Change in state of mater

Matter exists in three states viz., solid, liquid and gas. The change from one state to another can be brought about by the application or withdrawal of heat.

Change in State of Matter

The water can be in the form of solid ice or liquid water or gaseous steam. The process in which a solid changes into liquid on heating is called melting. For example, ice changes into water. The change of a liquid into a solid on cooling is known as freezing. The process in which a liquid changes into vapour on heating is called vaporisation. e.g., water changes into water vapour or steam. Some materials may change directly from a solid to a gas. This is called sublimation. Solid carbon dioxide changes to carbon dioxide gas as it warms up. Another substance which sublimes is Iodine. When vapour condenses to form a liquid the change of state is called condensation. Steam changes to water as it condenses.

Latent Heat

The latent heat of a substance is defined as the amount of heat absorbed by a unit mass of the substance to change its state without change of temperature. The heat absorbed during the change of state of a substance is used to overcome the force of attraction between the molecules of a substance. The kinetic energy of the molecules does not increase and hence there is no raise in temperature during the change of state of the substance. Two of the more common forms of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting or freezing) and latent heat of vaporization (boiling or condensing). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and to gas.

Cooling due to evaporation

• When we put a little ether or petrol at the back of our hand and wave it around, we observe that the spirit evaporates rapidly and our hand feels very cold. The sprit takes the heat of vaporization from our hand. The hand loses heat and gets cooled. Similarly, water vaporizing from the leaves of the trees cools the surrounding air.
• A liquid evaporates when it changes into gas. Evaporation occurs at the surface of a liquid. During evaporation, only high energy molecules overcome the attraction of their neighbouring molecules and leave the liquid. In this way, the liquid loses its most energetic molecules, while the less energetic molecules are left behind.
• The average kinetic energy of the remaining molecules is therefore, reduced. This results in a fall of temperature of the liquid which gets cooled. The rate of evaporation of a liquid depends on its surface area, temperature and the amount of vapour already present in the surrounding air.
• On a rainy day, wet clothes take longer time to dry, because large amount of vapour already present in the air, slows down the evaporation. Similarly, during high fever, a cloth soaked in cold water is kept on the forehead the water evaporates rapidly and takes heat from the head and the body.
• Dogs keep their tongue usually out in summer. Water evaporates from the tongue and keeps it cool. Water in an earthen pot remains cool in summer. Water comes out of the pores of the vessel and evaporates. Therefore water remains cool in an earthen vessel by evaporation. After sometimes, when the pores get blocked by the dissolved material in water, the earthen pot becomes useless.
• Evaporation of sweat or perspiration from our skin causes a cooling effect.

Fusion of Ice Experiment

We take a test tube with clean water and a thermometer is placed in the test tube. The test tube is placed in a freezing mixture bath. The water level in the test tube is well below the level of the freezing mixture. While stirring water slightly and carefully, the thermometer readings for every 30 seconds are recorded till the temperature falls a few degrees below 0 °C. A graph is drawn by taking time along the X-axis and temperature long the Y axis. The portion AB represents the liquid state. At B the change of state takes place from liquid to ice at 0 °C. At C entire liquid is changed to ice. Here during the change of state the temperature remains constant. Below C it is in the solid state (ice). The flat portion of the graph represents the time during which the water solidifies. Here both solid and liquid states exist together. This is the melting or freezing point. During this time heat continues to be lost from the substance as it changes from liquid to solid but there is no fall in temperature.

When water changes into solid; its volume increases. When a substance melts, heat is gained. When it freezes, heat is lost

Working of Refrigerators

When a liquid evaporates it takes in heat energy and cools its surroundings. When the gas condenses back to a liquid, the latent heat is released. This is used to take heat from inside a fridge, and release it outside. A liquid which evaporates easily is called volatile liquid. Freon is a volatile liquid used in most fridges. The liquid evaporates in the coils around the ice box or cold plate inside the fridge. This causes cooling. The Freon gas formed is pumped away and pressurised in the condenser on the back of the fridge. Here the Freon gas condenses back into liquid. As it condenses it releases the heat energy it has taken in. So heat energy has been taken from food and other things inside the fridge and released outside it.

If we leave the fridge door open, the pump has to work hard and more heat will be released into the kitchen which will eventually become hotter.

Freezing Mixtures

A mixture of compounds that produces a low temperature is called freezing mixture. A freezing mixture consists of powdered ice, common salt and ammonium nitrate. Temperatures lower than 0° C can be produced by mixing certain salts such as Sodium Chloride, Ammonium Chloride, Magnesium Sulphate etc. with ice. When salt is mixed with ice, some ice melts taking heat from the salt. The temperature of the mixture decreases. Now salt gets dissolved in the water formed. The necessary heat for this is extracted from the mixture itself and consequently the temperature of mixture falls below zero. With the freezing mixture of salt and ice in the ratio 1 : 3, temperatures as low as -13°C can be obtained

Latent heat of fusion

The latent heat of fusion of a substance is the quantity of heat required to convert unit mass of the solid at its melting point to the liquid state at the same temperature. The S.I unit of Latent heat is J kg-1.

Ice at 0°C is more effective in cooling a substance than water at 0°C. This is due to the fact that for melting at 0°C each kilogram of ice takes its latent heat of 3.34 x10⁵ J from the substance and hence cools the substance more effectively. On the other hand water at 0°C cannot take latent heat from the substance. This concept is valid for most of the liquids and their solids.

Latent heat of vaporization

Latent heat of vaporization of a liquid is the amount of heat required to convert unit mass of a liquid at its normal boiling point into vapour at the same temperature.

The burns caused by steam are much more severe than those caused by boiling water though both of them are at the same temperature of 100°C. This is due to the fact that steam contains more heat in the form of latent heat (2.26 x 10⁶ J/kg) than boiling water. Here we note that the latent heats of fusion is maximum for ice and latent heat is maximum for steam. Hence steam and ice can be considered to be the best source and sink of heat respectively in a heat engine.

Impact of pressure Melting Points

When we take two pieces of ice and apply pressure and release them; we observe that the two pieces freeze together. This implies that melting point of a substance can be lowered by applying pressure. We take a slab of ice is and a put a metal wire over it. Two equal weights (5 kg) are fixed to its ends. The wire passes through ice slab due to the load applied to it. Just below the wire, ice melts at a lower temperature due to increase in pressure. When the wire has passed, the water above the wire freezes again. Thus the wire passes through the slab and the slab does not split. This phenomenon of refreezing is called regelation.

Here, we have to note down that if a substance contracts on melting, as in the case of Ice, its melting point is lowered by an increase of pressure. If a substance expands on melting, as in the case of a paraffin wax, its melting point is raised by an increase of pressure.

Skates, Sledges and Snowballs

Since the edges of the skates are fine, the pressure applied on ice is sufficient to melt it. Water thus formed due to melting acts as a lubricant and enables the skates to move freely over ice. Due to regelation the water formed is again converted into ice. Thus free motion of skates with good grip is achieved. The same explanation holds good for sledges and snow balls.

Impact of impurities on Melting Points

We put some salt or other impurity into a beaker of water and heat it until it boils. Measure the boiling point and observe that it is above 100°C. It shows that the boiling point of liquid is raised by adding impurities. Again, we take some pieces of ice in a beaker and sprinkle some salt on the ice. Stir until the ice melts and measure its temperature. We observe that it is less than 0°C. The presence of impurity lowers the melting point.

Impact of Pressure on Boiling Points

The boiling point of a liquid is lowered under reduced pressure and increased under increased pressure. The atmospheric pressure is less on the top of a mountain and therefore water boils at a lower temperature. This temperature is too low to cook food properly.

It means that a longer time is required for cooking in hill stations. The time required for cooking vegetables and other foods can be greatly reduced if the boiling point of water is raised. This can be done by the use of a pressure cooker. A pressure cooker consists of a strong vessel of an aluminium alloy or stainless steel sealed so tightly that steam can be confined inside it with a pressure of about 2 atmospheres. The boiling point of water at this pressure will be about 120°C. When foods are cooked under these conditions there is a considerable saving of fuel and time. Since the cooking time is reduced the food value (vitamins and minerals) is retained better. Any possible oxidation of food material is also prevented because cooking takes place in an atmosphere of steam instead of air. The pressure cooker solves cooking problems at high altitudes also.