CBSE Class 9 Science Practical Skills – Melting Point of Ice and Boiling Point of Water

CBSE Class 9 Science Practical Skills – Melting Point of Ice and Boiling Point of Water

EXPERIMENT

AIM
To determine the melting point of ice and the boiling point of water.

OBJECTIVE A
To determine the melting point of ice.

MATERIALS REQUIRED
Ice cubes, filter paper, beaker, wire gauze, tripod stand, burner, thermometer, stirrer, clamp stand.

THEORY
Ice is the solid form of H₂0. Its melting point is 0 °C i.e., the forces of interaction between the molecules in the solid form of H₂0 can be broken down at 0 °C and solid H₂0 gets converted into liquid water.

PROCEDURE

1. Take some ice cubes. Dry them using the filter paper and put them quickly in a beaker.
2. Place the beaker on a wire gauze kept over a tripod stand.
3. Suspend a thermometer, into the ice cubes, with the help of a clamp stand.
   
4. Heat the ice cubes and stir continuously for uniform heating.
5. Note the temperature (t₁) when ice starts melting.
6. Heat continuously till ice melts completely. Note this temperature (t₂).
7. Record your observations in the table.

OBSERVATIONS
Observation Table

Mean value of temperatures =  \(\frac { { t }_{ 1 }+{ t }_{ 2 } }{ 2 }\)
Boiling point of water = ……………..°C.

RESULT
The melting point of ice =………… °C.

PRECAUTIONS

1. Ice should be dry before using for melting point determination.
2. The bulb of the thermometer should be dipped in ice and should be surrounded on all sides with ice.
3. Maintain a uniform temperature, by continuous stirring.
4. Temperature should be measured by keeping eyes in line with the level of mercury.

OBJECTIVE B
To determine the boiling point of water.

MATERIALS REQUIRED
Distilled water, boiling tube, rubber cork with two bores, delivery tube, clamp stand, pieces of pumice stone/ porcelain pieces, beaker, thermometer, burner.

THEORY
H₂0 exists in three different physical states-solid ice, liquid water and water vapour. In the liquid state, the forces of interaction are less and therefore, water exists as a liquid at room temperature. The boiling point of water is 100 °C, i.e. the vapour pressure of water becomes equal to atmospheric pressure at 100 °C at sea level.

PROCEDURE

1. Take about 50 ml distilled water in a boiling tube, and add few pieces of pumice stone/porcelain to it.
2. Fix a cork with two bores in the mouth of the boiling tube and fix a thermometer in one bore and delivery tube in the other.
3. Clamp the tube to the stand.
   
4. Place a beaker below the open end of the delivery tube to collect condensed water.
5. Heat the boiling tube by preferably rotating the flame for uniform heating.
6. Note the temperature (t₁) when water starts boiling.
7. Heat continuously till the temperature becomes constant, and water keeps on boiling. Note this temperature (t₂)
8. Record your observations in the table.

OBSERVATIONS
Observation Table

Mean value of temperatures =  \(\frac { { t }_{ 1 }+{ t }_{ 2 } }{ 2 }\)
Boiling point of water = ……………..°C.

RESULT
The boiling poin of water = ……….. °C

PRECAUTIONS

1. Use distilled water only. Impure water and hard water have boiling point greater than 100°C.
2. The bulb of the thermometer should be slightly above the liquid.
3. Add porcelain pieces before heating to avoid bumping.
4. Heat water by rotating the flame.
5. Note the temperature by keeping the eyes in line with the level of mercury.

INTERACTIVE SESSION

Examiner :
Define boiling point of a liquid.
Examinee:
Boiling point of a liquid is the temperature at which the vapour pressure of liquid is equal to the pressure of the gas above it.

Examiner :
What do you understand by vapour pressure of a liquid?
Examinee:
The pressure that the vapours exert on the surface of the liquid at equilibrium is called vapour pressure of that liquid.

Examiner :
What is the difference between evaporation and boiling?
Examinee:

Examiner :
How does the boiling point of water change at high altitudes as compared to at sea level? Why?
Examinee:
The boiling point decreases at higher altitudes because the atmospheric pressure at higher altitudes is less and less energy is required to make vapour pressure equal to atmospheric pressure.

Examiner :
Two liquids A and B have boiling points equal to 35 °C and 70 °C respectively. Which has a higher vapour pressure? Explain why.
Examinee:
Liquid ‘A’ has higher vapour pressure. A liquid which has a higher vapour pressure would require lesser heat energy, so that the vapour pressure becomes equal to atmospheric pressure.

Examiner :
Out of water and ether, which has a higher vapour pressure?
Examinee:
Ether has a higher vapour pressure. The intermolecular interaction between ether molecules is less and therefore it vapourizes easily and has a higher vapour pressure.

Examiner :
On adding a non volatile solute to water, what happens to its boiling point?
Examinee:
The boiling point of water increases.

Examiner :
Why cooking in a pressure cooker is faster?
Examinee:
In a pressure cooker, the pressure is high and therefore, boiling point of water increases and cooking is faster.

Examiner :
What is the melting point of pure ice?
Examinee:
It is 0 °C or 273 K.

Examiner :
How does addition of a non volatile solute effect the melting point of ice?
Examinee:
The melting point of ice decreases.

Examiner :
Can you change a liquid into vapours below its boiling point?
Examinee:
Yes, through the process of evaporation which is a surface phenomenon. For example, drying of clothes at a lower temperature.

Examiner :
Which out of ionic and covalent solids have a lower melting point?
Examinee:
The covalent solid has a lower melting point.

Examiner :
Why do they have a lower melting point?
Examinee:
It is so because covalent compounds have low intermolecular forces of interaction, which can be broken down easily.

Examiner :
Why do ionic compounds have higher melting points?
Examinee:
Ionic compounds have high melting points due to electrostatic forces of interaction existing between the ions.

Examiner :
Non polar compounds are volatile. Give reason.
Examinee:
Non polar compounds have weak van der Waals’ forces existing between the molecules, and they are therefore, volatile.

Examiner :
Why are ionic compounds crystalline?
Examinee:
Ionic compounds are crystalline in nature as they have ions arranged in a specific three dimensional nature.

Examiner :
Using melting points, how can we characterise a substance?
Examinee:
For a pure substance, the melting point will be sharp, i.e. the compound melts completely within a range of 2 °C.

Examiner :
Would the boiling point of distilled water and hard water be the same?
Examinee:
No, becasue hard water contains non volatile impurities (Ca and Mg salts) and therefore hard water will have a higher boiling point.

Examiner :
For clearing snow, CaCl₂ is used. Why?
Examinee:
CaCl₂ lowers the melting point of snow and therefore, snow melts at a lower temperature as compared to its melting point and can therefore, be cleared off.

NCERT LAB MANUAL QUESTIONS

Question 1:
Why is the bulb of thermometer kept above the surface of water while determining the boiling point of water?
Answer:
While determining the boiling point of water the bulb of the thermometer is kept above the surface of water, as steam contains only water vapour and the effect of any non-volatile impurity, if present in water, will be otherwise nullified.

Question 2:
Why does the temperature remain unchanged until the entire solid changes into liquid even if we are heating the solid?
Answer:
The temperature remains unchanged until the entire solid changes into liquid even on further heating as the heat provided is used to overcome the forces of attraction present between the particles of a solid and to change the state from solid to liquid.

Question 3:
Why do we fix a two holed-cork in the round bottom flask while determining the boiling point of water?
Answer:
A two holed-cork is fixed in the round bottom flask while determining the boiling point of water

1. to allow the water vapour to escape from the flask, otherwise the pressure inside the flask becomes high and may lead to bursting of the round bottom flask and
2. to fix the thermometer and read the temperature when water boils.

PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA

Question 1:
In the experiment, “Determination of melting point of ice”, the reading of the thermometer must be noted when
(a) temperature starts rising
(b) temperature becomes constant
(c) ice starts melting
(d) whole of the ice gets melted.

Question 2:
When we observe the melting of ice, the melting point of ice is a constant temperature at which
(a) both ice and water are present
(b) only water is present
(c) only ice is present
(d) first ice and then only water.

Question 3:
Two students Arpit and Rakshita are asked to arrange the apparatus to determine the boiling point of water. They arranged the apparatus as shown below by figures A and B respectively.

The diagram in which the apparatus is correctly arranged is
(a) A only
(b) B only
(c) both A and B
(d) neither A nor B.

Question 4:
Reema, Archna, Shakti and Ilia made the arrangement I, II, III, IV for determination of boiling point of water. Which one of them has made the correct set up?

(a) I
(b) II
(c) III
(d) IV

Question 5:
When a thermometer is kept in the ice the reading shows that
(а) temperature keeps increasing
(b) temperature keeps decreasing
(c) temperature increases first and then decreases
(d) temperature first decreases and then remains constant at 0 °C.

Question 6:
A student set-up an apparatus for finding the melting point of ice. When half of the ice melted, the temperature shown by thermometer is
(a) more than 0 °C
(b) less than 0 °C
(c) zero degree Celsius
(d) none of the above.

Question 7:
A thermometer has 20 equal divisions between 90 °C and 100 °C marks. A student while determining the boiling point of water finds that the mercury thread becomes stationary at the 19th mark above 90 °C. He should record the boiling point of water as
(a) 90.19 °C
(b) 99.5 °C
(c) 109 °C
(d) 119 °C

Question 8:
A student takes some water in a beaker and heats it over the flame for determining its boiling point. He keeps on taking its temperature readings. He would observe that the temperature of water
(a) keeps on increasing regularly
(b) keeps on increasing, irregularly
(c) first increases slowly, then decreases rapidly and eventually becomes constant
(d) first increases gradually and then becomes constant.

Question 9:
In the determination of boiling point of water, correct reading in the thermometer is noted when
(a) water starts boiling.
(b) whole of the water evaporates.
(c) temperature starts rising.
(d) temperature becomes constant.

Question 10:
At 0 °C or 273 K, the physical state of water is observed as
(a) solid
(b) liquid
(c) vapour
(d) both solid and liquid.

Question 11:
In the determination of boiling points of water, it is advised to put the bulb of the thermometer above the water rather than in water, it is to
(a) reduce the error due to atmospheric pressure
(b) make sure that boiling point obtained is accurate even when water sample contains non-volatile impurities dissolved
(c) reduce the error due to expansions of glass because of heat
(d) obtain the, boiling point accurately even in a shorter time

Question 12:
For the accurate determination of the boiling point of water, we use
(a) tap water
(b) distilled water
(c) boiled water
(d) hard water (solution).

Question 13:
Two compounds A and B have boiling points as 40 °C and 170 °C. The nature of the compounds are respectively
(a) ionic, covalent
(b) ionic, ionic
(c) covalent, ionic
(d) covalent, covalent.

Question 14:
The inferences drawn by the temperature versus time graph are

(а) during melting, temperature of substance does not change
(b) temperature rises after all amount of ice melts
(c) at a specific temperature water starts boiling and temperature remains the same during the conversion of water into steam
(d) all of these.

ANSWER KEY
Multiple Choice Questions/VSA

1. (b)
2. (a)
3. (b)
4. (c)
5. (d)
6. (c)
7. (6)
8. (d)
9. (d)
10. (d)
11. (b)
12. (b)
13. (c)
14. (d)

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