Sastry CBSE

Viva Questions with Answers on Determination of Melting point

1. Define melting point.
Ans. It is defined as the constant temperature at which the solid and the liquid phases of sub¬stance coexist.
2. How is the determination of melting point useful ?
Ans. It helps us to :
(i) identify unknown substances ;
(ii) know whether a compound is pure or not.
3. How does the determination of melting point help us know about the purity of the compound ?
Ans. Melting point indicates the purity of a substance. If a substance contains moisture or some other impurity, then its melting point is usually lowered. A sharp melting point indicates a pure substance.
4. What is sharp melting point ?
Ans. Melting point of a solid is said to be sharp if it melts completely within a range of 1°C.
5. Why do pure solids possess sharp melting point ?
Ans. A pure solid has same force of attraction between particles at different places and hence melts at a constant temperature.
6. What is the effect of impurities on the melting point of solids ?
Ans. Impurities lower the melting point of a solid.
7. Can we heat the capillary tube directly for the determination of melting point ?
Ans. No, because direct heating would result in uneven and fast heating.
8. Can any other liquid be used in place of liquid paraffin to determine the melting point ?
Ans. Yes, concentrated H2S04 or silicone oils can be used to determine the melting point.
9. Why is the melting point of benzamide more than that of acetamide ?
Ans. Benzamide and acetamide contain same functional group, but the molecular mass of benzamide is more than that of acetamide. As a result benzamide has stronger intermolecular forces and hence has higher melting point.
10. Why different solids have different melting points ?
Ans. Melting point depends upon intermolecular forces existing in the solid state. Since different solids have intermolecular forces of different strength, their melting points are different.

Chemistry Lab ManualNCERT Solutions Class 11 Chemistry Sample Papers

Determination of Melting point

The melting point of a substance may be defined as the temperature at which the substance changes from the solid state to the liquid state. It is a very useful physical constant because a pure substance melts at a definite temperature and has a sharp melting point while an impure substance has a lower melting point and melts over a wide range. Therefore, determination of melting point is a very convenient method to check the purity of a solid substance. Moreover, melting point determination can be used to identify a substance by comparing its melting point with the melting points of known substances.

Experiment 

To determine the melting point of the given solid substance.

Requirements

100 ml beaker, thermometer, iron stand, clamp, tripod stand, stirrer, thin-walled capillary tube 8 to 10 cm long and 1 to 2 mm diameter, spatula. Liquid paraffin.

Procedure

1. Powder the crystalline substance. Take a capillary tube and seal its one end by heat¬ing (Fig. 3.1). For filling the substance make a heap of the powdered substance on the porous plate. Push the open end of the capillary tube into the heap. Some substance will enter into it. Now tap the sealed end of the capillary tube on the porous plate gently. Fill the capillary tube up to 2-3 mm.

2. Attach the capillary tube to a thermometer which is immersed in a bath of liquid paraffin. The surface tension of the bath liquid is sufficient to hold the capillary tube in position.

3. Heat the beaker slowly and go on stirring the liquid in the beaker so that the temperature remains uniform throughout. For this, a glass loop stirrer is moved up and down. When the temperature is within 15° of the melting point of the pure substance, the flame is lowered. Now, the temperature is allowed to rise slowly.
4. The temperature is noted when the substance starts melting. The temperature is noted again when it is completely melted. The average of the two readings gives the melting point of the substance.

Precautions

1. Use dry and powdered sample for the determination of melting point.
2. Keep the lower end of the capillary tube and the thermometer at the same level.
3. Packing of the powder should be uniform without any big air gaps in between the solid particles.
4. Heating should be gradual and the bath should be stirred regularly to maintain uniform temperature.
5. The bulb of the thermometer and the capillary sticking to it should not touch the side or the bottom of the beaker.
6. Do not use rubber band for attaching the capillary tube to the thermometer.

Observations

Temperature at which the unknown substance begins to melt = t₁°C
Temperature at which the substance completely melts = t₂°C
Melting point of the unknown substance = \(\left( {\frac{{{t_1} + {t_{2\:}}}}{2}} \right)\) °C

Table: Melting Points of Some Organic Compounds

Chemistry Lab ManualNCERT Solutions Class 11 Chemistry Sample Papers

Viva Questions With Answers On Basic Laboratory Techniques

Chemistry Lab ManualNCERT Solutions Class 11 Chemistry Sample Papers

1. Why is a Bunsen burner provided with air holes ?
Ans. To regulate the supply of air.

2. What type of flame would you use for general heating purpose ?
Ans. A non-luminous oxidising flame is used for general heating as it gives maximum heat due to complete combustion of hydrocarbons.

3. What is the use of a fume cup-board ?
Ans. It is used to perform those experiments which involve the production of poisonous gases or vapours.

4. Why is a broad flame used for bending a glass tube ? ’
Ans. If a narrow flame is used, folds are formed at the bend.

5. Why does glass not possess a sharp melting point ?
Ans. Glass is an amorphouse solid. It does not have a regular arrangement of constituent particles. Hence, it does not have a sharp melting point.

6. Which type of glass softens readily, soda lime glass or borosilicate glass ?
Ans. Soda lime glass.

7. Why is it required to round off the freshly cut edges of glass tube ?
Ans. Freshly cut edges of glass tube are sharp. They might injure fingers while handling.

8. Why should the tube be rotated while heating ?
Ans. The tube is rotated while heating in order to ensure uniform heating from all sides.

9. Why is the red hot tube bent slowly ?
Ans. Red hot tube is very soft. It might flatten if it is bent suddenly. Slow process of bending prevents flattening of glass tube.

10. What is the role of glycerine in the process of boring ?
Ans. Glycerine is used to lubricate the borer. This gives a smooth hole on boring.

11. Why should the diameter of the borer be less than the diameter of the tube to be inserted in the hole ?
Ans. This is done to ensure that the tube fits tightly in the hole.

12. What type of glass is preferred for drawing out a jet ? 
Ans. Soda lime glass (soft glass) is used for drawing out a jet because it has lower melting point and hence softens easily.

13. Why is the glass tube of smaller diameter choosen for drawing out a jet ?
Ans. In order to draw a jet from tube of larger diameter the tube will have to be stretched too far which is not convenient.

14. What is the use of wash bottle ?
Ans. It is used for getting a thin stream of water required for washing or transferring a precipitate.

 

 

CBSE Chemistry Basic Laboratory Techniques

Chemistry Lab ManualNCERT Solutions Class 11 Chemistry Sample Papers

In the chemical laboratory a student is required to carry out from simplest operations like bending of glass tube, cutting glass tube, boring a cork, to complex process of analysing sub-stances qualitatively and quantitatively. A general acquaintance with such operations thus becomes obvious before taking up actual experiments. As most of these processes involve heat¬ing so knowledge of using a burner is essential.

Bunsen Burner

It is a common heating device used in laboratory and consists of following parts :
1. Base, made of cast iron. It keeps the burner in a stable upright position.
2. Gas-inlet tube. It fits horizontally into the side of the base and can be connected to the gas tap through a rubber tube.
3. Nipple, made of brass rod and has a fine pin-hole running through it. At its lower end, the nipple is screwed into the base. At the upper end, it carries the burner base.
4. Burner tube, a metallic tube with two opposite air holes near its lower end. It is screwed to the nipple and carries the air regulator.
5. Air adjusting disc is a metallic ring that loosely fits on the lower end of the burner tube. It is pierced with two holes that exactly correspond to the two air holes of the burner tube. It can be rotated to regulate supply of air into the burner tube by par¬tially or wholly closing the air holes.

Working of Burner

The rubber tubing is connected to the gas tap and the burner is lighted. As the gas escapes through the nipple, there is a fall of pressure. As a result of which air is sucked in through the air holes. The mixture of air and combustible gas bums at the top with a flame. Depending upon the quantity of air mixed flame can be luminous or non-luminous.
Oxidising flame or non-luminous zone is hottest. It is this portion that should be used for the purpose of heating. Luminous zone is the brightest part of the flame. It is reducing in character and is used for reducing process, such as in charcoal cavity test, match stick test and borax bead test of some radicals.

Cutting a Glass Tube

Cutting a glass tube is primarily required for making U-shaped tubes, delivery tubes and other purposes. Hence a student must know how to cut glass tube of required length for specified purpose without injuring hands. For cutting a glass tubing, proceed in the following manner :
1. Select a glass tubing free of cracks.
2. Place it on the bench, hold it firmly and make a single deep scratch with triangular file. Do not apply too much pressure.
3. Place the thumbs on each side of the scratch at equal distances from it. Apply gentle pressure and give a quick bending motion towards you until it breaks smoothly (Fig. 2.2).
4. Sometimes, the ends of the tube are not smooth and might bruise your fingers (Figs. 2.2 and 2.3). These can be further smoothened by rotating the ends in a flame for 2-3 minutes. The edges will be smoothened (Fig. 2.3). Allow it to cool while holding in your hand.

Precautions
1. Make a single deep scratch at the desired length with one stroke of the file.
2. To avoid injury hold the glass tube with the help of a thick piece of cloth.
3. Do not heat the end for long time. It may seal the end or make it narrower.

Bending a Glass Tube

For bending a glass tube proceed as follows :
Hold the glass tubing between the thumb and fingers, introduce it lengthwise in the luminous flame of burner. Keep the tube rotating till it softens.

2. Now apply gentle pressure so that it bends by its own weight. When the desired angle is formed, remove the tubing from the flame.
3. Place the bent limb on the asbestos sheet. Press it gently so as to make it coplanar.
Allow the tubing to cool.

Precautions

1. Select a glass tube of sufficient length to keep your hands safe from heat. Do not try to bend very small glass tubes of the lengths less than 20 cm.
2. While heating, the glass tube should be rotated in order to ensure uniform heating.
3. Never bend the glass tubing by force. By doing so, the tubing may break.

Drawing a Glass Jet

1. Take a delivery tube of required length and diameter. Hold it with both hands and place it lengthwise in flame (Fig. 2.5).

2. Keep rotating the tube so as to ensure uniform heating, continue heating till it softens.
3. Take the tubing out of the flame and gently pull the two ends apart. The middle portion is drawn out to a thickness of about 2 mm.
4. Cool and cut the narrow portion with a triangular file and two jets will be obtained. Finally round the ends of the jets by heating in a flame for a short while.

Precautions

1. While drawing a jet, pull apart the two ends of the red-hot tube slowly so that it becomes thin unformly.

Boring a cork

Boring a cork is required for setting up an apparatus for the preparation of gas and for carrying / out distillation etc. Above all, it is required for fitting up a wash bottle. For perfect boring of the
cork, the following steps are involved :
1. Softening of the Cork. It is essential as a cork gets hard on keeping. In order to soften a cork, wet it with water. When it becomes more flexible and does not crack readily, then press it in a cork-presser which is a mechanical device and if it is not available, simply press the wetted cork under your shoes after wrapping the cork in a piece of paper.

2. Selection of the Borer. Choose a borer slightly smaller in diameter than that of the tube to be fitted in the cork. This will ensure tight fitting of the tube.

3. Boring of the Cork. Place the cork on the table with its narrow end upward. Mark the position of the borer on both the sides of the cork to ensure straight hole. Holding the cork ‘tightly with left hand, apply force on the borer with a twisting motion. Apply some glycerine to the borer if it is a rubber cork. Glycerine acts as lubricant for the hard rubber cork. When half of the cork has been bored, take the borer out and reverse the cork. Start the process of boring taking care that the borer remains vertical throughout. Remove the borer after the cork has been bored from one face to the other. Remove the pieces of the cork inside by inserting the needle.

Note. For fitting up a wash bottle, it is necessary to bore two holes in the cork. The two holes are bored in the same way as done for single hole but with a precaution that the two holes should not be very close to each other. If the distance is very small, then the thin cork layer may break.

Fitting the Glass Tube in the Bore. Wet the cork with water. Wet the end of the tube also with water. Hold the cork in one hand say left hand and tube in the right hand. It should be noted that the tube should be held closely from the wetted end. Insert the tube into the bore giving a rotatory motion as shown in Fig. 2.9.

Precautions

1. Select bores of diameter slightly smaller in size than that of the tube to be inserted in the hole.
2. Make a mark on both sides of the cork.
3. To obtain a smooth hole, drill half the hole from one side and another half from the other side of the cork.
4. Since the rubber is hard, the end of the tube’ to be inserted is usually dipped in caustic soda solution or glycerine before fitting in the hole.

Wash Bottle

A wash bottle is a container in which distilled water is taken. With the help, of wash bottle a fine stream of water can be obtained for washing the precipitate and for other purposes. It has the shape as shown in Fig. 2.10.

A flat-bottom flask of 500 ml is taken. Appropriate cork with two bores is fitted into it. The two tubes, one bent at an angle of 120° and second at 60°, are passed through the two bores. This is done in a manner so that the upper portions of the two tubes lie in a straight line as shown in the Fig. 2.10. The upper portion of the 120° angled tube is held in mouth whereas a jet is fitted to the tube angled at 60°. On blowing out air with the mouth through one tube, water comes out from the other tube with force as shown in Fig. 2.11.

Precautions
1. The edges must be rounded off.
2. The longer arm of the tube bent at 60° should be only very slightly above the bottom of the flask so that it can be used even when it contains only a small amount of water.
3. All connections must be air tight.

Polythene Wash Bottle

Now-a-days polythene wash bottles are preferably used in the laboratory. It consists of flexible plastic material bottle, fitted with a plastic tubing having a jet at its outer end. On squeezing the bottle fine stream of water comes out of the jet. It can be used to give washings or to remove the precipitates from a beaker etc.

Cleaning Of Glass Apparatus

In order to get good results, apparatus must be cleaned properly before use. Sometimes washing with simple water serves the purpose but if the apparatus is greasy etc. then rinsing with cone. HCl or HNO₃is recommended. It is then freely washed with water under the tap. Chromic acid, prepared by dissolved 5 g of potassium dichromate in 100 ml of cone. H₂S0₄, is another reagent which can be used for removing grease and dirt from the apparatus.

Caution. Chromic acid is very corrosive, therefore, physical contact with it should be avoided.

 

Introduction to CBSE Class 11 Chemistry Lab Manual

Hypothesis become theories and theories attain rank of laws after withstanding rigorous experimental tests. Feasibility of a process is confirmed in the laboratory. Qualitative and quantitative analyses give complete chemical picture of the substance. It is with these considerations in mind we proceed to learn what is there in a chemistry laboratory.

Chemistry Laboratory

A chemistry laboratory is a workshop for chemists. Here students learn the techniques of the preparation, identification and estimation of chemical substances. Before starting experiment, a student must know from where to get the apparatus required for the given experiment and the placement of the chemicals to be used. A student must know the proper use of each equipment and the precautions to be observed while working in the laboratory. A chemistry laboratory is provided with the following fittings with which the student must become familiar.

1. Demonstration Table
Before starting experiment, the teacher gives instructions and demonstrates the concerned experiment on demonstration table. In chemistry laboratory, no seats are made available to the students, so students stand around demonstration table and note the instructions from teacher.
2. Students’ Working Table
A number of wooden or concrete tables are provided for working. Each seat is provided with :
(а) Reagent shelves. Reagents or chemicals to be used are placed on the reagent shelf. These are the reagents which are commonly used. For example, all dilute and concentrated acids such as H₂S0₄, HCl, HNO₃, etc. and bases like NaOH, NH₄OH, etc.
(b) Sinks and water taps. A sink and a water tap is fitted between every two reagent shelves. On either side of the sink, usually two taps are fitted for supply of water.
(c) Gas taps. These taps are fitted on the seats for supply of petrol gas to the burners. Sometimes kerosene is used for producing gas in place of petrol.
3. Side Shelves
Mostly there are two big shelves fitted on the walls of the laboratory. Reagents and chemicals, which are less frequently used, are placed in these shelves. Sometimes solid chemicals are placed in a separate shelf.
4. Fume Cup-board
There is at least one fume cup-board in the comer of the laboratory. All experiments giving out poisonous gases Or vapours are performed in this cup-board.
5. Balance Room
It is a small room attached to each laboratory. Here, a number of balances are kept for weighing the substances. .
6. Exhaust Fans
Two exhaust fans are provided at the two corners of the laboratory for the removal of the poisonous gases and vapours from the laboratory.

Common Laboratory Apparatus

The apparatus which is commonly used by Xlth class student is described below :
1. Beakers. Beakers of different sizes such as 150 ml, 200 ml made of soft glass or corning glass. Beakers are used for taking various liquids.
2. Test Tubes. Test tubes of different sizes are available. Small test tubes used for salt analysis known as centrifuging tubes and boiling tubes are also available.
3. Conical Flask. It is used in volumetric analysis for carrying out titration.
4. Funnel. It is used for filtration or for pouring solutions.
5. Measuring Flask. It is used in quantitative analysis when we have to prepare a solution with a particular volume. There are flasks of 50 ml, 100 ml and 250 ml capacity. There is a mark on the stem of the flask upto which the liquid is taken to complete the volume.
6. Glass-Rod. It is used for stirring purposes. It is also used as an aid for transferring the liquid into the funnel.
7. China Dish. It is a small vessel made of porcelain. It is used in crystallisation, for concentrating a solution.
8. Wire Gauze. It is placed above the flame of the burner so that the glass vessel being heated does not touch the flame directly and hence is prevented from breaking.
9. Tripod Stand. It is used for supporting a china dish or a beaker so that it can be heated from below.
Other apparatus with which a student must familiarize are test tube holder, test tube brush, crucible tongs, spatula, watch glass, clamp stand, burette, pipette, water bath, sand bath and centrifugal machine.

Instructions To Work In Laboratory

To work in the laboratory, a student must follow the following rules :
1. A student must have a practical note-book, rough note-book for instructions, a pen or pencil, a laboratory coat and other equipment such as a platinum wire, fractional weights as required.
2. Always come prepared for the experiment. This will help in understanding the experiment better.
3. Always listen to the teacher’s instructions carefully and note down the important
points and precautions to be followed. .
4. After the instructions, collect the apparatus from the laboratory assistant in queue.
5. Thoroughly clean the apparatus to be used.
6. Do only the experiments assigned, unallotted experiments should not be done.
7. Do your experiment honestly without caring for the final result. Record the observa¬tions on a rough note-book instead of writing on the pieces of paper.
8. Plan your work so that it is finished in the stipulated time.
9. Be economical with the reagents. Only small quantities of the reagents are to be used.
10. Handle the glass apparatus very carefully. In case of any breakage, report it to your teacher at once.
11. Dispose of all waste liquids in the sink and allow water to run for sometime by opening the water tap.
12. Keep your seat clean. If an acid or other corrosive chemical is spilled, wash it off with water.
13. Clean your apparatus after the experiment and return it to the laboratory assistant.
14. In case of any injury or accident or breakage of the apparatus, report it to the teacher immediately.
15. Wash your hands with soap after the experiment.

Some Important Precautions

To avoid unnecessary risk or injury during laboratory work, the students are advised to observe the following precautions :
1. Do not touch any chemical with hand as some of them may be corrosive.
2. Never taste a chemical. It may be poisonous.
3. Do not place the chemical on the palm of your hand.
4. Do not keep the reagent bottles open.
5. Do not roam here and there in the laboratory without work.
6. Do not put any object into the reagent bottle.
7. Do not bring inflammable liquids such as alcohol, ether near the flame.
8. Do not take the reagent from the shelf to your seat.
9. Do not disturb the arrangement of reagents placed on the shelf.
10. Do not use cracked glass apparatus such as beakers for heating purposes.
11. Do not keep water tap running when not required.
12. Do not throw solid waste materials like filter paper pieces, test-tube pieces, etc. in the sink. Throw them in the waste box only.
13. Do not heat beakers or china dish directly on flame. Always make use of wire gauge.

Practical Note-Book

All the experiments that are conducted in the laboratory are recorded in a practical note-book. It is compilation of whole work done by the student, so it must be well maintained, protected from mechanical and chemical damage. For keeping up-to-date record of experiments following points should be kept in mind :

1. The name of the experiment should be entered along with the date of carrying out that experiment.
2. Requirements should be mentioned next to the title given.
3. Theory and principle of the experiment should be given in precise manner.
4. This should be followed by procedure in which experiment is to be conducted. Then a summary of precautions to be taken care are mentioned. Finally mention the general calculations for the experiment.

If we make a table of the points to be written on left hand and right hand side of the note­book, it will look somewhat like the one given below :

Keep following points in consideration regarding your practical note-book :

1. Do not tear pages from note-book.
2. Do not over write if a mistake has been committed in recording, put a line over it and write the correct word or figure again.
3. Number the pages of your note-book.
4. Complete the index, indicating the experiment, its serial number, page number on which it is written.
5. Keep your note-book neat and tidy and covered with brown paper.

First Aid Emergency Treatment In The Laboratory

A chemistry laboratory encompasses different types of chemicals, apparatus. Any lack of attention on the part of student may cause accident. Accidents may occur by chance also. In any case prompt action should be taken to give first aid to the victim and then should be hospitalised if the need be. The probable accidents and their first aid emergency treatment are given below :

Go to the doctor after getting first aid.

Chemistry Lab ManualNCERT Solutions Class 11 Chemistry Sample Papers