a random manner, some molecules may collide. Some may lose kinetic energy and some may collide and increase kinetic energy, enough to overcome the attractive forces of their neighbouring molecules.
As the higher kinetic energy molecules have escaped, the average kinetic energy of the liquid has been reduced. This means the liquid left behind has been cooled and there is a corresponding temperature drop.
This is the principle used by evaporative air conditioners and perspiration.
Condensation is the opposite process to evaporation. This is the cooling of a gas to a liquid. When water vapour molecules collide with a cold can of Coke, giving up so much kinetic energy, they condense into a liquid.
Condensation is a warming process. The kinetic energy lost by the gas molecules warms the surface that they strike. A steam burn is more dangerous than a boiling water burn at the same temperature. Steam gives up energy when it condenses to the liquid that wets the skin.
When energy is continually withdrawn from a liquid, molecular motion slows until the forces of attraction becomes enough to cause them to fuse. The molecules then vibrate about equilibrium positions and form a solid. This is called freezing.
If foreign material such as sugar or salt is added to water, the freezing temperature is lowered. The foreign ions get in the way of the water molecules and stop them forming ice molecules, Antifreeze is often used for this purpose.
The thermal energy which a particle absorbs in melting, vaporising or sublimation or gives out in freezing, condensing or sublimating is called Latent Heat because it does not produce a change in temperature
During a change of state, there is no change in temperature until all of the substance has changed state. If we study boiling water and steam that are both at 100 o C, they both have the same average kinetic energy.
The molecules in steam however, has much more potential energy as they are free to move and are not held together. When water turns to steam, no temperature rise is observed as the energy absorbed goes into increasing the potential energy.
The energy required to change 1 kg of a substance from the solid to liquid state is called the latent heat of fusion (L f ) . This also refers to the amount of heat released when a liquid is turned to solid. For water, the L f = 3.34 x 10 5 J.
The water will now increase in temperature at the rate of 1 o C for every 4 kJ of heat added. When the temperature reaches 100 o C, the temperature again remains constant until all of the water is turned to steam.
The energy required to change 1 kg of a substance from the liquid to gaseous state is called the latent heat of vaporisation (L v ) . This also refers to the amount of heat released when a gas is turned to liquid.
The heat required to change the state of a substance can also be expressed mathematically. Q = mL
Pressure of a gas is due to the particles bombarding the walls of the container. Each collision with the wall causes a momentum change (as there is a change in direction). The force on the wall by one molecule = average rate of change of pressure.
If the temperature increases, the average KE of the particles increases
The increase in velocity of the particles leads to a greater rate of collisions and hence the pressure of the gas increases as the collisions with the side have increased
Also the change in momentum is greater, therefore greater force
4. Volume occupied by the gas particles themselves is assumed to be negligible compared with the volume of the container as you can compress a gas.
5. There are no forces acting between the particles themselves or between the particles and the wall of the container except during collisions. There are large distances between particles F g is negligible.
6. Time between collisions is large compared with the time for collisions. There are no forces between particles particles travel in straight lines and constant v and then collide elastically and then changes v . Forces act on particles only during collision collision time is small.