Chemistry FBISE SSC-1 Keybook Chapter 5

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  1. Q. Why do mountaineers carry with them the pressure cooker?

Ans.     Mountaineers carry pressure cooker with them because external pressure is low on the mountains. Due to which food does not cook easily. Cooker helps to increase the internal pressure and food cooks properly.

 

  1. Do mountaineers need to cook food quickly?

Ans.     Yes, mountaineers need to cook food quickly because there is shortage of time and energy resources.

 

  1. Food cooked in ordinary kettles at higher attitude remains semi-cooked why?

Ans.     Since at higher attitude external pressure becomes low due to which boiling point decreases. That’s why food remains semi-cooked.

 

  1. Q. How does pressure cooker help in proper cooking of food?

Ans.     Pressure cooker helps increase in external pressure on water. Which in turn increases the boiling point of water. Due to which food cook properly.

 

  1. Why pressure cooker saves time in the kitchen?

Ans.     Pressure cooker increase boiling point of water, this saves the time in kitchen.

 

  1. Freeze-dried foods are light-weight and can be conveniently re-constituted by adding water. How?

Ans.     When food are dried water convert from liquid to gaseous phase. Due to which food become light. And when we add water it becomes to its original state.

 

  1. Why gases are highly compressible?

Ans.     Gases are highly compressible. This is due to the presence of large empty spaces between the gas molecules. On applying pressure distances between the gas molecules decreases, therefore its volume decreases.

 

  1. Discuss:

(a)        Pressure:

Force exerted by a gas on unit area of a container is called its pressure. Gas exerts an equal pressure in all directions. The SI Unit of pressure is the Pascal (Pa):

1 atm   = 760 mm Hg

= 760 torr

Or

1 atm   = 101.325 kpa

= 1.01325 x 105 pa

(b)        Mobility:

Gas can flow and can be transported through pipes. They also leak more rapidly out of small hole. This is due to tendency of a gas to expand and fill the entire available space.

 

(c)        Density:

Gases have relatively low densities under normal conditions. This is because the molecules are much farther in the gas. When a gas is cooked its density increases because its volume decreases.

For example density of oxygen at 20oC is 1.4 g/dm3 and at 0oC is 1.5/dm3

 

  1. State Boyle’s law?

Ans.     Boyle’s law states that the volume of a fixed amount of a gas at a given temperature is inversely proportional to the applied pressure.

P ∝

P =   x constant

P x V = Constant

Example 5.1

            Ethene is used as anesthetic gas. The pressure on 2.5 dm3 of ethane changes from 1.05 to 2.10 atm. The volume of ethane becomes 1.25 dm3 if the temperature remains constant. Explain this change using Boyle’s law.

Solution:

According to the Boyle’s law, product of pressure and volume is constant at any two sets of conditions.

P x V = constant

Calculate P x V for the two sets of condition and compare

Before change P1 x V1             = 1.05 atm x 2.5 dm3

= 2.625 atm dm3

After change P2 x V2    = 2.1 atm x 1.25 dm3

= 2.625 atm dm3

                                                P1 V1    =          P2 V2

Thus the calculated result agrees with the pressure – volume relationship according to the Boyle’s law.

1.

 

A student obtained following data in an experiment at 20ᵒC:

P (atm)V (dm3)
0.3500.707
0.5510.450
0.7620.325
0.9510.261
1.2100.205

Explain pressure-volume relationship using this data and the Boyle’s law

Solution:

According to the Boyle’s law, product of pressure and volume is constant at any set two sets of the conditions. Calculate PxV for the two sets of condition and compare.

 

P (atm)V (dm3)P x V (atm.dm3)
0.3500.7070.350×0.707=0.247
0.5510.4500.551×0.450=0.247
0.7620.3250.762×0.325=0.247
0.9510.2610.951×0.261=0.247
1.2100.2051.210×0.205=0.247

Thus the calculated result agrees with the pressure-volume relationship according to the Boyle’s law.

 

  1. Ammonia gas is used as refrigerant 0.474 atm. Pressure is required to change 2000 cm3 sample of ammonia initially at 1.0 atm to 4.22 dm3 at constant temperature. Show that this data satisfies Boyle’s law.

Solution:

According to the Boyle’s law product of pressure and volume is constant at any two sets of conditions.

P x V = Constant

Convert cm3 into dm3 and calculate P×V for the two sets of condition and compare.

P1         =          0.474 atm

P2         =          1.0 atm

V1        =          2000 cm3 = dm3 = 2dm3

                V2        =          4.22 dm3

Before change P1 x V1 =          0.474 atm x 2 dm3

=          0.948 atm dm3

After change P2 x V2   =          1 atm x 4.22 dm3

=          4.22 atm dm3

P1 x V1 ≠          P2 x V2

Thus the calculated result does not agree with the pressure-volume relationship according to the Boyle’s law.

 

  1. State Charles’s law?

Charles’s law state that the volume of given mass of a gas varies directly with absolute temperature at constant pressure.

V ∝ T

V = T x Constant

= Constant

Example 5.2

Table shows data of volume of a gas and its temperature for the given mass of a gas at 900 mm Hg

 

Temperature (oC)Volume (cm3)
0107.9
5109.7
10111.7
15113.6
20115.5

Explain volume-temperature relationship using Charles’s law

Solution:

According to the Charles’s law, ratio of volume to absolute temperature is constant for any set of conditions.

Convert oC temperature to Kelvin temperature by adding 273.

T = oC + 273

Find for each set of conditions and compare.

 

Temperature (oC)Volume

(cm3)

Temperature (K)
0 oC107.9273=0.3952
5 oC109.7278= 0. 3946
10 oC111.7283= 0. 3947
15 oC113.6288= 0. 3944
20 oC115.5293= 0. 3942

 

The ratio is fairly constant. Thus volume of the gas varies directly with the absolute temperature as stated by Charles’s law.

 

  1. A chemist obtained data shown in the table in an experiment at 1 atm. Explain volume temperature relationship using Charles’s law
Temperature (ᵒC)Volume

(cm3)

25ᵒC117.5
30ᵒC119.4
35ᵒC121.3
40ᵒC123.3

 

Solution:

According to the Charles’s law, ratio to absolute temperature is constant for any set of conditions.

Convert oC temperature to Kelvin temperature by adding 273

T =       oC + 273

Find for each set of conditions and compare

 

Temperature (oC)Volume

(cm3)

Temperature (K)
25 oC117.5298=0.3943
30 oC119.4303= 0. 3941
35 oC121.3308= 0. 3938
40 oC123.3313= 0. 3936

The ratio is fairly constant. Thus volume of the gas varies directly with the absolute temperature as stated by Charles’s law.

 

  1. A bacterial culture isolated from sewage produces 36.4 cm3 of methane (CH4) gas at 27 oC and 760 mmHg. This gas occupies 33.124 cm3 at 0 oC and same pressure. Explain volume and temperature relationship from this data.

Solution:

According to the Charles’s law, ratio of volume to absolute temperature is constant for any set of conditions.

Convert oC temperature to Kelvin temperature by adding 273

Find for each set of conditions and compare

Temperature (ᵒC)Volume

(cm3)

Temperature (K)
27ᵒC36.4300= 0.1213
0ᵒC33.124273= 0.1213

The ratio is fairly constant. Thus volume of the gas varies directly with the absolute temperature.

 

  1. A perfect elastic balloon filled with helium gas has a volume of 1.25×103 dm3 at 1.0 atm and 24 oC on ascending to a certain attitude where temperature is 15oC the volume of balloon becomes 1.208×103 dm3. Show that this data satisfies the Charles’s law.

Ans.     According to the Charles’s law, ratio of volume to absolute temperature is constant for any set of conditions.

Convert oC temperature to Kelvin temperature by adding 273

T = ᵒC + 273

Find for each set of conditions and compare

Temperature (oC)Volume

(dm3)

Temperature (K)
25 oC1.25×103298= 4.1946
15 oC1.208×103288=4.1945

The ratio is fairly constant. Thus volume of the gas varies directly with the absolute temperature as stated by the Charles’s law.

 

Give reasons:

(i)         When you put nail polish remover on your palm, you feel a sensation of coldness.

Ans.     Nail polish remover evaporate very quickly. Since evaporation is a cooling process, so we feel a sensation of coldness.

 

(ii)        Wet clothes dry quickly in summer than in winter?

Ans.     According to kinetic molecular theory molecules which have greater kinetic energy than average can break away from the surface. Due to which in summer molecules leave surface faster than in winter.

 

Look at figure and predict the boiling points of the four liquids at normal atmospheric pressure.

 

Ans.

LiquidBoiling point
Chloroform60 oC
Ethanol80 oC
Water100 oC
Ethanoic Acid120 oC

 

  1. Why there is difference between boiling point of water on top of the Mount Everest and in a pressure cooker?

Ans.     A liquid boils when its vapour pressure is equal to the pressure exerted on the liquid by its surroundings.

At Mount Everest atmospheric pressure is 34 kPa, so water boils at 70 oC. In pressure cooker atmospheric pressure is 2 atm (202.65 kPa) so water boils at 120 oC.

 

  1. The boiling point of water on the top of Mount Everest is 70 oC, while at Murree 98 oC. Explain this difference.

Ans.     A liquid boils when its vapour pressure is equal to the pressure exerted on the liquid by its surroundings.

At Mount Everest atmospheric pressure is lesser than on Murree. Due to which there is difference between boiling points.

 

  1. If you try to cook an egg in boiling water while camping at an elevation of 0.5 km in the mountain, you will find that it takes longer than it does at home. Explain why?

Ans.     A liquid boils when its vapour pressure is equal to the pressure exerted on the liquid by its surroundings. At sea level water boils at 100 oC. At elevation of 0.5km atmospheric pressure become low, due to which boiling point also become low. That’s why egg takes longer time to cook at an elevation of 0.5 km in the mountain than it does at home.

 

  1. Define distillation?

Ans.     Distillation is the process in which a liquid is heated to vapourize it and the vapours are cooled to condense them back to the liquid in a different container.

 

  1. Define:

(a)        Melting Point:

The temperature at which a solid turns into a liquid is called melting point.

(b)        Freezing Point:

The temperature at which a liquid changes into the solid is called freezing point.

 

  1. Sodium Chloride, an ionic compound, has a high melting point of 801 oC, whereas molecular solid such as ice has relatively low melting point of 0oC. Explain this difference.

Ans.     Sodium chloride, an ionic compound has very strong attractive forces due to which its melting point is high i.e. 801oC. Whereas molecular solid such as ice has low attractive forces that’s why they have relatively low melting point of 0 oC.

 

  1. Define sublimation.

Ans.     Sublimation is a process in which solid directly changes into gas.

 

  1. Quartz is the crystalline form of silicon dioxide (SiO2). It is a hard, brittle and colourless solid. When quartz is heated above its melting point (about 100 oC) and cooled rapidly, an amorphous solid called quartz glass results. Figure shows two dimensional representation of quartz glass and quartz crystal. Identify each. Give reasons.

 

(a)                                                        (b)

(a)        Quartz crystal because particles are arranged orderly, repeating three-dimensional pattern.

(b)        Quartz glass because particles are not arranged in a definite pattern.

 

  1. Differentiate between amorphous and crystalline solids by writing appropriate sentences in the empty boxes.

 

 

  1. Define Allotropes.

Ans.     The different forms of an element in the same physical state and phase are called allotropic form or allotropes.

 

  1. What do you know about bucky ball?

Ans.     In bucky ball, 60 carbon atoms form a hollow sphere. Carbon atoms are arranged in pentagons and hexagons just like a soccer ball.

 

  1. Explain the allotropic form of phosphorus?

Ans.     There are two allotropic form of Phosphorus.

  • White Phosphorus (ii) Red Phosphorus

White Phosphorus

The white phosphorus consists of tetra-atomic molecules, P4 that are not bonded to each other.

Red Phosphorus

In red phosphorus, P4 molecules are bonded to one another in long chains.

 

  1. Explain the allotropic form of sulphur?

Ans.     Sulphur consists of molecules that contain eight atoms, S8. Sulphur also exist in several crystalline and an amorphous form. Rhombic and monoclinic forms are its important crystalline allotropes. Plastic sulphur is the amorphous form.

 

Copy and make comparison of physical states of matter by writing appropriate sentences in the empty boxes.

 

SolidLiquidGas
Has a definite shape and volumeHas definite volume but no definite shapeHas no definite shape and volume
Particles are closely packed together and have strong intermolecular forcesParticles are not so closely packed together but have strong intermolecular forcesParticles are not close packed together and have not strong intermolecular forces
Particles have fixed position and they only vibrateParticles slide past one anotherParticles move about freely and randomly in all direction

 

Review Questions

Q.1      Encircle the correct answer

(i)         Which statement about the particles of a solid is not correct?

  1. They move at great speeds
  2. They are arranged in regular patterns.
  3. There is a very little space between the particles.
  4. The forces of attraction between the particles arc strong.

(ii)        The boiling points of sonic elements arc given below:

a.     Argon

b.     Nitrogen

c.     Oxygen

d.     Xenon

-186°C

-196°C

-183°C

-108°C

A mixture of the above gases at -210°C was heated by 20°C. Which of the element will still be in the liquid state?

  1. Oxygen only
  2. Argon, oxygen and xenon
  3. Argon, nitrogen and xenon A
  4. Xenon only
  5. Oxygen and Xenon

 

(iii)       The following table shows the melting and boiling points of four substances. Which substance is a solid at room temperature?

SubstanceMelting Point (0°C)Boiling Point(0°C)
A-186-183
B-6120
C0100
D36130

(iv)       In which of the following processes speed of the particles decreases?

  1. melting
  2. boiling
  3. sublimation
  4. condensation
  5. evaporation
  • Consider the following table:
Sr. No.XYX/Y
11.002730.0037
21.373730.0037
31.734730.0037
42.105730.0037

Which of the following relationships does this table represent?

  1. x ∝ y b. x = y c. x ∝ 1/y         d. x=1/y

(vi)       A liquid boils when its vapour pressure., becomes equal to

  1. 760 cmHg b. 1Pa c. 101.325kPa d. 0.1atm

(vii)      Acetone has characteristic fragrant odour and is used to make finger nail polish. It melts at -94°C and boils at 56°C. What is the physical state of acetone at 25°C and 1atm?

  1. gas b. liquid           c. solid             d. cannot be predicated

(viii)     Water normally boils at 100°C, but it is possible for water to boil at room temperature. What variable would you have to change to do this:

  1. increase external pressure
  2. decrease external pressure
  3. decrease temperature
  4. none of these.

(ix)       Bromine has a melting point of -7°C and a boiling point of 59°C. What is the physical state of bromine at 100oC

  1. gas
  2. liquid
  3. solid
  4. cannot be predicted

 

(x)        Which is not the property of crystalline solids?

  1. have well defined shape
  2. have orderly arrangements of particles
  3. have repeating three dimensional arrangement of particles
  4. are generally soft

Answers:

i. Aii. Biii. Div. Dv. A
vi. Cvii. Bviii. Bix. Ax. D

 

Q.2      Give short answers

(i)        Explain why volume of a gas decreases on increasing pressure on it at constant temperature.

Ans.     As pressure increases, the molecules of the gas compressed, reducing the amount of spaces between molecules. This is due to the presence of large empty spaces between molecules which decreases the volume.

 

(ii)        How does temperature effect vapour pressure of a liquid?

Ans.     Vapour pressure is the pressure exerted by a liquids vapour. When temperature increases the kinetic energy of molecules become more and more. Hence when temperature increases vapour pressure increases and when temperature decreases vapour pressure decreases.

 

(iii)       Water boils at 120 oC in a pressure cooker, why?

Ans.     Pressure cooker is equipped with a valve that controls the pressure inside the pot. This valve generally exerts a pressure of 2 atm. Therefore, the valve does not allow water vapours to escape until the pressure inside the pot reaches 2 atm. Because vapour pressure of water becomes 2 atm when the temperature reaches 120 oC. So water boils at 120 oC in a pressure cooker.

 

(iv)       Is evaporation a cooling process?

Yes evaporation is a cooling process. Molecules which have greater kinetic energy than average can break away from the surface. The molecules with the highest kinetic energy escape first. The molecules in the liquid have a lower average kinetic energy. Hence temperature of liquid fall down.

 

(v)        Can you make water boil at 70 oC

Yes we make water boil at 70 oC by decreasing atmospheric pressure. When atmospheric pressure is 101.325K pa water boil at 100ᵒC. When atmospheric pressure decreases to 34 kPa water boils at 70 oC.

 

(vi)       Express the pressure 400 mm Hg in kPa?

760 mm Hg      =          1 atm

760 mm Hg      =          101,325 kPa

I mm Hg          =          101.325 kPa

760

400 mm Hg      =          10.325 kPa x 400

760

400 mm Hg      =          53.23 kPa

 

Q.3      Explain the effect on the volume of a gas by change in the

(a)        Pressure:

According to Boyle’s law,       V ∝

The volume of a gas at constant temperature is inversely proportional to pressure. Hence when pressure increases volume decreases and when pressure decreases volume increases.

 

(b)        Temperature:

According to Charles’s law,

V∝T

Volume of the gas varies directly with the absolute temperature. Hence when temperature increases volume increases and when temperature decreases volume also decreases.

 

Q.5      Explain the following properties of gases.

 

(a)        Diffusion:

The movement of molecules from a higher concentration to a lower concentration is called diffusion.

Molecules of gas diffuse in all direction.

 

(b)        Effusion:

The escape of gas molecules through the hole one after the other without collision is called effusion.

 

Q.5      Explain the terms

(a)        Evaporation:

Conversion of a liquid to a gas or vapour at all temperature is called vaporization or evaporation.

Evaporation is a cooling process. Liquid evaporate faster when heated.

 

(b)        Vapour pressure:

The pressure exerted by the vapours of a liquid in equilibrium with its liquid vapour pressure. The equilibrium is a state when rate of vaporization and rate of condensation is equal to each other but in opposite direction.

 

Liquid       Vapours

(c)        Boiling point:

The temperature at which vapour pressure of a liquid becomes equal to the external or a atmospheric pressure is called boiling point. At sea level liquid boil when its vapour pressure becomes 760 mm Hg or 101.325 kPa.

 

Q.6      Explain the effect of external pressure on boiling point.

Ans.     Liquid boil when their vapour pressure is equal to the pressure exerted on the liquid by its surroundings. So when external pressure increases boiling point also increases and when external pressure decreases boiling point also decreases.

 

Q.7      Differentiate between amorphous and crystalline solids.

 Amorphous SolidsCrystalline Solids
1.An amorphous solid lacks ordered arrangement of its particlesA crystalline solid is composed of orderly, repeating three-dimensional arrangement of particles.
2.Does not have a well-defined arrangement of its particlesHave a well-defined arrangement of its particles
3.Have non-repetitive arrays of particlesHave repetitive arrays of particles
4.Melts over a wide range of temperatureHave sharp melting point.

 

Q.8      Explain the allotropic form of any two solids.

Ans.     Allotropic form of Solids

            Diamond

In diamond each carbon atom is bonded to four others, creating a rigid compact array. This makes diamond hardest known substance.

Graphite:

In graphite carbon atoms are arranged in layers of hexagonal arrays. Weak bonds exist between the layers that allow them to slide over one another. This makes graphite soft.

 

Q.9      Plots of vapour pressure versus temperature of four liquids are given in the following figure.

(a)        Find the boiling point of each liquid when the atmospheric pressure is 1 atm.

 

Liquid   Boiling Point
   Chloroform     61 oC
   Ethanol     78 oC
   Water     100 oC
   Ethanoic Acid     119 oC

(b)        At what temperature ethanol will boil when the atmospheric pressure is 51 kPa.

At 101.3 kPa boiling point of ethanol = 78 oC

1 kPa   = _78_

101.3

51 kPa =   _78_   x 51

101.3

51 kPa =   39.3 oC

 

(c)        How can you make water to boil at 80ᵒ C?

At 1 oC = 760 mm Hg

100

80 oC    = 760 x 80

100

80 oC = 608 mm Hg

When pressure is 608 mm Hg water boils at 80 oC.

 

(d)        At what temperature Chloroform will boil when pressure is 50 kPa

At 101.3 kPa boiling point of Chloroform = 61 oC

1 kPa   = _61__ oC

101.3

50 kPa = __61_ x   50 oC

101.3

50 kPa =   30.11 oC

When external pressure is 50kPa Chloroform boils at 30.11oC.

 

(e)        Can you boil Chloroform at 0 oC

            Chloroform boils at 61oC =273+61 = 334K, and 0oC =273K.

At 334K temperature, pressure =101.3kPa

At 1K temperature, pressure =             101.3 kPa

334

At 273K temperature, pressure = 101.3 ×273kPa

334

= 82.82 kPa

To boil chloroform at 0 oC, external pressure should be 82.82kPa.

 

(f)        Predict the boiling point of Chloroform at 600 mm Hg.

At 760 mm Hg boiling point of Chloroform = 61 oC

1 mm Hg     =   61 oC

760

600 mm Hg =  61 oC x 600

760

600 mm Hg =  48.16 oC

Boiling point evaporation lower the temperature of a liquid.

 

Q.11    Why does evaporation lower the temperature of a liquid?

According to kinetic molecular theory molecules which have greater kinetic energy that average can break away from the surface. The molecules with the highest kinetic energy escape first. The molecules in the liquid have a lower average kinetic energy. Hence temperature of liquid falls down.

 

Q.12    The air in a perfectly elastic balloon occupies 885 cm3, during the fall when the temperature is 20 oC. During the water, the temperature on a particular day is -10 oC, the balloon occupies 794.30 cm3. If the pressure remains constant. Show that the given data proves the volume temperature relation according to the Charles’s law.

 

Ans.     According to Charles’s law, ratio of volume to absolute temperature is constant for any set of conditions.

= Constant

Convert ᵒC temperature to Kelvin by adding 273

T = t oC + 273

 

 

Temperature (oC)Volume

(cm3)

Temperature (K)
20 oC88520+273=293=3.0204
-10 oC794.39-10+273=263= 3.0204

 

The ratio is fairly constant. Thus volume of the gas varies directly with the absolute temperature as stated by the Charles’s law.

 

Q.13    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(oC)
2.70 oC
3.7100 oC
5.7300 oC

Show that gas thermometer obtained results according to Charles’s law

Ans.     According to Charles’s law, ratio of volume to absolute temperature is constant set for any conditions.

= constant

Convert oC temperature to Kelvin by adding 273

 

Temperature (oC)Volume

(cm3)

Temperature (K)
0 oC2.70+273=273=0.0099
100 oC3.7100+273=373= 0.0099
300 oC5.7300+273=573= 0.0099

 

The ratio is fairly constant. Thus volume of the gas varies directly with the absolute temperature as stated by the Charles’s law.

 

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 990 cm3. After the piston moves up, the volume is 90cm3. The fuel-air mixture initially has a pressure of 01 atm and final pressure 11.0 atm. Do you think this change occurs according to the Boyle’s law?

Ans.     According to Boyle’s law, product of pressure and volume is constant at any set of conditions.

P x V = Constant

Calculate P x V for the two sets of condition and compare.

Convert pressure cm3 to dm3 dividing by 1000

P1 = 990cm3 = dm3 = 0.99 dm3

                                P2 = 90cm3 =  dm3 = 0.090dm3

Before change P1 x V1 = 1 atm x 0.99 dm3 = 0.99 atm dm3

After change P2 x V2 = 11 atm x 0,090 dm3 = 0.99 atm dm3

Thus the calculated result agrees with the pressure-volume

relationship according to the Boyle’s law

 

Q.15    A sample of neon that is used in neon sign has a volume of 1500 cm3 at a pressure of 636 torr. The volume of the gas after it is pumped into the glass tube of the sign is 1213.7 cm3 when it shows a pressure of 786 torr. Show that this data obeys Boyle’s law.

Ans.     According to the Boyle’s law, product of pressure and volume is constant at any two sets of conditions.

P x V = Constant

Calculate P x V for the two sets of condition and compare

Convert torr into atm dividing by 760 and cm3 to dm3 dividing by 1000

P1 = 636 torr = atm = 0.8368 atm

P2 = 786 torr = atm = 1.0342 atm

V1 = 1500 cm3 = dm3 = 1.5 dm3

V2 = 1213.74 cm3 = dm3=1.21374 dm3

Before change P1 x V1             = 0.8368 atm x 1.5 dm3

= 1.255 atm dm3

After change P2 x V2    = 1.0342 atm x 1.21374 dm3

= 1.255 atm dm3

Thus the calculated result agrees with the pressure-volume relationship according to the Boyle’s law.

 

Q.16    Instrumentation changes as science progresses, comments on it.

Ans.     Scientific instruments play important role in checking and bring change in theories. They provide a great capacity for increasing the power of observation and making induction process easier. For example Torricelli invented a device called barometer. He measured atmospheric pressure by barometer. Robert Boyle modified barometric tube into J-shaped tube. He studied what happened when more mercury added to the open end. This means instrumentation improves as science progresses.

 

 

ThinkTank

  1. Following table shows data collected from an experiment by a student.
Volume (dm3)Pressure (mmHg)
400353.5
320442
240589
200707

Do you think the student collected data carefully or carelessly? Explain.

According to the Boyle’s Law, product of pressure and volume is constant at any two sets of conditions. Calculate P×V for the two sets of conditions and compare the results.

P1×V1 before change      = 353.5atm×400dm3

= 141400atm.dm3

P2×V2 after change          = 442atm×320dm3

= 141440atm.dm3

P3×V3 after change          = 589atm×240dm3

= 141360atm.dm3

P4×V4 after change          = 707atm×200dm3

                                                                                    =141400atm.dm3

Since, P1×V1 ≠ P2×V2 ≠ P3×V3 ≠ P4×V4 , thus the calculated results do not agree with the pressure-volume relationship according to the Boyle’s Law. Thus the student collected data carelessly.

  1. The water level in an aquarium decreases slowly even though the tank does not leak. What change of state is occurring?

Ans. Evaporation:

The volume of the liquid gradually decreases and finally no more of the liquid is left. This is because liquids constantly change into gas or vapours even when the temperature is less than the boiling point of a liquid. In evaporation, some molecules in the liquid break away and enter the gas or vapour state.

  1. What types of attractive forces do you expect between the molecules of HF and HCl?

Ans.      Hydrogen Flouride:

The H-bond in HF is so strong that the molecules remain associated through H-bonding even in the gaseous state.

Hydrogen Chloride (HCl):

Hydrogen chloride atoms will most likely form dipole-dipole forces between slight positive hydrogen end and slight negative end of Chlorine atoms.

  1. Name two substances that are solids at 25o Name two substances that are liquid at 25oC.

Ans.     Magnesium chloride (MgCl2) and silver sulphate (AgSO4) are solids at 25oC. Methanol (CH3OH) and Chloroform(CCl4) are liquids at 25oC.

  1. Identify the process occurring in each of the following:
  2. Mothballs slowly disappear.

Ans.       Sublimation

  1. A cold windshield becomes covered with ice when struck by raindrops.

Ans.       Freezing

  1. An autoclave is used to sterilize surgical equipment. it is far more effective to produce steam by autoclave than steam produced from boiling water in the open atmosphere, because it generates steam at a pressure of two atmospheres. Explain why an autoclave is such an efficient sterilization device?

Ans.     At standard conditions and pressure, (i.e. 1atm) water boils at 100oC, while at 2atm, water boils at 120oC. That is why, in autoclave water boils at higher temperature and produces more steam in less time. Therefore autoclave is more efficient than boiling in open air.

  1. The following table shows the melting points and boiling points of four substances.
SubstanceMelting Point (oC)Boiling Point(oC)
A-123-79
B-1758
C52305
D-6120
  1. Write the physical state of each substance at room temperature and 1 atm.

Ans. The substance A is a gas because both, the melting point and boiling point, are less than the room temperature.

The substance B is a liquid at room temperature

because melting point is lesser than room temperature

but boiling point is higher than the room temperature,

this means the substance has already melted but not

reached its boiling point.

The substance C is a solid at room temperature because both melting and boiling points are greater than the room temperature.

The substance D is a liquid at room temperature because melting point is less than room temperature but boiling point is more than room temperature.

  1. Which substance exists as a liquid for the longest range of temperature?

Ans. Substance D exists as a liquid for the longest range of temperature.

  1. Describe what will happen to the substance B when it is heated from 0oC to 100o

Ans. Substance B will initially be a liquid at 0oC. When it is

heated, temperature will rise till it reaches 58oC. At this

point, temperature will stop rising, and substance will

continue to absorb energy and change its state to gas. Total

energy absorbed by it for changing its state from liquid to

gas is called latent heat of vapourization. After all of the liquid has converted into gas, temperature will rise again till 100oC.

  1. Describe what will happen to the substance D when it is cooled from 100oC to -10oC?

Ans.     Substance D is initially liquid at 100oC. When it is cooled, temperature will fall until it reaches its melting point at -6oC. At this point, temperature fall will stop but liquid will continue releasing energy and change its state from liquid to solid. Total energy it will release to become solid is called latent heat of fusion. After all of it has been converted to solid, it will cool again to -10oC.