It is a state of matter in which the constituent particles are loosely bound by intermolecular forces. Liquid change their shape with a fixed volume.
Typical Properties of Liquids
Some typical properties of liquids are given below:
2) Boiling Point
3) Vapour Pressure
4) Surface Tension
5) Freezing Point
6) Density etc.
Define and Explain Evaporation Experimentally.
It refers to the conversion of a liquid to a gas or vapour at all temperatures but less than the Boiling point of the liquid is called Evaporation or vaporization.
In evaporation, we have to examine the movement of molecules in liquid. The molecules of a liquid are not motionless. The molecules which have low kinetic Energy move slowly, while others with high K.E move faster. This is clearly observed in an open container of Ether or Acetone. The volume of the liquid gradually decreases and finally no more liquid is left behind. This is because of Evaporation. Thus this spontaneous change of liquid into itsvapours is called Evaporation and it continues at all temperatures.
- Evaporation is a Cooling Process?
- Liquids Evaporate faster when heated?
- Evaporation Causes Cooling
Evaporation Causes Cooling reason is that when high energy molecules leave the liquid and low energy molecules are left behind, the temperature of the liquid falls and heat moves from surrounding to liquid as a result temperature of surrounding also decreases. Hence- Evaporation is a cooling process and lower the temperature process of liquid.
- ii) Liquids Evaporate faster
Higher the temperature, faster is the rate of evaporation and vice versa. This is because added heat increases the Kinetic Energy of the molecules. Hence liquids evaporate faster when heated.
The pressure exerted by the vapours of a liquid in equilibriumwith its liquid at a given temperature is called vapourPressure.
Let us consider a liquid enclosed in a container. Air is evacuated and the vessel is sealed. The liquid then starts evaporating and its molecules collide with each other. Some of the molecules are recaptured by the liquid phase. This process is called Condensation. When two opposing process proceed exactly the same rate then the system is said to be in a dynamic state of equilibrium and the number of molecules in the vapour phase exerts a definite pressure which is called the vapour pressure of the liquid as shown in the figures.
Effect of Temperature on Vapour Pressure
Vapour pressures of some liquid at various temperatures.
Table: Vapour pressure of some liquids at various temperatures
Vapour Pressure (kPa) of Several Substances at Various temperatures
|0 C||20 C||40 C||60 C||80 C||100 C|
We conclude from the table that the V.P of liquids changes with temperature. This is because an increase in temperature of a liquid increases the K.E of the molecules and as a result vapour pressure increases.
The temperature at which vapour Pressure of a liquid becomes equal to the external or atmospheric Pressure i.e 760mm Hg or 101.325 KPa at sea level is called Boiling Point.
The B.Point of some of the liquids are given below
|B. Point (C)||100||61||35||56||78||119|
When a liquid is heated in an open container, average K.E of the molecules, increases and atmosphere exerts pressure on the liquid surface. Therefore, the temperature of the liquid gradually increases. This heating process overcome the attractive forces and increases the V.P of the liquids. At a certain temp, the V.P of the liquid becomes equal to the external or atmospheric Pressure. At this stage, bubbles of vapours from throughout the liquid, rise to surface and escape into the air and liquid starts boiling. This is called B.P of the liquids. The Graph clearly shows the variation in vapour Pressures at 101.325 KPa of four liquids with Boiling Points.
Variation in vapour pressures with temperature of four liquids.
Effect of External Pressure on Boiling Point
Boiling Points depend upon the External Pressure over the surface of the liquid. At Sea level, External Pressure is 1 atm = 760mm Hg = 101.325 KPa. As we go at higher altitude i.e a top of Mount Everest, the external pressure decreases to 34 KPa. So at sea Level, the normal B.P of water is 100°C at 101.325KPa while at higher altitude at Mount Everest, water boils at 70°C with 34 KPa external Pressure. Hence B.P is high at high External Pressure and vice versa which is clearly shown in the figures below.
In Solid State atoms, molecules or ions are very closely packed having definite shape and volume. They are in compressible and do not flow but vibrate about their fixed positions as shown in the figure.
The typical properties of solids are:
- Melting point/melting
- Freezing Point/Freezing
- Sublimation etc.
Melting and Freezing
During heating, the particles of a solid start vibrating with greater Kinetic Energy. This makes the molecules a little bit apart from each other. This process increases the volume of solids. On future heating a stage reaches when the particles leave their fixed positions and start moving in the form of a liquid state as shown in the figure. This is called melting.
The temperature at which a solid turns into a liquid is called melting point.
On cooling, the liquid freezes. Thus freezing of a liquid is the reverse of melting. So the temperature at which a liquid changes into the solid is called Freezing Point.
Themelting and freezing of substances occurs at the same temperature. At this
temperature, the liquid and solid substances are in equilibrium each other.
The Melting Point of a solid depends on the strength of attractive forces that hold particles together in the fixed positions. Stronger the forces higher will be the melting point. Ionic solids comparatively having high M.P than covalent solids.
The vaporization of a solid substance into the vapour phase without passing through the liquid phase is called sublimation.
Iodine, Benzoic Acid, Ammonium, Chloride naphthalene etc.
When impure sample of Iodine molecule is heated on sand bath. After some time dark violet black crystals of Iodine deposit the underside of the glass water placed on the top of a beaker containing Solid Iodine. The iodine vapour Sublimates from Iodine crystals in the bottom of the beaker and Condenses to form crystals on the glass watch as shown in the figure.
Sublimation of violet-black crystals of iodine
Types of Solids
On the basis of structure the solids have been divided into two classes based on their macroscopic appearance.
- a) Amorphous Solids
- b) Crystalline Solids
- Amorphous Solids
Amorphous is derived from Greek and “Amorphos” which means without shape. Hence Amorphous Solids are those in which atoms, ions or molecules are not arranged in a definite pattern, e.g. glass, Plastics, rubber etc..
Amorphous solids do not melt at a definite temperature but gradually soften when heated.
- Crystalline Solids
The solids in which atoms, molecules or ions are arranged in a regular repeating three- dimensional well- ordered pattern are called as crystalline solids. Crystals have regularshape. Particles forming the crystals are packed in a very exact and ordered pattern as shown in figure below.
Cubic crystalline shape ofNaCl
The phenomenon of the existence of an element in different forms which have different physical properties but same chemical properties is known as ALLOTROPY and different forms will be called ALLOTROPES or ALLOTROPIC FORMS.
Allotropes of Carbon
Carbon has many Allotropic forms which are divided into two classes.
- Crystalline Allotropic forms: Diamond Graphite and Bucky Ball
- Non-crystalline Allotropic forms e.g: coal, coke, charcoal,Lamp black etc
Crystalline Allotropes of Carbon
Carbon has three crystalline allotropes. The arrangements and properties are given below:
It is the hardest known Substance in diamond; each carbon atom is covalently bonded to four other C-atoms which are arranged in the form of tetrahedron. Diamond is used for cutting glass and polishing hard surface because of rigid compact array as shown in the figure.
In Graphite, Carbon atoms are arranged in layers of hexagonal arrays. In these layers, each C-atoms is joined covalently by weak each Vender Waal’s forces. These layers slip over each other and makes Graphite Soft.
Graphite is used as electrodes, lubricant in machines, and black pigment.
- Bucky Ball
It is new allotropic form which consists of forty to hundred carbon atoms. These atoms are arranged in a hollow cage like structure. Simplest molecule of Bucky Ball is made up of sixty C-atoms. Carbon atoms are arranged in Pentagons (Five member ring) and Hexagons (six member ring) just like a soccer Balls shown in the structure with the formula C60.
Allotropes of Phosphorus
Phosphours can exist in at least six different solid allotropic forms, three are common i.e white, Red and black phosphorus Phosphorus is a non-metal but here two are given.
- White Phosphorus
It is a very reactive, poisonous, volatile, waxy allotrope which is soluble in benzene and carbon disulphide. It exist in the form of tetra atomic molecules (P4) and form a tetrahedral structure as shown below:
- Red Phosphorus
It is much less reactive and poisonous than white Phosphorus. The tetra atomic molecules of red Phosphorus combine to form macromolecules in long chains.
Allotropes of Sulphur
Sulphur consists of molecules that contain eight atoms, S8covalently bonded with each other. Sulphur exist two crystalline allotropes i,e. Rhombic and monoclinic sulphurs while one non crystalline (Amorphous) Sulphuri,e Plastic sulphur.
Compare the three Physical States of Matter.
Distinction between the three states of matter on the basis of particle model
|1.||Shape and volume||Solids posses definite shape and definite volume||Liquid do not have definite shape but definite volume.||Gases have neither a definite shape nor a definite volume.|
|2.||Compress-ibility||Solids are least compressible. It is due to the presence of small empty space between their molecules.||It is slightly more that in solids.||Gases are highly compressible.|
|3.||Energy||Molecules possess least energy.||Molecules possess moderate values of energies.||These molecules are highly energetic and have lot of energy in them.|
|4.||Molecular Motion||The molecules possess vibratory motion.||Molecules have linear as well as vibratory motion and are restricted to the boundary of the liquid||Molecules have complete freedom of motion. They move in all directions.|
|5.||Intermolecular forces||Mutual forces of attraction between the molecules are maximum.||Forces of attraction between the molecules are small.||Forces of attraction between molecules are almost negligible.|
|6.||Packing of particles||The molecules are very closely packed||The molecules loosely held.||The molecules are at a very large distance apart.|
|7.||Diffusion||No tendency to diffuse||Slow tendency to diffuse||High tendency to diffuse|
|8.||Density D = m/v||High||Less than that of solids||Very low densities|
SOCIETY, TECHNOLOGY AND SCIENCE
- Scientists use the power of reasoning to explain their observation. For instance when a balloon is filled with air, it expands. A scientist would explain it by saying that air molecules are free to move inside their container. There is no attractive or repulsive force between the molecules. As a result gas expands until it takes the shape of its container. Therefore, air expands to fill the interior of the balloon evenly.
- In early 1600s, Galileo argued that suction pumps were able to draw water from a well because of the force of vacuum” inside the pump. After Galileo’s death, the Italian mathematician and physicist E. Torricelli proposed another explanation. He suggested that the air in the atmosphere has weight. The force of atmosphere pushing down on the surface of water drives the water into the suction pump when it is evacuated. In 1946 Torricelli invented a device called barometer. He measured atmospheric pressure is 760mm Hg. Torricelli’s work soon caught the attention of British scientist Robert Boyle. He modified barometric tube into a J-shaped tube. By adding mercury to the open end of the tube, he trapped a small volume of air in the sealed end. He studied what happened to the volume of the air as he added more mercury to the open end. Boyle’s from the studies discovered the pressure-volume relationship. J-tube was further modified and another device known as manometer was developed that can measure the pressure of any gas. This means instrumentation improves as science progresses.
- Freeze-dried foods are light-weight and conveniently re-constituted by adding water. When salt is applied on meat it draws out considerable amount of water from the meat. After this meat is frozen and placed in a chamber attached to a vacuum pump. By lowering the pressure below the vapour pressure of ice, the ice crystals sublimate and the meat is dried without the loss of its flavour. Dried meat needs no refrigerator because bacteria such as salmonella which cause food poisoning cannot grow on salt and in the absence of moisture. Thus curing with the salt helps preserve meat.
- Explain why volume of a gas decreases on increasing pressure on it at constant temperature?
Answer According to Boyles Law volume and pressure are inversely proportional to each other at constant temp. High pressure on gas molecules increases the intermolecular forces as a result of which volume of gas molecules decreases.
- How does temperature effect vapour pressure of a liquid?
- Water boils at 120°C in a pressure cooker, why?
Pressure Cooker is a closed container in which on heating B.P of water increases which help to increase the external pressure. A liquid can be made to boil at any temperature by changing the external pressure. So water boils at 120°C in a pressure Cooker because inside the pressure Cooker, external pressure increases.
- Is evaporation a cooling process?
Yes. evaporation is a cooling process. For more detail see question 14 (i)
- Can you make water boil at 70°C?
Yes. A liquid can be made to boil at any temperature by changing the external pressure. Lower the external pressure, lower, will be the boiling point of water. Water boils at 1000C with 532 mm Hg Pressure.
- Express the pressure 400mm Hg in kPa?
760mmHg = 101.325 Kpa
400 mm Hg of presuure =
= 53.33 KPa
Hence 400 mmHg = 53.33Kpa
Q12. The air in a perfectly elastic balloon occupies 855cm3, during the fall when the temperature is 20°C. During the winter, the temperature on a .particular day is -10°C, the balloon occupies 794.39cm3. If the pressure remains constant. Show that the given data proves the volume temperature relation according to the Charles Law.
V1 = 885cm3
T1= 20°C + 273 = 293K
V2 = 794.39cm3
T2 =-10°C + 273 = 263K
According to Charles Law = =
3.02 cm3K -1= 3.02 cm3K -1
Hence it proves Charles law
Q13. In the past, gas volume was used as a way to measure temperature using devices called gas thermometers. An experimenter obtains following data from gas thermometer.
|Volume (dm3)||Temperature (°C)|
According to Charles Law = = =
V1= 2.7dm3 T1 = 0°C + 273 = 273 K
V2 = 3.7dm3 T2= 100°C + 273 =373K
V3 = 5.7dm3 T3 = 300°C + 273 = 573K
by putting the values
0.00989 dm3 K-1 = 0.00992dm3 K-1 = 0.00994 dm3 K-1
9.89 cm3 K-1 = 9.92cm3 K-1= 9.94 cm3 K-1
Q.14 In automobile engine the gaseous fuel-air mixture enters the cylinder and is compressed by a moving piston before it is ignited. If the initial cylinder volume is 990cm3. After the piston moves up the volume is 90cm3. The fuel- air mixture initially has a pressure of 1. 0 atm and final pressure 11.0 atm. Do you think this change occurs according to the Boyles law.
V1 = 990cm3 V2 = 90cm3
P1 = 1 .0atm P2 = 11 .0 atm
According to Boyles Law
P1 V1 = P2V2
Putting the values
- atm x 990cm3 =11.0 atm x 90cm3
990atm cm3 = 990 atm.cm3
Q15. A sample of neon that is used in a neon sign has a volume of 1500cm3 at a pressure of 636 torr. The volume of the gas after it is pumped into the glass tube of the sign is 1213.74cm3 when it shows a pressure of 786 torr. Show that this data obeys Boyle’s law?
V1= 1500cm3 P1= 636 torr
V2 = 1213.74cm3 P2= 786 torr
According to Boyel’s Law
P1V1 = P2V2
636 torr x 1500cm3 = 786 torr x 1213.74 cm3
954000 torr cm3 = 954000 torr cm3
Q16. Instrumentation changes as science progresses, comment on it?
The search of man for collecting knowledge and integrating it is called Science. So true knowledge and its integration can be progressed more with the help of instruments. Some examples will prove the truth.
- Exact heat changes during chemical reaction can be measured with the help of digital thermometer.
- pH of solution, urine and blood can be measured exactly by pH-meter not by pHpapers
- Stethoscope uses doctor to listen heart beats and breathing
- Barometer used for measuring air pressure easily
- Mass spectrometer used to find out the atomic mass of an atom
- Geologist easily detect the presence of oil and gas under earth with the help of instruments.
- Micro organism easily seem by micro scope for better analysis
- 1st date moon easily seen by powerful telescope
- In medicine, use of instruments plays a vital role in the micro analysis of blood, urine, cancer, breaking bones etc. In short, Instrumentation changes as science progresses.
ELT ASSESSMENT EXERCISE 5.1
Related To Gaseous State
- A student obtained following data in an experiment at 20 C° Explain pressure- volume relationship using this data according Jo Boyle’s Law.
According to Boyles Law
Pressure x Volume = Constant
0.350 atm x 0.707dm = 0.248 atm. dm3
0.551 atm x 0.450dm3 = 0.248 atm. dm3
762 atm x 0.325dm3 = 0.248 atm. dm3
0.951 atm x 0.261 dm3 =0.248 atm. dm3
1.210 atm x 0.205 dm3= 0.248 atm. dm3
Thus the calculated results agrees with the Pressure-Volume relationship according to the Boyles Law.
- Ammonia gas is used as refrigerant 0.474 atm (P1) pressure is required to change 2000 cm3 (V1=2 dm3) sample of ammonia initially at 1.0 atm (P2) to 4.22 dm3 (V2) at constant temperature. Show that this data satisfies Boyle’s Law?
According to Boyles Law
V1= 2000cm3 = 2dm3
P1 = 0.474 atm
P2 = 1.0 atm
V2 = 4.22dm3
Putting all the values in above equation
P1V1 = P2V2
0.474 atm x 2 dm3 =1.0 atm x 4.22 dm3
0.948 atm. dm3≠ 4.22 atm dm3
The given data does not satisfied the Boyles Law
ELT ASSESSMENT EXERCISE 5.2
- At Mount Everest, the atmospheric pressure is 34 KPa which is about 8850 m above the sea level. While at Murree atmospheric pressure is 99.298 KPa which is comparative near to sea level. So at high altitude, the atoms pressure decreases, obviously B.P of water also decreases. Hence B.P of water at Mount Everest is 70°C while at Murree98°C.
- An egg in boiling water cooked longer while camping at an elevation of 0.5 Km in the mountain because at high altitude as compare 10*sea level, external pressure decreases which will decrease the Boiling point of the water. So it will take longer time than it does at home.
SELF ASSESSMENT EXERCISE 5.3
- When you put nail polish remover on you palm, you feel a sensation of coldness.
- Wet clothes dry quickly in summer than in winter.
Nail Polish remover contains an organic solvent “Acetone”. It contains weak intermolecular forces and having high V.P. These high energy molecules escape from your palm and one feels sense of cooling. The other reason is that heat of body is used up to evaporate nail polish remover.
Greater the temperature of surrounding, greater the K.E possessed by the molecules and greater the escaping tendency from the surface of the liquids and Vice Versa. In summer, surrounding temperature is higher as compared to winter. So, wet clothes dry quickly in summer than in winter.
SELF ASSESSMENT EXERCISE 5.6
Sodium chloride, an ionic compound, has a high melting point of 801°C. Whereas molecular solid such as ice has relatively low melting point of 0°C. Explain this difference.
In Sodium chloride, an ionic crystalline solid has strong electrostatic forces of attraction are present between positively charged ions with many negatively charged ions and Vice Versa while in molecular solids i.e ice, weaker intermolecular forces are present. So M.P of sodium chloride (801°C) is higher than molecular solids (0°C) that is ice.