Liquid state of matter

Liquid State

Ø  Intermediate state between solid and gas

Ø  Neither completely disordered nor completely ordered

Ø  The constituent particles of a liquid are in a state of continuous motion but at a slower speed than the gas molecules.

Ø  Fluids like gases; diffuse likes gases but at slower rates.

1.       Which is incorrect?

a.       A liquid has a definite freezing point.

b.       A liquid has a definite size.

c.        A liquid has a definite surface tension.

d.       A liquid has molecules held by hydrogen bonding.

Evaporation

Spontaneous changing of liquid into vapour at any temperature

Ø  Surface phenomena

Ø  A fraction of molecules having kinetic energy high enough escape the liquid from the surface.

Ø  Endothermic process - causes cooling

Heat – average kinetic energy of constituent particles of a body

Reason for cooling by evaporation: During evaporation particles with high kinetic energy escape into the vapour state and particles with lower kinetic energy are left in the liquid.

e.g. a drop of ether at room temperature on hand feels cold

-sweating occurs during summer to cause cooling by evaporation

Factor affecting evaporation

·         Nature of liquid (intermolecular force of attraction): Greater the intermolecular force of attraction lower the evaporation

·         Temperature: Higher the temperature higher the evaporation rate

·         Surface area: Higher the surface area higher the evaporation

·         Presence of impurities: The presence of nonvolatile impurities decreases the rate of evaporation.

2.       Evaporation of water is

a.       A process in which no heat change occurs.

b.       A process accompanied by a chemical reaction.

c.        Exothermic change

d.       Endothermic change

1.       Cooling is caused by evaporation. Give reason.                        2

2.       Mention two suitable examples to show evaporation causes cooling. 2

3.       Give reason: evaporation takes place from the surface of the liquid.   1

4.       State the physical principle behind the following phenomenon: A drop of ether on your skin disappears fast and the skin feels cool.                                                                               2

Condensation

- Conversion of vapour into liquid generally on cooling

- Condensation of water vapour in the atmosphere leads to rainfall resulting completion of the water cycle.

Vapour pressure

Ø  The pressure exerted by the vapour of liquid when the rate of evaporation is equal to the rate of condensation at a given temperature in a closed vessel.

Ø  Vapour pressure of water at a given temperature is also called aqueous tension.

Factors affecting vapour pressure

Ø  Nature of liquid

Ø  Temperature (directly proportional)

Ø  Presence of impurities

Non-volatile impurities decrease the vapour pressure of a liquid. Non-volatile particles on the surface reduce evaporation at a given temperature.

5.       Define the term aqueous tension.                                                    1

6.       Define aqueous tension. Why is it subtracted from the total pressure to determine the pressure of a dry gas?   2

7.       What is the effect of temperature on the vapour pressure of a liquid? 1

8.       What happens to the vapour pressure of the liquid if nonvolatile solute particles are dissolved in it?

 

Boiling point

Ø  The temperature at which a liquid boils at a given atmospheric pressure (generally 1 atm pressure)

Ø  A liquid boils when its vapour pressure equals the atmospheric pressure.

Ø  Bulk phenomena

Ø  Temperature remains constant throughout the boiling process. Because the heat supplied during the boiling process is used up by the molecules to change into a vapour state. Or the heat energy is taken by the vapouring molecules (particles) and the temperature liquid body remains constant.

Factors affecting the boiling point

Ø  Nature of liquid

Ø  Atmospheric pressure

 

3.       Boiling point of a liquid depends on all of the following except

a.       Viscosity                                                 c. External pressure

b.       Volume of the liquid taken                                d. Vapour pressure

4.       Which of the following does not affect boiling point?

a.       intermolecular forces                         c. external pressure

b.       nature of liquid                                    d. the initial temperature of the liquid

5.       Vapour pressure of a liquid increases with

a.       increase in intermolecular forces                    c. increase in pressure

b.       increase in polarity of molecules                      d. increase of temperature

9.       What do you mean by boiling point?                                              1

10.   In terms of vapour pressure what do you mean by boiling point?     1

11.   At what condition does a liquid boil?                                              1

12.   Point out the effect of increasing atmospheric pressure on the boiling point of a liquid.           1

13.   Why does the boiling point of a liquid rise with increasing pressure? 1

14.   Does atmospheric pressure affect the boiling point of a liquid? Explain.

15.   Boiling point of a liquid rises with increasing atmospheric pressure. Give reason.      2

16.   Boiling points of liquids P and Q are 70⁰C and 90⁰C respectively. Which of the two has a higher vapour pressure at 71⁰C? Give reason. 2

17.   Explain why the boiling point of water is lower at a higher altitude? 2

18.   Does the temperature of boiling liquid increase, decrease or remain constant even though the heat is continued? Give reason.                                                                                          1

Comparison of Evaporation and boiling

Boiling and evaporation are somewhat similar processes but they differ in many respects. The main differences may be outlined as

Evaporation	Boiling
1.       Evaporation takes place spontaneously in all temperatures.	1.       Boiling takes place only at a definite temperature called boiling point at which the vapour pressure of the liquid is equal to atmospheric pressure.
2.       Evaporation is a surface phenomenon, takes place only at the surface of the liquid.	2.      Boiling is a bulk phenomenon, takes place at the surface as well as below the surface.
3.       Generally evaporation is a silent process.	3.       Generally boiling is a noisy process.

19.   Differentiate between evaporation and boiling.                                2

20.   Write the differences between evaporation and boiling.

 

Surface tension

Force acting along the surface of a liquid at a right angle to any line of unit length. Unit: dyne cm^-1(SI system) or N m^-1(CGS system)_.

 

Factors affecting surface tension

Ø  Intermolecular force (nature of liquid)

Ø  Temperature

Effects/Applications of surface tension:

Ø  Falling water drops are spherical

Ø  Capillary action

Ø  Hot/warm food is tastier than cold

Ø  It’s easier to remove dirt from the clothes/dishes with hot water than cold

Ø  Soap/detergents act by increasing the surface tension of water

 

21.   Define surface tension. Write its unit.                                            2

22.   How is the surface tension of a liquid originated?                            2

23.   What is the effect of temperature on surface tension?                       1

24. Raindrops are spherical. Why?

25.   Give reason: falling liquid drops are spherical.                                2

26.   How does surface tension work to form the spherical shape of a liquid drop?

27.   Why is mercury droplet spherical?                                                 2

28.   Name the physical property behind the rise of liquids in a capillary tube.

29.   Write any two physical properties of liquids caused by surface tension.

30.   The meniscus for mercury in a glass tube is concave downward. Explain.

31.   The meniscus for mercury in a glass tube is concave downward. Why?

32.   It is more efficient to wash clothes in hot water than in cold water. Why?

Viscosity

Ø  Internal resistance to the flow of liquid.

Ø  Internal resistance or friction produced by one layer of liquid flowing over an adjacent layer with different velocities.

Ø  Coefficient of viscosity- the force per unit area required to maintain unit velocity gradient.

Ø  Unit of coefficient of viscosity: CGS- dynes s cm^-1 or poise; SI- Newton s m^-2 or Poiseulli

Factors affecting viscosity

Ø  Intermolecular force

Ø  Temperature

6.       Sugar solution is highly viscous due to

a.       Large molecular mass                                         c. Chain structure

b.       High intermolecular H- bond                             d. All of them

7.       When a liquid is heated

a.       Viscosity increases.                             c. Surface tension increases.

b.       Surface tension lowers.                      d. Surface tension is not affected.

33.   What do you understand by the term viscosity?                              2

34.   What do you mean by viscosity?

35.   Define the term coefficient of viscosity.

36.   What is the effect of temperature on viscosity?                                2

37.   Why glycerine is more viscous than water?                                    2

38.   Give reason alcohols can flow easily but honey cant.                        1

Liquid crystals and their applications

In general, liquids are isotropic and crystalline solids are characterized by anisotropic nature. There exist some liquids with properties somewhat in between the conventional liquid and crystalline solids. Such liquids are called liquid crystals. This is also considered the fourth state of matter. In general liquid crystals can flow like a liquid but their constituent particles are arranged in a crystal-like regular structure.

Liquid crystals have many important applications.

·         Their main application is in the liquid crystal displays (LCD) screen of the television, computer monitor, laptops, mobiles, watches, video games etc.

39.   Define liquid crystals and give their important applications.            2

 

8.       Which of the following statements is not true?

a.       A liquid is difficult to compress.

b.       Poise is a unit of surface tension.

c.        Evaporation causes the cooling of a liquid.

d.       Dyne cm^-1 is the unit of surface tension.

*** This is not a complete note. It is just to guide you. It is recommended to study prescribed textbooks along with this material. ***