LAW OF CONSERVATION OF MASS

Explanations that how law of conservation of mass is obeyed in burning of a candle

When a candle burns, it forms carbon dioxide, water and some residue of melted wax. It seems that mass of candle is lost due to chemical change.
If we weigh the candle before burning and weigh all the products after
Burn the candle, we observe that there is no change in mass. We can verify this by Partigon experiment.

 A hard and a long glass tube open at both end is taken. Through the upper part a mixture of quick lime and soda lime is packed over a copper gauge.Through the lower end of the tube a candle placed over a porous cork placed  inside the tube. The entire arrangement (the tube with lime mixture and a candle) is placed on the left arm of the balanced and weighed carefully. The candle is now lighted and is quickly placed on the tube again, closing with porous cork.
After some time, when the candle burnt out completely, it has been found that the left arm of the balanced beam is tilted downward. It shows that there is a gain in mass during burning of the candle. This is because during combustion, carbon – dioxide and water produced are absorbed by the lime and sodalime. Again some oxygen from outside air through the porous of the cork has been used in the combustion increased the weight. Thus we observe that,
Mass of candle + mass of oxygen = mass of carbon dioxide + mass of water vapour
Or the total mass of the product = total mass of the reactants.
Thus, the law of conservation of mass is verified.

A little ash only remains as residue on burning a piece of magnesium wire.
Explain whether mass is conserved in this case.

Burning a piece of magnesium in air (or oxygen) is a chemical change. If we weight
the magnesium and the combined oxygen before reaction and after reaction, then there is no change in their mass.

The following experiment verify the law of conservation of mass.

Experiment : A hard test tube filled with oxygen and a tight fitted stopper is taken .
A small piece of magnesium is introduced in the closed tube contain oxygen and magnesium piece carefully. Now heat the test tube strongly by a Bunsen burner.
Magnesium piece burns brilliantly in the oxygen.

After cooling we find a white residue in the test tube. Weight the test tube again after reaction .We observe that the weight of the test tube before reaction and after complete reaction is the same. The magnesium piece reacts with oxygen form magnesium oxide.
                                                     2Mg + O₂ = 2MgO
Thus the weight of magnesium and oxygen before reaction and the weight of magnesium oxide after reaction is same. This verifies the law of conservation of mass.

MASS AND WEIGHT- DIFFERENCES

Difference between Mass and Weight

Mass	Weight
The mass of a body is the matter contained in it .	The weight of a body is the product of its mass and acceleration due to gravity.
The mass of a body is always a constant quantity and does not change anywhere.	The weight of a body is a variable quantity and depends on the value of acceleration due to gravity.
Mass is a scalar quantity.	Weight is a vector quantity.
The mass of a body remains same at all places.	The weight of a body is zero at the centre of the earth.

Some general questions that a student may know for his/her knowledge

1.If the mass of a body on the earth is 10 kg then on moon the mass is 10 kg
The reason behind this is that mass remains same at all places .
Again the weight of the body changes at different on the earth with the variable value of acceleration due to gravity. It will be mg where m is the mass and g is acceleration due to gravity whose value on the earth is calculated approximately 9.8 m/sec², therefore weight =mg=10 x 9.8=98 N and its weight on the moon will be mg =10 x1.6=16 N because the value of g for the moon is 1.6 m/sec².

2. Is there any change in the weight of a body when it is taken from Kolkata
to Darjeeling explain this statement - The value of ‘g’ changes from place to place on the surface .The value of ‘g’ decreases with the rise of altitude(height). Since Darjeeling is situated at a higher altitude then Kolkata, hence the value of ‘g’ is less than Darjeeling with respect to Kolkata. Thus the body will experience less weight at Darjeeling  then Kolkata.

Every thing in the universe obeys some laws regarding nature so mass is also a part of the universe it also obeys the same law.

Law of conservation of mass : The law of conservation of mass deals with the relation between the mass of reactants and the products during the chemical change. It was postulated by A. Lavoisier.

The law of conservation of mass states that during any physical or chemical change , the total mass of the product is equal to the total mass of the reactants
                                                                    Or
matter is neither created nor destroyed during any physical or chemical change.

Verification of law of conservation of mass

Landoll’s experiment : In this experiment, take two solutions of sodium chloride and silver nitrate separately in the two limbs of H shaped tube (Landoll’s tube). Seal the tube and weight it. After weighing shake the tubes thoroughly to mix the solutions. The reaction occurs between the solution of silver nitrate and sodium chloride. A white precipitate of silver chloride is formed as ‘
    AgNO₃ + NaCl = AgCl +NaNO₃
After the reaction weight the tube again. We will observe that the weight remains practically unchanged. This   verify the law of conservation of mass.

MATTER AND ENERGY

Matter : Anything which occupy some space , can be divide, and it can be perceived
By our sense organs (eye, nose, ear, tongue, and skin ) is called a matter.

Mass : Mass of a body is the total quantity of matter contained in the body.
Description : Each and every substance in the world , whether living or non-living is a matter. Mass is a constant quantity of a matter .
Mass of a body is measured with the help of a common balance .
Mass is a scalar quantity i.e it has only magnitude but no direction .
The C.G. S unit of mass is gm and S.I unit is Kg.

The fundamental properties of matter 
  
The fundamental properties of a matter is given below;
·       Volume : Every matter living or non- living have a definite volume
·       Porosity : Every matter posses very minute porus , this property is called porosity.
·       Gravitation : Any two matter attract each other .
·       Divisibility : All matter can be divided into very small parts .
·       Inertia : An inainimate matter cannot , by itself , change its state of rest or Motoion.
·       Cohesion : Different molecule of the same substance attract each other.
·       Density : The mass of unit substance is called its density.

Weight of a body , how it is measured ,and its unit

 Weight : The force with which the earth attracts the body towards its own centre is                           called the weight of the body.
The weight of a body is measured by a spring balance .
Weight of a body, being a force is a vector quantity. We know from Newton’s law of motion that force = mass x acceleration or  P = mf.
In this case, the acceleration of the body is the acceleration due to gravity.Hence ,         if W be the weight of the body, then from the above definition W = mg , where ‘g’ is the acceleration due to gravity.
If  g = 1, then m =W. Thus mass of a body is numerically equal to the weight of a body at a place of unit acceleration . At the centre of the earth g = 0 hence weight of a body is zero at the centre of the earth.
   
 Units : Weight being a force, its S.I unit is Newton(N) and C.G.S unit  is dyne .             The gravitational unit of weight in S.I unit is Kilogram-force (Kgf) or Kilogram weight .  In C.G.S system the unit of weight is gramme force (gf)
They are related as

1Kgf =1000 gf = 9.8 N
1gf =980 dynes , and
1 N = 10⁵ dynes where g =9.8 m/s² 

METHOD OF MEASUREMENT OF DIFFERENT PHYSICAL QUANTITIES (3)

Measurement of time


Time is measured by a wall clock, wrist watch, table clock. To measure a

fraction of time , stop watch is used.



Method of measurement of time by a clock


In our day to day life , time is measured by pendulum clock, whose function is based on the oscillation of a pendulum. In this type of clock, a metal rod is suspended from a rigid support. A heavy metal ball is suspended from the end of the rod. The time taken by the metal ball in one oscillation is regarded as 1 second. It consists of a circular dial which divided in 60 equal divisions.

Each division represents 1 minute. It is further divided into 12 equal parts, where each parts represents 1 hour. Hours are marked from 1 to 12 . In this clock long arm is the arm of minute and the short arm is the arm of hours. A spring is wound up with the help of key in old type of clock and by battery in digital clocks or battery operated clocks. The potential energy is converted to kinetic energy and rotates the arms of the clock.



Method of measurement of interval of time with the help of stop watch


Stop watch is used to measure the time interval between two events. It consists of a circular dial which is divided in 60 equal divisions. Each division represents 1 second. A long, light needle called second’s hand pivoted at the centre, rotates over the circular scale. A big circular dial also contains a small circular dial to read the time in minutes . The smaller dial graduated in 30 or 60 divisions and the small needle called the minute’s hand rotates over the scale. When the seconds hand completes one rotation, the minutes hand moves through one division or 1 minute.

A knob is provided at the top of the frame for winding the watch. By pressing the

Knob in succession, the watch can be started, stopped and both the needles can be

Brought to searting point for future use.

METHOD OF MEASUREMENT OF DIFFERENT PHYSICAL QUANTITIES (2)

Method for the measurement of volume of a liquid

The space occupied by a body is called the volume of that body.
The volume of a liquid in the laboratory is measured by the following
Measuring devices.
1.    Measuring cylinder
2.    Measuring flasks
3.    Burette
4.    Pipette
Measuring cylinder is commonly used to measure the volume of a liquid. It is 
a glass cylinder of uniform radius. The outer wall is graduated in cubic centimetre
and millimeter from the bottom upwards. It is available in different sizes. Before
measuring the volume of a liquid through measuring cylinder it is cleaned and
kept fixed vertically. The liquid whose volume has to measure is poured in it
and the reading of upper layer of the liquid is noted. This reading is the volume
of the liquid.


To determine the volume of an irregular solid heavier than and insoluble in water
                                 With the hep of measuring cylinder

To find the volume of an irregular shape of solid body, we take a graduated
cylinder. Fill the cylinder partly with water. Note the reading of the lower level of
water(say x₁ cc ). Now tie the given solid with a thin string and lower it gently into the  cylinder so  that the solid is completely immersed in water. The level of water is rises again.
Note the new reading of the water level raised (say x₂cc).
Since the body displaces water equal to its volume , Hence the volume of
Solid body = second reading – first reading
Thus , volume of solid = (x₂ - x₁) cubic centimeters.

To find the density of an irregular shaped body insoluble in water.

Density: The density of a substance is defined as its mass per unit volume 
Density is a scalar quantity and represented by the letter ‘d ’. If the mass of a body be M gms, and the volume of the body is V cubic cms, then
                                               d = M/V. Its unit is gm per cm³. SI unit is kg /m³.
To find the density of an irregular body , first find the mass of  the body by common  balance . Now find the volume of the body by the following steps.
To find the volume of an irregular shape of solid body, we take a graduated
cylinder. Fill the cylinder partly with water. Note the reading of the lower level of
water(say x₁ cc ). Now tie the given solid with a thin string and lower it gently into the cylinder so that the solid is completely immersed in water. The level of water is rises again.
Note the new reading of the water level raised (say x₂cc).
Since the body displaces water equal to its volume , Hence the volume of
Solid body = second reading – first reading
Thus , volume of solid = (x₂ - x₁) cubic centimeters.
Dividing Mass of the body by volume we get the density of the body.

Measurement of volume of a regular shaped body is determined 

To find the volume of a regular shaped body its dimensions such as length, breadth, height, radius etc, are measured . Then use the following relations:

Volume of cuboid = length x breadth x height
Volume of cube = (side)³
Volume of sphere =4 / 3 π (radius)³

METHOD OF MEASUREMENT OF DIFFERENT METHOD OF MEASUREMENT OF DIFFERENT PHYSICAL QUANTITIES

Method for the measurement of length with the help of ordinary scale.

Ordinary scale is a device used to measure the length of an object. 
Ordinary scale is generally made up by wood, metal or plastic. It is graduated in centimeters, millimeters. 1 centimeter is divided into 10 equal parts, each part is 1 millimeter.  
To measure the length of a line say AB, we place the scale in such a way that it remains perpendicular to the line AB. Now note the reading of the starting point and the end point B from the scale.
Then, Length of line AB = Reading of end B – Reading of end A
As  per example: Let the starting point A of the line AB is at 2 cm, coincide with the scale. If the end point B of the line does not coincide exactly with the graduation of the scale, the reading has to be taken by eye approximation method. While taking the reading  eye should be placed directly above the point B. Let the end B coincide with the scale at 6.5 cm .
 Therefore length of the line AB = 6.5cm -2cm = 4 .5 cm.

Method for the measurement of mass with the help of a common balance

Common balance is an instrument used to measure the mass of a body. 
By common balance we find the mass of a body by comparing its mass with some known standard weights. Generally we put the body on the left pan of the balance whose mass is to be determine and the standard weights are placed on the right pan. When the beam of the balance becomes horizontal then the mass of the body is equal to the standard weight. 

Method for the measurement of mass with the help of a spring balance

Spring balance is a type of balance used to measure the weight of a body.
It consists of a spring kept in a metal tube. It is partially exposed in front of the metal tube. The upper end of the spring is rigidly fixed. The lower end of the spring is attached with a metal rod. One hook is attached with the lower end of the metal rod. The body whose weight is to be determined is hanged from this hook. A pointer attached to the spring moves on a graduated scale (usually in grams or kilograms and grams ). The position of the pointer indicates the weight of the body.