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CHAPTER 4
THE LAWS OF CHEMICAL COMBINATIONS AND
THE IDEA OF ATOMS

In Chapter 2 we have learned that elements in a compound combine in a definite

proportion by weight. We can experimentally determine the proportions by which the
elements combine to form a compound. One such experiment will be described using
magnesium and oxygen from the air.
4.1. To find the Proportion by Weight of Magnesium and Oxygen in Magnesium
Oxide
When a known weight of magnesium is heated in air, it combines with oxygen of the air
to form magnesium oxide. The difference between the weights of magnesium and the
oxide formed from it is the weight of oxygen. From the results, the proportion by weight
of magnesium and oxygen in magnesium oxide can be calculated.

Fig. 4.1 Ignition of magnesium

A clean, dry crucible and its lid are first weighed. A length of magnesium ribbon
(about 20 cm) is cleaned by rubbing it two or three times with sand paper. This
magnesium ribbon is made into a tight coil and placed in the crucible which is then
covered it with its lid.
The weight of the magnesium and crucible with its lid is then determined. The
covered crucible is then placed on a tripod, using a pipe-clay triangle to support it. It is
then carefully heated until the magnesium ignites.
During heating, the lid must be lifted occasionally with a pair of tongs to let air
in. Care must be taken not to allow the white fumes (magnesium oxide) to escape.
After the magnesium has completely burnt, the flame is removed and the crucible
cooled down. Then a drop or two of water is added and the crucible again heated. From
this stage onwards, the crucible must be strongly heated to make sure that the combustion
of the magnesium is completed.
Heating is then discontinued and the crucible cooled down. The crucible, lid and
magnesium oxide are then weighed. The process of heating, cooling and weighing must
be repeated until there is no more increase in weight.
Treatment of results
 44

Weight of crucible and lid = W1 g

Weight of crucible, lid and magnesium = W2 g
Weight of crucible, lid and magnesium oxide = W3 g
 Weight of magnesium = (W2 -W1) g
 Weight of oxygen combined with magnesium = (W3 - W2) g
Therefore,
Weight of magnesium : Weight of oxygen
(W2 – W1) : (W3 - W2)
The ratio of the two elements (W2 - W1) : (W3 - W2) is consistently 3:2 .
Constancy of the composition by weight of magnesium oxide
At least three experiments should be done by using three different weights of
magnesium.
The results of all these experiments will show that the ratio of the weight of
magnesium to that of oxygen is always 3 : 2.
Therefore, irrespective of the starting weight of magnesium, the weight ratio of
magnesium to oxygen in magnesium oxide is always constant.
Hence, it can be concluded that in magnesium oxide the magnesium and oxygen
are always combined in a definite proportion by weight.
4.2 Comparison of the Total Weight of the Products of a Chemical Reaction
with the Total Weight of the Reactants
We have learned that in chemical reactions the reactants change chemically. Now we
may ask a question; " Is there any change in weight when substances react to form new
products?"
The answer to this question may be obtained by comparing the total weight of the
products of a reaction with the total weight of the reactants.
Some pairs of reactants are listed below. Use solutions of any pair of reactants for
each experiment.
Magnesium sulphate and barium
chloride Silver nitrate and sodium
chloride
Lead (II) nitrate and potassium iodide

Fig. 4.2. Verification of law of conservation of mass

 45

A conical flask is taken and one of the solutions of any pair given in the list is
placed in it just to cover the bottom. A piece of thread is tied around the mouth of a small
test tube which is half filled with the other solution of the pair. The test tube is held in
position in the conical flask by means of the thread and tight fitting cork as shown in the
diagram.
The whole apparatus is weighed. Let the weight be W1 g.
The flask is tilted so that the two solutions mix. A visible chemical change will
take place. The cork ensures that all the substances remain inside the flask.
The whole apparatus is again weighed after the reaction. Let the new weight be
W2 g.

We can verify that W1 g = W2 g

We can repeat the experiment with the solutions of the remaining pairs of
reactants. In each of these reactions it is found that the weight of the reactants is equal to
the weight of the products.

4.3. Constancy of Mass in Chemical Reaction

We have seen in the above discussions that in chemical reactions, the weight of
the reactants is always equal to the weight of the products. So we can say that, in a
chemical reaction, the total weight of the substances always remains the same. That is,
the total mass remains constant throughout a chemical reaction.
This discovery leads to the idea that though substances change into other
substances, the amount of matter in the substances, that is the mass, does not change.
This is because no new matter is created, nor is any of the old matter destroyed, during
the reaction. This is a law of nature known as the law of conservation of mass.

The law of conservation of mass states that :

Matter can neither be created nor destroyed.
Hence the total mass of the substances before and after a reaction is always the
same.

4.4 Constancy of Composition by weight of the Compounds

Irrespective of the method used for its preparation, the components in a given compound
always combine in definite proportions by weight. For Example, the weight proportion
of hydrogen and oxygen in water is always 1:8. The proportion by weight of carbon and
oxygen in carbon dioxide is always 3:8.

The law of definite proportions

The constancy of composition of compounds is another law of nature known as the law
of definite proportions.,
The law states :
The same compound always contains the
same elements in the same proportion by
weight.
The importance of the above two laws
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(1) These two laws, especially the law of definite proportion reinforce the idea that
all substances are made up of very small particles or atoms.
(2) The law of conservation of mass is important because we can make use of it in
writing chemical equations.
4.5 The Laws of Chemical Combination and the Idea of Atoms
Why do elements combine in definite proportions by weight? Why does the total mass of
the substances in a chemical reaction remain constant?.
These questions can be easily answered by assuming that :
Matter is composed of extremely small par-
ticles called atoms.
We may further assume that :
Atoms can neither be created nor destroyed
in chemical reactions and that they combine
in simple proportions to form small particles
of compounds called molecules.
With these assumptions we can explain chemical reactions as describe in the
example below.
Combination of magnesium and sulphur
Magnesium is a metal. Sulphur is a non-metal. They combine to form magnesium
sulphide on heating together.
magnesium + sulphur magnesium sulphide

It is found that the weight ratio of magnesium to sulphur in magnesium sulphide

is always 3:4.
To explain this we will assume that ;
(a) Magnesium is composed of magnesium atoms. These atoms have the some mass.
(b) Sulphur is composed of sulphur atoms. These sulphur atom also have the same
mass among them.
(c) The mass ratio of one magnesium atom to one sulphur atom is 3:4.
(d) One atom of magnesium combines with one atom of sulphur to form one
molecule of magnesium sulphide.
(e) The atoms of magnesium and those of sulphur cannot be further subdivided.
1. According to the assumptions (a) and (b), the atoms of magnesium and sulphur
may be represented by the following diagrams.

Fig. 4.3 Diagrammatic representation-of atoms

 47

2. When magnesium and sulphur react to form magnesium sulphide, each atom of
magnesium combines with an atom of sulphur to form a molecule of magnesium
sulphide as represented below.

Magnesium sulphur Magnesium sulphide

atom atom molecule

Now what is the mass ratio of magnesium and sulphur in a molecule of

magnesium sulphide?
Since a molecule is formed by the union of a magnesium atom and a sulphur
atom, the mass ratio of magnesium to sulphur in the molecule is 3:4.

3. Now we can assume that the molecules of magnesium sulphide are similar with
one another. So the structure of magnesium sulphide can be represented as follows :

Since (1) the mass ratio magnesium to sulphur in each molecule of

magnesium sulphide is 3:4, and
(2) all the molecules of magnesium sulphide are similar to one another
it is clear that the mass ratio of magnesium to sulphur in the whole
compound is also 3:4.
Therefore, the law of definite proportions can be explained by the idea of atoms.
The law of conservation of mass can be explained as follows :

In the combination of magnesium and sulphur, no atoms are destroyed and no

new atoms are created. The only change that occurs is the change in the arrangement of
atoms. We have seen that each atom of magnesium combines with one atom of sulphur
forming a molecule of magnesium sulphide. There is no change in the total number of
atoms during the reaction. If there is no change in the total number of atoms, there will
be no change in the total mass.
Thus, the mass remains constant in a chemical reaction. This agrees with the
statement of the law of conservation of mass.
The idea of atoms described above was first put forward by John Dalton in 1803.
We shall now study his atomic theory.

4.6 Dalton's Atomic Theory

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John Dalton (1766-1844), an English school teacher, developed the idea of the atomic
theory from the experimental evidence of the two laws, namely ;
(1) The law of definite proportions, and
(2) The law of conservation of mass.
These two laws indicate the existence of atoms in all substances even though
these atoms cannot be seen.
The basic principles of Dalton's atomic theory are summarized as follows :
(1) Matter is made up of extremely small particles called atoms.
Atoms are indestructible during chemical reactions.
(2) Atoms can neither be created nor destroyed.
(3) Atoms of one particular element have the same mass, shape, size and
other properties.
(4) Atoms of different elements have different masses and different
properties.
(5) Atoms of different elements combine in simple ratios to form molecules
of compounds.
(6) Molecules of the same compound have the same mass, shape and other
properties.
(7) Molecules of different compounds have different masses and different
properties.

4.7 The Law of Multiple Proportions

Based on his atomic theory, Dalton had predicted the existence of another chemical law.
This law is called the Law of Multiple Proportions.
If two elements combine to form more
than one compound, the different
weights of one element that combine
with it a fixed weight of the other
elements are in a ratio of small whole
numbers.
Now, let us look at some experimental data.

Example 1 : Two oxides of carbon

Carbon combines with oxygen forming two oxides. They are carbon dioxide and carbon
monoxide.
In an experiment it was found that the weights of carbon and oxygen in the two
oxides were as follows :
oxide carbon oxygen
carbon dioxide 2.4 g 6.4 g
carbon monoxide 3g 4g
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Calculation
Take l g of carbon for the fixed weight
Calculating the weights of one element. Calculate the weight of oxygen
of oxygen that combine that combines with 1 g of carbon in each of the
with a fixed weight of oxides.
carbon
 Wt. of oxygen that
combines with 1g of = = 2.583 g
carbon in carbon dioxide

 Wt. of oxygen that

combines with 1g of = = 1.333 g
carbon in carbon monoxide
Finding the ratio of
the weights of oxygen 2.583 : 1.333

Dividing by the smallest 2.583 : 1.333

number 1.333 : 1.333
1.937 : 1
Rounding off to the
nearest whole number 2 : 1
2:1 is a small whole number ratio or a simple ratio.

Question : Using the given data in the above example, calculate the ratio of the
weights of carbon that combine with a fixed weight of oxygen.

Example 2: Three oxides of chromium

Chromium and oxygen combine to form three oxides. In an experiment the weights of
chromium and oxygen in each oxide are as follows :
oxide chromium oxygen
1st oxide 5.2 g 1.6 g
2nd oxide 20.8 g 9.6 g
3rd oxide 1.3 g 1.2 g

Calculation
Take 1 g of chromium for the fixed weight
Calculating the weights of one element and calculate the weights of
of oxygen that combine oxygen that combine with 1 g of chromium
with a fixed weight of
chromium
 Wt. of oxygen that
combines with 1g of = = 0.3077 g
chromium in 1st oxide
 50

 Wt. of oxygen that

9.6
combines with 1g of = = 0.4615 g
chromium in 2nd oxide, 20.8

 Wt. of oxygen that

1.2
combines with 1g of = = 0.9232 g
chromium in 3rd oxide 1.3

Finding the ratio of

the weights of oxygen 0.3077 : 0.4615 : 0.9232
in oxides (1), (2), (3)
Dividing by the smallest 0.3077 0.4615 0.9232
: :
number 0.3077 0.3077 0.3077
1 : 1.4998 : 3.0000
Rounding off to the
nearest decimal 1 : 1.5 : 3
place
Multiply by 2 to obtain the
smallest whole number 2 : 3 : 6
2:3:6 is a small whole number ratio or a simple ratio.
These experimental data support the truth of the law of multiple proportions.

4.8 Explanation of the Law of Multiple Proportions by the Atomic Theory

According to Dalton's Theory, the atoms are indivisible. When two elements, A and B,
combine to form more than one compound, an integral number of A atoms will unite
with an integral number of B atoms. The possible combinations may be AB, AB 2, AB3,
A2B, A2B3, etc.
Support A and B combine to form three compounds, namely AB, AB2,and AB3.
If we keep the weight of A in these compounds to be constant, it can be easily
seen that the weights of B are in. the ratio of 1: 2: 3.
Since this is a simple ratio, the above chemical combinations are in agreement
with the law of multiple proportions.

SUMMARY
In this chapter, the laws of chemical combinations such as the law of
conservation of mass , the law of definite proportions, Dalton’s atomic theory, the law of
multiple proportions have been defined with appropriate examples. Worked out examples
are also given so that students may understand the application of these laws in chemical
combination problems.

Questions and Problems

1. Two elements, A and B, combine to form a compound. In the compound the
composition by weight of A and B is 3:4.
(a) What is the weight of the element B that combines with 12 g of A?
(b) What is the weight of the element A that combines with 16 g of B?
(c) What are the weights of A and B in 21 g of the compound?
 51

2. A compound contains 20% of oxygen.

(a) What is the weight of oxygen in 40 g of the compound?
(b) What is the weight of the compound that contains 5 g of oxygen?
(c) What law leads to the reasoning for your calculation?
3 Five experiments were done to find out the proportion by weight of magnesium
and oxygen in magnesium oxide.
The following results were obtained in each experiment.
Experiment magnesium oxygen
1 1.5 g 1.0 g
2 1.2 g 0.8 g
3 4.8 g 3.2 g
4 2.4 g 1.6 g
5 6.0 g 4.0 g
(a) By using the results of the experiments, determine the proportion by weight
of magnesium to oxygen in magnesium oxide.
(b) Which law of chemical combination is in agreement with the above results?
4. Using the proportion by weight of magnesium to oxygen obtained in question (3),
answer the following :
(a) When magnesium is burnt in air, magnesium oxide is formed. If 3 g of
magnesium is burnt completely what is the weight of magnesium oxide?
(b) When magnesium is heated in steam, magnesium oxide is formed. After
complete combustion of magnesium, 15 g of magnesium oxide is obtained.
What is the weight of magnesium used in this experiment?
(c) What law leads to the reasoning for your calculation?
5. (a) Complete combustion of one gram of carbon always results in the formation
of exactly 3.667 g of carbon dioxide.
State the law governing this result.
(b) Complete the following statements in accordance with the law governing
the results in (a).
In carbon monoxide.
(i) 1 g of carbon combines with 1.33 g of oxygen
(ii) 2g …. …. ( ) g ….
(iii) 3 g …. …. ( ) g ….
(iv) 4g …. …. ( ) g ….
6. When x gram of one substance reacts with y gram of another substance, two
products are formed. The weights of the two products are a gram and b gram
respectively. The weight relation of the above chemical reaction is
x + y = a + b
What law is in agreement with the above relation of weights?
7. When x gram of a compound is heated, it gives two substances. The weights of
the two substances are y gram and z gram respectively. In the light of the law of
conservation of mass write an equation to show the relation by weight of the
reactants and the products.

8. When mercury (II) oxide is heated it loses weight due to the escape of a gas.
(a) What are the products obtained by heating mercury (II) oxide?
 52

(b Why does mercury (II) oxide lose its weight on heating?

(c) Does the above observation agree with the law of conservation of mass?
Give reasons.
9. If a piece of magnesium is burnt in air it gains weight.
(a) Why does magnesium gain weight on burning?
(b) Does this observation agree with the law of conservation of mass?
Give reasons.
10. Carbon forms two oxides in which the weight of oxygen combines with 1 g of
carbon are 1.33 g and 2.66 g respectively. What law is in agreement with this
observation? Give reasons.
11. In three oxides of an element the weights of oxygen that combine with a fixed
weight of the element are 1. 13 5 g, 2.2 7 g and 3.405 g respectively.
(a) Find the ratio of the weights of oxygen that combine with a fixed weight of
the element.
(b) What law is in agreement with the above results?
12. A metal forms two chlorides. In one, 1.727 g of the metal combined with 1.03 g
of chlorine. In the other, 2.065 g of the metal combined with 2.465 g of chlorine.
Show that the formation of these compounds is in agreement with the law of
multiple proportions.
13. An element A combines with bromine forming two bromides. It is found that 7 g
of one bromide contains 4.2 g of bromine and 13 g of the other bromide contains
9 g of bromine. Show that these values agree with the law of multiple
proportions.
14. An element combines with oxygen to form two different compounds. The
percentages of the element and oxygen in the two compounds are as follows :
compound element oxygen
1st compound 11.1% 88.9%
2nd compound 5.883% 94.117%
Show that the results agree with the law of multiple proportions.
15. An element combines with oxygen to form two oxides which contain 71.42
percent and 55.556 percent of the element respectively. Show that these data
agree with the law of multiple proportions.
16. Illustrate the law of multiple proportions using the following data.
(a) compound element oxygen
st
1 compound 79.87% 20.13%
2nd compound 88.81 % 11.19%
(b) compound element oxygen
1st compound 92.825% ( )
nd
2 compound 86.6% ( )
(c) compound element bromine
1st compound 25.92% 74.08%
nd
2 compound 18.92% 81.08%
(d) compound element hydrogen
1st compound ( ) 20%
nd
2 compound ( ) 7.69%
(e) compound element chlorine
1st compound 5.264 g 6.699 g
 53

2nd compound 6.172 g 11.89g

(f) compound element oxygen
(1) 8.572 g 4.286 g ( )
(2) 14.3 10 g 5.724 g ( )
17. In an experiment, 1.99 g of copper (11) oxide was found to contain 1.59 g of
copper. In another experiment 5 g of oxygen was found to combine with 19.87 g
of copper. State the law governing these results and show how the results
illustrate the law.
18. It was found by experiment that 5.5 g of an oxide of carbon contained 1.5 g of
carbon. Also it was found that 3 g of carbon combined with 4 g of oxygen. State
the law governing these results and show how the results the law.
19. An element forms three oxides. The compositions of the three oxides are :
oxide element oxygen
st
1 oxide 92.83 g 7.17 g
2nd oxide 90.66 g 9.34 g
rd
3 oxide 86.62 g 13.38 g
Show that the compositions of the three oxides agree with the law of multiple
proportions.
20. The compositions of three oxides of nitrogen are as follows :
15 g of the first oxide contains 7 g of nitrogen.
15.2 g of the second oxide contains 5.6 g of nitrogen.
6.75 g of the third oxide contains 1.75 g of nitrogen.
Show that the data are in accordance with the law of multiple proportions.
21. An element forms three compounds in which the percentages of the elements are
51.13, 25.86 and 20.73, respectively. Show that these values illustrate the law of
multiple proportions.
22. Write TRUE or FALSE for the. following statements.,
(a) The law of conservation of mass states that matter can neither be created nor
destroyed.
(b) The law of definite proportion states that the same compound always
contains the different elements in the different proportion by weight.
(c) Atoms can be created and destroyed.
(d) Atoms of different element have different masses and different properties.
(e) Molecules of the same compound have the same mass, shape and other
properties.
(f) The atoms of magnesium can be further subdivided .
(g) The total mass of the substances before and after a reaction is always the
same.
23. Fill in the blanks with a suitable word or words.,
(a) Atoms can neither be ………. nor destroyed.
(b) Matter is made up of extremely small particles called ……….
(c) One atom of magnesium combines with one atom of ………. to form one
molecule of magnesium sulphide.
(d) The same compound always contains the same elements in the same
………. by weight.
 54

(e) If two elements combine to form more than one compound, the element that
combine with a fixed weight of the other elements are in a ………. of small
whole numbers.
(f) Dalton’s atomic theory is based on ………… laws.
(g) Atoms of different elements combine in …… to form molecules of
compounds.
(h) Magnesium reacts with ……… to form magnesium sulphide.
24. Select the correct word or words given in the brackets.
(a) Atoms are indestructible during, (biological, chemical, physical) reactions.
(b) Atoms of different elements have (same, definite, different) masses and
different properties.
(c) The atoms of magnesium and those of sulphur (can, cannot, may) be further
subdivided.
(d) The mass (ratio, weight, proportion) of magnesium to sulphur in each
molecule of magnesium sulphate is 3:4.
(e) Molecules of the (same, different, some) compounds have the same mass,
shape and other properties.
25. Match each of the items given in list A with the appropriate item given in
list B.
List A List B
(a) Carbon combine with (i) the law of definite
oxygen forming two oxides proportion and the law of
conservation of mass
(b) Magnesium and sulphur (ii) the same compound always
on heating gives the compound contain the same elements
in the same proportions by
weight
(c) Dalton's atomic theory is on (iii) carbon dioxide and carbon
two laws monoxide
(d) The law of definite proportions (iv) the law of conservation of mass
(e) Matter can neither be created (v) magnesium sulphide
nor destroyed
26. Answer the following questions.
(a) Write down any four assumptions of Dalton’s atomic theory.
(b) Define the law of multiple proportion.
(c) Are atoms of the same element are of the same kind ?
(d) Are the properties of iron atoms same or different ?
(e) Are the properties of a molecule same or different from its component
atoms ?
(f) What kind of atoms are present in the same element ?
(g) Why do different elements have different properties ?
(h) Why does the same element have the same properties?
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