History of chemistry

Class 11 Chemistry, unit 1, book pages 5–13

Practise 43 MCQs Take the unit quiz Review 15 cards

Chemistry grew out of ideas that were first imagined before they were tested, from Democritus' indivisible atoms to Dalton's atomic theory. This unit uses four famous thought experiments (Democritus, Maxwell's demon, Schrodinger's cat and Al-Ghazali's burning cotton) to show how ideas are built in the mind and then checked. It ends with inductive and deductive reasoning, and shows where each of them can go wrong, with examples from chemistry.

Learning outcomes (2)

Describe how Al-Ghazali's burning cotton thought experiment highlights the challenges of inductive reasoning.
Give examples of deductive reasoning with respect to chemistry.

Thought experiments

A thought experiment is an experiment performed in the laboratory of the human mind. We study a specific situation, observe what would happen in it, and reach a conclusion. The difference from a real experiment is that the former is done in the mind, whereas the latter is done in the real world.

Thought experiments play a vital role in developing scientific theories and principles.

Thought experiment: An experiment carried out in the mind: a specific situation is imagined, what happens in it is worked out, and a conclusion is drawn, without any apparatus.

1.1The discovery of atomOften examined

In 500 BCE, the Greek philosopher Leucippus was the first to think about the atom, so he is known as the father of atomic philosophy. In 430 BCE, his student Democritus thought that matter is made of uniform and indivisible particles called atomos, a Greek word meaning uncuttable. This is the first thought experiment about the atom in recorded history.

Democritus further thought that atoms also existed for feelings and the human soul. He held that the shape and size of atoms are responsible for the properties of matter. The sour taste of a substance is due to needle-shaped atoms, and the white colour of matter is due to smooth-shaped atoms.

Before Democritus' thought experiment there were two Greek philosophies about change in matter. One said that change is just an illusion, and the other said that change is a reality. Democritus argued that changes in matter are due to the combination and separation of indivisible particles.

Experimental background of atomic chemistry

In 1808, the English chemist John Dalton turned Democritus' thought experiment about the atom into a scientific theory, called Dalton's atomic theory. He wrote a book on a new system of chemical philosophy, and presented an experimental picture of how compounds are formed from elements.

Later, the chemists Gay-Lussac and Amedeo Avogadro supported Dalton's atomic theory, and so the experimental foundation of atomic chemistry was laid. In this way Democritus' thought experiment was turned into a real experiment, and atomic philosophy became atomic chemistry.

1.2Maxwell's demon experimentOften examined

In 1867, the Scottish scientist James Clerk Maxwell devised a thought experiment on gases. He imagined two containers, A and B, filled with the same gas at the same temperature. The two containers share a small, frictionless, molecular-sized window in the partitioning wall.

A little demon (an invisible spirit) sits at the window. It is smart enough to guard the window and to detect the speed of the gas molecules in the two containers. The demon selectively lets fast-moving molecules pass from container A to container B, and slow-moving molecules pass from container B to container A.

Maxwell's demon: fast molecules pass from A to B and slow molecules from B to A, through the window that the demon guards.

In this way, all high kinetic energy molecules fill container B, and low kinetic energy molecules stay in container A. Theoretically, this creates a temperature difference between the two containers, one becoming hotter and the other colder, without any expense of energy. However, this cannot happen without the cost of energy, and it goes against the second law of thermodynamics.

Second law of thermodynamics: Heat flows naturally from a hotter body to a colder body, and this increases the entropy of the system.

Solution to Maxwell's demon

The problem with Maxwell's demon was that the demon was ideally considered to need no energy to open and close the window. In fact, the demon's action is not possible without the use of energy, and that use of energy increases the entropy of the system.

If the demon is considered to use energy to open and close the window, then energy is used and the entropy of the system increases, which follows the second law of thermodynamics. In this way the Maxwell's demon experiment helps us consider all factors while studying the behaviour of physical systems in physical chemistry.

1.3Schrodinger's cat experimentOften examined

Erwin Schrodinger (1887 to 1961) was an Austrian scientist. He explained his cat experiment in a discussion with Einstein while addressing the problems of quantum mechanics, so the experiment played an essential role in the development of quantum mechanics.

He imagined a cat enclosed in an opaque box with a poison, a Geiger counter and a radioactive material. An outside observer would say that the cat is in two states, both alive and dead at the same time. The actual reason is that the radioactive material is also in two states: it decays and does not decay at the same time.

He thought that when the radioactive material decayed, the Geiger counter received the radiation emitted by it and pushed the hammer of the container holding the poison. As a result, the container opened, the poison came out and killed the cat. If the radioactive material did not decay, the cat would remain alive.

Schrodinger's cat: the outcome for the cat depends on whether the radioactive material decays. Until the box is opened, neither outcome is known from outside.

So one cannot say from outside the box whether the radioactive material decays or not, and so whether the cat is alive or dead, until the box is opened or the material and the cat interact with the real world.

Schrodinger's paradoxical thought experiment can be related to his equation, which laid the foundation of quantum mechanics and hence quantum chemistry. The equation gives the idea that atoms and sub-atomic particles exist in two states (particle and wave) simultaneously. The two states become one state when they are observed and measured.

1.4Imam Al-Ghazali: historical background

Abu Hamid Al-Ghazali was a Muslim theologian and mystic. He was born in 1058 AD in Iran and died in 1111 AD. After completing his education he moved to Iraq, where his teaching and debating received unanimous approval, and he got the name Imam of Iraq.

He then turned away from his worldly status and rank, and started to see life from a different perspective. He left Baghdad and went to Damascus for a short time, and then to Jerusalem. From there he wrote a famous book, The Revival of Religious Science, and he started working on disciplining his soul and improving his nature.

He is considered one of the most influential philosophers of the world and the most prominent Islamic philosopher. His famous work Tahafut al-Falsafa (the incoherence of the philosophers) is considered a breakthrough in the history of philosophical thinking of that time. It challenged the philosophical thought of the Neoplatonic thinkers, who believed that perfection can be attained in this world, with logic and reason.

1.4.1Al-Ghazali's burning cotton experimentOften examined

Al-Ghazali put forward an idea of necessary causation, which says that the link between cause and effect is necessary and always true. As an example, he argued that when a piece of cotton is placed near a burning flame, the cotton will burn. He further added that if another similar piece of cotton is brought very close to a similar burning flame, the cotton will burn again.

He thought that the burning flame was the cause of the burning of the cotton piece, so there is a causal link between the flame and the cotton. He concluded that a burning flame necessarily burns the nearby piece of cotton, and so this theory is called necessary causation.

Necessary causation: The view that one thing is necessarily the cause of another, so that the effect always follows the cause. The book's example is that clouds are the cause of rainfall, because we can never witness rainfall without clouds.

1.4.2Al-Ghazali's departure from necessary causationOften examined

Al-Ghazali performed an experiment in which he brought a piece of cotton close to a burning flame. After some time the cotton burnt. To him, this meant that the burning flame was the cause of the burning of the cotton.

He gave another example: a person who could not open his eyes. One day, miraculously, the person became able to open his eyelids and see the beauty of the world. He thought that the opening of the eyelids was the cause of seeing things around him.

Al-Ghazali later departed from his earlier point of view. He imagined that what we observe around us is the continuation of events, not a kind of direct causation. He said that sticking to the theory of necessary causation would reject the concept of God's will.

He put forward the logic that all things have a nature and act in a habitual way, because nature follows specific principles. He further added that God created the universe (nature), and that He can suspend the habitual succession of events according to His wish.

He argued that the blind man could not understand that those were rays of light striking his eyes and making him able to see around. The blind man had confused the succession of events with causation.

David Hume, another philosopher, supported Al-Ghazali's approach of the continuation of events. He added that there is no natural glue (causal link) that holds the events around us together. Rather, they occur one after another.

Worked example: succession of events or causation?

Using the blind man example, explain the difference between a succession of events and causation, and apply it to the burning of cotton near a flame.

The blind man opens his eyelids and then sees the world. He thinks that opening the eyelids caused the sight.
In fact rays of light striking his eyes make him see. The opening of the eyelids only came before the seeing.
So one event followed another (a succession). That does not show that the first produced the second (causation).
Apply this to the cotton. We see the flame, then the burning. By Al-Ghazali's later view this is the habitual course of events, which follows the principles of nature that God created.
God can suspend this habitual course, so the burning is not necessarily caused by the flame. Hume agreed that there is no natural glue between events.

A succession of events is not proof of a necessary cause

1.4.3Al-Ghazali's thought experiment: a challenge to inductive reasoningOften examined

Al-Ghazali's thought experiment has the following inductive reasoning:

The flame burns a piece of cotton.
The piece of cotton burns when kept near the flame.
The burning flame is the cause of the burning piece of cotton.

The burning cotton experiment of Al-Ghazali was a challenge to inductive reasoning. He concluded that when a cotton piece is kept near a burning flame, it must always burn. This conclusion goes beyond the cases observed, questions the inductive argument, and tells us that repeated observation cannot always be true.

He had missed some important factors in data collection, such as moisture, chemical reactions, the flow of air and the composition of the cotton piece. This thought experiment weakened our dependence on inductive reasoning and underscored the importance of deductive reasoning.

1.5Reasoning

According to Garrett, 'reasoning is a stepwise thinking with a purpose and goal in mind'. Reasoning is therefore a specialized thinking that enables a person to understand, in the mind, the link between the cause and the effect of an event. It is also the reasoning that lets somebody solve a problem in a systematic way, on the basis of past experiences and present observation.

There are two types of reasoning: inductive and deductive.

Inductive reasoning moves from specific observations to a general conclusion. Deductive reasoning moves from a general statement to a specific conclusion.

1.5.1Inductive reasoningOften examined

In inductive reasoning, generalized conclusions are drawn from specific observations. In general, we may encounter difficulties while deriving a set of conditions or a situation from a specific statement, because inductive logic may involve some uncertainty. To reduce that ambiguity we may go beyond the given information, and we must work with an impartial mind.

Inductive reasoning: Reasoning that goes from specific observations to a generalized conclusion. Its conclusion is a possibility, not a certainty.

Concept assessment exercise 1.1: inductive reasoning on the reactivity of halogens

Make inductive reasoning on the reactivity of halogens in the periodic table.

Collect specific observations. Chlorine displaces bromine from a solution of bromide, and bromine displaces iodine from a solution of iodide. Each displacement is a redox reaction in which the more reactive halogen takes the electrons.
$Cl X_{2} + 2 Br X^{-} 2 Cl X^{-} + Br X_{2} Br X_{2} + 2 I X^{-} 2 Br X^{-} + I X_{2}$
Look for a pattern. In each case a halogen higher in Group VII displaces the halogen below it, so the upper halogen is the more reactive one. Fluorine, at the top of the group, is the most reactive.
Generalize from the pattern. The reactivity of the halogens decreases as we go down Group VII.
State the limit of the conclusion. It rests on a few observations, so it is a probable conclusion. A reaction that was not tested could still differ, which is why inductive reasoning carries some uncertainty.

Reactivity: F X_{2} > Cl X_{2} > Br X_{2} > I X_{2} (decreases down the group)

Worked example: inductive or deductive?

Decide whether each argument is inductive or deductive and say how certain its conclusion is. (a) All elements of Group VII have similar chemical properties; fluorine and bromine lie in Group VII; so fluorine and bromine have similar chemical properties. (b) Three different pieces of cotton were held near a flame and each burned; so every piece of cotton held near a flame burns.

(a) Look at where the argument starts. It starts from a general statement about all of Group VII and applies it to two particular elements.
$general \to specific$
Going from a general statement to a specific conclusion is deductive reasoning. If the premises are true, the conclusion must be true.
(b) It starts from three particular observations and ends with a statement about every piece of cotton.
$specific \to general$
Going from specific observations to a general conclusion is inductive reasoning. The conclusion goes beyond the cases observed, so it is only probable. Moisture, air flow or the composition of the cotton could change the result (Al-Ghazali's challenge).

(a) deductive, certain if the premises are true; (b) inductive, only probable

1.5.2Deductive reasoningOften examined

Deductive reasoning is a research approach that involves testing a hypothesis. It uses a general statement to make specific conclusions. Deductive reasoning is also called top-down logic. It tells us that if the given general statement is true, then the specific conclusion coming out of it must be true.

Deductive reasoning: Reasoning that uses a general statement to reach a specific conclusion. If the general statement is true, the specific conclusion must be true.

The following examples, with reference to chemistry, make deductive reasoning clear. Three of them are given below: the stability of chlorine, balancing chemical equations, and the chemical properties of elements.

1.5.2Stability of chlorineOften examined

Chlorine is an element that belongs to the halogen group (Group VII). We can apply deductive reasoning to it in the following way:

The elements that obey the octet rule can exist freely.
The chlorine element ( $Cl X_{2}$ ) follows the octet rule.
The chlorine element ( $Cl X_{2}$ ) has an independent existence.

Statements 1 and 2 are the premises and statement 3 is the conclusion. The premises are the general rule and the specific case. A single chlorine atom has seven valence electrons, one short of an octet. Two chlorine atoms share one pair of electrons to form $Cl X_{2}$ , and in this way each atom has an octet and the molecule can exist on its own.

Worked example: a deductive argument for fluorine

Use the same deductive pattern as for chlorine to show that fluorine,

F X_{2}

, can exist freely.

General statement (premise 1): elements that obey the octet rule can exist freely.
Specific case (premise 2): a fluorine atom has seven valence electrons ( $2 s^{2} 2 p^{5}$ ), one short of an octet. Two fluorine atoms share one pair, so each has $6$ lone-pair electrons $+ 2$ shared electrons.
$6 + 2 = 8$
Check the premise before concluding: each atom of $F X_{2}$ has eight electrons around it, so $F X_{2}$ follows the octet rule.
Conclusion: $F X_{2}$ has an independent existence.

Fluorine molecule: each atom has three lone pairs and shares one pair (accent), giving eight electrons around each atom.

F X_{2} follows the octet rule, so it can exist freely

1.5.2Balancing chemical equations

Consider a chemical reaction in which hydrogen gas reacts with oxygen gas to form water. The deductive reasoning tool for making a balanced chemical equation under the law of conservation of mass can be broken down as follows:

A balanced chemical equation follows the law of conservation of mass.
The following chemical equation is balanced: $2 H X_{2} + O X_{2} 2 H X_{2} O$ .
The given chemical equation obeys the law of conservation of mass.

The first statement is the general statement, the second is the specific case and the third is the conclusion. Check the specific case by counting atoms. The left side has $4$ H atoms and $2$ O atoms. The right side has $2 \times 2 = 4$ H atoms and $2$ O atoms. The atoms are conserved, so the equation is balanced.

1.5.2Chemical properties of elements

We can relate the deductive reasoning process to the chemical properties of elements in the following way:

All elements belonging to the same group have similar chemical properties.
Sodium and potassium lie in the same group (Group I).
Sodium and potassium have similar chemical properties.

Statement 1 is the general statement, statement 2 is the specific case, and statement 3 is the specific conclusion that follows from them.

Concept assessment exercise 1.2: trend in atomic radius along a period

Apply deductive reasoning on the trends in atomic radius along a period in the periodic table.

General statement. Across a period, the atomic radius decreases from left to right. The nuclear charge increases while electrons are added to the same shell, so the outer electrons are pulled closer to the nucleus.
Specific case. Lithium, beryllium, boron, carbon, nitrogen, oxygen and fluorine are all in period 2, so they have electrons in the same shell.
Conclusion. Lithium has the largest atomic radius of the period and fluorine the smallest, and the radius falls at each step across the period.
$r (Li) > r (Be) > r (B) > r (C) > r (N) > r (O) > r (F)$

r (Li) > r (Be) > \dots > r (F) : the radius decreases across period 2

Key terms

Thought experiment: An experiment carried out in the mind: a specific situation is imagined, its outcome is worked out, and a conclusion is drawn.
Atom (atomos): A uniform, indivisible particle of matter, proposed by Democritus. The Greek word means uncuttable.
Dalton's atomic theory: The 1808 scientific theory of John Dalton, which turned Democritus' thought experiment into an experimental picture of compounds formed from elements.
Entropy: The disorder of a system.
Second law of thermodynamics: Heat flows naturally from a hotter body to a colder body, and this increases the entropy of the system.
Superposition: The state of a system that is in two states at the same time, such as the cat in Schrodinger's box before the box is opened.
Quantum mechanics: The field of physics whose foundation was laid by Schrodinger's equation, which gives the idea that atoms and sub-atomic particles exist in two states (particle and wave) at once.
Necessary causation: The view that one thing is necessarily the cause of another, so that the effect always follows the cause.
Continuation of events: Al-Ghazali's later view that events follow one another in a habitual succession, not through direct causation.
Reasoning: Stepwise thinking with a purpose and goal in mind, which helps a person understand the link between cause and effect and solve problems systematically.
Inductive reasoning: Reasoning that moves from specific observations to a generalized conclusion. The conclusion is probable, not certain.
Deductive reasoning: Reasoning that uses a general statement to reach a specific conclusion. If the general statement is true, the specific conclusion must be true.
Octet rule: The tendency of an atom to have eight electrons in its outer shell, which makes it stable.
Halogen group (Group VII): The group of fluorine, chlorine, bromine and iodine.
Atomic radius: The size of an atom measured from its nucleus to its outer electrons. It decreases from left to right across a period.

Choose the correct answerp. 12–13

The word atom comes from Greek language which means

Abeautiful
BindivisibleAnswer
Csmooth
Dsharp

Why

The word atom is made of 'a', meaning not, and 'tomos', meaning to cut, so it means indivisible or uncuttable. Beautiful, smooth and sharp are not the meaning of the word. Smooth is only the shape that Democritus gave to the atoms of white matter.

Leucippus is known to be the mentor of Democritus and the father of atomic philosophy. Who was the first to use the word atom?

ALeucippus
BJames Clerk Maxwell
CDemocritusAnswer
DErwin Schrodinger

Why

The book credits Leucippus with first thinking about the atom and with originating atomism, but it credits Democritus with the word 'atomos' for the uniform, indivisible particles of matter. Maxwell and Schrodinger are linked with later thought experiments, not with the word.

Note. The stem follows the book's own description: Leucippus originated atomism and Democritus used the word 'atomos' for the particles. The answer follows the word, which is linked to Democritus.

iii

Who put forward the atomic theory?

ADaltonAnswer
BJ C Maxwell
CAl Ghazali
DE Schrodinger

Why

In 1808, John Dalton turned Democritus' thought experiment into a scientific theory, Dalton's atomic theory. Maxwell and Schrodinger are linked with other thought experiments, and Al-Ghazali was a philosopher who studied causation.

Who observed the movement of molecules from one container to the other in Maxwell's thought experiment?

AMaxwell
Bthe demonAnswer
CMaxwell's students
DMaxwell's mentor

Why

In the thought experiment the demon sits at the window, detects the speed of the molecules and decides which ones pass from one container to the other. Maxwell devised the experiment, but he is not the observer inside it, and his students and mentor play no part in it.

The Maxwell's demon experiment violates which of the following law?

Alaw of conservation of mass
BBoyle's law
Cfirst law of thermodynamics
Dsecond law of thermodynamicsAnswer

Why

The demon would make heat flow in a way that increases the difference of temperature without any expense of energy, which goes against the second law of thermodynamics. The conservation of mass and Boyle's law do not deal with heat flow, and the first law concerns energy, whereas the book's objection is the second law.

The Schrodinger's thought experiment is believed to have contributed to evolving the well-known field of physics called

Aplasma physics
Bparticle physics
Cstatistical mechanics
Dquantum mechanicsAnswer

Why

Schrodinger discussed his cat experiment while addressing the problems of quantum mechanics, and his equation laid the foundation of quantum mechanics. Particle physics is tempting because the experiment mentions sub-atomic particles, but the book names quantum mechanics.

vii

What were the findings for an outside observer in Schrodinger's cat thought experiment?

Athe cat was dead
Bthe cat was alive
Cthe cat was both alive and dead simultaneouslyAnswer
Dthe cat was neither alive nor dead

Why

The book says that an outside observer would say the cat is in two states, both alive and dead at the same time, until the box is opened. Options A and B are the outcomes that are known only after the box is opened, and option D is not described in the experiment.

viii

The famous book of Al-Ghazali, 'Tahafut-al-Falsafa', challenged the philosophical thought of Neoplatonic thinkers which believed that

Aperfection and happiness are achievable in this worldAnswer
Bno happiness and perfection exist in this world
Cthere is life hereafter
Dman is mortal

Why

The book says that the Neoplatonic thinkers believed that perfection can be attained in this world, with logic and reason, and that Tahafut challenged this belief. Option B is the opposite of their view, and options C and D are not the subject of the passage.

Al-Ghazali thought experiment was based on burning

Apaper
BcottonAnswer
Cwood
Dcoal

Why

The experiment is the burning of a piece of cotton placed near a burning flame. Paper, wood and coal are not used in it.

What is correct statement about Al-Ghazali's approach? I. he thought that every event must have a cause. II. he thought that there is succession of events, not causation. III. he thought that God can suspend the habitual continuation of events.

AI and II only
BII and III onlyAnswer
CIII only
DI, II and III

Why

Statements II and III describe Al-Ghazali's later approach: events follow one another in a habitual succession, and God can suspend that succession, which is how miracles happen. Statement I describes his earlier view of necessary causation, which he later departed from, so it is not his approach in the end.

Note. Statement I is an accurate description of Al-Ghazali's early view of necessary causation. The answer B reads 'approach' as his final position, which is what the book describes in section 1.4.2.

Give short answerp. 13

What was an atom to Democritus?

Solution

To Democritus an atom was a small particle of matter that is uniform and indivisible. The word 'atomos' means uncuttable. He thought that all matter is made of such particles, and that the particles differ in shape and size.

How did Democritus connect atoms to feelings and the properties of matter?

Solution

Democritus thought that atoms also existed for feelings and the human soul. For the properties of matter, he held that the shape and size of atoms are responsible. For example, the sour taste of a substance is due to needle-shaped atoms, while the white colour of matter is due to smooth-shaped atoms.

iii

What was the link of life of the cat with radioactive material?

Solution

The fate of the cat depends on the radioactive material. If the material decays, the Geiger counter receives the radiation and pushes the hammer of the container of poison; the poison comes out and kills the cat. If the material does not decay, the poison stays sealed and the cat remains alive.

Until the box is opened, the material is in two states, so the cat is in two states too: both alive and dead at the same time.

How can we relate Schrodinger's cat experiment with quantum mechanics?

Solution

Schrodinger explained his cat experiment while addressing the problems of quantum mechanics, and his equation laid the foundation of quantum mechanics and hence quantum chemistry. The equation gives the idea that atoms and sub-atomic particles exist in two states (particle and wave) simultaneously. The two states become one when they are observed and measured. The cat, alive and dead until the box is opened, is a picture of this idea.

Define the theory of necessary causation. How did Al-Ghazali prove it?

Solution

Necessary causation is the theory that the link between cause and effect is necessary and always true: one thing is necessarily the cause of another, so the effect always follows the cause.

Al-Ghazali supported it with the burning cotton experiment. A piece of cotton placed near a burning flame burnt. When a similar piece was brought close to a similar flame, it burnt again. From these repeated cases he concluded that the burning flame necessarily burns the nearby cotton.

Define inductive and deductive reasoning.

Solution

Inductive reasoning draws a generalized conclusion from specific observations. Its conclusion is a possibility, not a certainty.

Deductive reasoning uses a general statement to reach a specific conclusion. If the general statement is true, the specific conclusion must be true.

Long questionsp. 13

Explain thought experiment of J C Maxwell.

Solution

The set-up

Maxwell imagined two containers, A and B, filled with the same gas at the same temperature. They are separated by a partitioning wall with a small, frictionless, molecular-sized window, and a demon guards the window.

Maxwell's demon: fast molecules pass from A to B and slow molecules from B to A, through the window that the demon guards.

What the demon does

The demon detects the speed of the molecules. It lets fast-moving molecules pass from A to B and slow-moving molecules pass from B to A. So the high kinetic energy molecules collect in B, and the low kinetic energy molecules remain in A.

The apparent result

A temperature difference appears between the containers without any expense of energy. Heat seems to flow from the colder container to the hotter one, which is against the second law of thermodynamics, because heat flows naturally from a hotter body to a colder body.

Why the demon does not win

The demon was considered to need no energy to open and close the window. In fact, its action is not possible without energy, and the use of energy increases the entropy of the system. Entropy therefore rises in agreement with the second law of thermodynamics.

The experiment reminds us to consider all factors, including the energy used by any device that sorts the molecules, when we study physical systems in physical chemistry.

Note. The book describes container A as becoming the hotter one. Following the sorting it describes, B collects the fast molecules and becomes the hotter container. The answer follows the sorting.

Explain the experimental background of atomic theory.

Solution

Greek thought

Democritus thought that matter is made of uniform and indivisible particles called atomos. Before him, there were two Greek philosophies about change in matter: one said that change is an illusion, and the other that change is a reality. Democritus argued that change is due to the combination and separation of indivisible particles.

From thought to experiment

In 1808, the English chemist John Dalton turned Democritus' thought experiment into a scientific theory, Dalton's atomic theory. He wrote on a new system of chemical philosophy and presented an experimental picture of how compounds are formed from elements.

Support for the theory

Later, Gay-Lussac and Amedeo Avogadro supported Dalton's theory. In this way the experimental foundation of atomic chemistry was laid, and the thought experiment of Democritus became a real experiment.

Thought experiment (Democritus): matter is made of indivisible atoms.
Dalton, 1808: the atomic theory, with an experimental picture of compounds formed from elements.
Gay-Lussac and Avogadro: support for the theory.
Result: atomic philosophy becomes atomic chemistry.

Describe what conclusions were drawn by Schrodinger from his thought experiment.

Solution

The main conclusion

A cat enclosed in an opaque box with poison, a Geiger counter and a radioactive material is in two states: both alive and dead at the same time, until the box is opened or the cat interacts with the real world.

Schrodinger's cat: the outcome for the cat depends on whether the radioactive material decays. Until the box is opened, neither outcome is known from outside.

Why the cat is in two states

The radioactive material is also in two states: it decays and does not decay at the same time. If it decays, the Geiger counter pushes the hammer, the poison is released and the cat dies. If it does not decay, the cat stays alive.

A conclusion about atoms

The equation behind the experiment gives the idea that atoms and sub-atomic particles exist in two states (particle and wave) at the same time. The two states become one when they are observed and measured.

The link with quantum mechanics

Schrodinger's paradoxical experiment is related to his equation, which laid the foundation of quantum mechanics and hence quantum chemistry. For this reason the experiment is believed to have played an essential role in the development of quantum mechanics.

How did Al-Ghazali conclude from a piece of burning cotton? Also relate his conclusions with his concept of God involvement in all natural laws.

Solution

The conclusion from the cotton

A piece of cotton placed near a burning flame burnt, and a similar piece placed near a similar flame burnt again. Al-Ghazali took the flame to be the cause of the burning and concluded that a burning flame necessarily burns the nearby cotton. This is his theory of necessary causation.

His departure from necessary causation

Al-Ghazali later thought that what we observe is the continuation of events, not direct causation. He said that sticking to necessary causation would reject the concept of God's will. All things have a nature and act in a habitual way, because nature follows specific principles.

The involvement of God

God created the universe (nature) and can suspend its habitual succession of events according to His wish. This is how miracles happen: when a person who could not open his eyes suddenly could see, the opening of the eyelids was not the cause of sight, but the wish of God. The blind man in his example confused the succession of events with causation.

Combined conclusion

Al-Ghazali concluded that events follow one another in a habitual order, the order that God has set, rather than being bound together by necessary causes. God can change that order at will, so the natural laws are under His control.

Explain that Al-Ghazali's thought experiment was a challenge to inductive reasoning.

Solution

The inductive argument

The flame burns a piece of cotton.
The piece of cotton burns when kept near the flame.
The burning flame is the cause of the burning piece of cotton.

The conclusion drawn

From these specific observations Al-Ghazali concluded that when a cotton piece is kept near a burning flame, it must always burn. This is a generalized conclusion, and it goes beyond the cases that were observed.

Why it challenges inductive reasoning

Inductive reasoning draws a general conclusion from specific observations, so the conclusion is only probable. Repeated observation cannot always be true, because a case that is not yet observed could differ. The thought experiment therefore questions the inductive argument itself.

The missed factors

Al-Ghazali had missed some important factors in data collection, such as moisture, chemical reactions, the flow of air and the composition of the cotton piece. Any of these could stop the cotton from burning, so the generalization is not reliable.

The result

This thought experiment weakened our dependence on inductive reasoning and showed the importance of deductive reasoning, in which a conclusion follows with certainty from a true general statement.

Take some examples from chemistry and apply them on inductive and deductive reasoning separately.

Solution

Inductive reasoning: the reactivity of the halogens

Specific observations: chlorine displaces bromine from a bromide solution, $Cl X_{2} + 2 Br X^{-} 2 Cl X^{-} + Br X_{2}$ , and bromine displaces iodine from an iodide solution, $Br X_{2} + 2 I X^{-} 2 Br X^{-} + I X_{2}$ .

Generalization: a halogen higher in Group VII is more reactive than the one below it, so the reactivity of the halogens decreases down the group. The conclusion is probable, since it rests on a few observations.

Deductive reasoning: three chemical examples

Stability of chlorine. General statement: elements that obey the octet rule can exist freely. Specific case: $Cl X_{2}$ follows the octet rule by sharing one pair of electrons. Conclusion: $Cl X_{2}$ has an independent existence.
Balancing an equation. General statement: a balanced equation follows the law of conservation of mass. Specific case: $2 H X_{2} + O X_{2} 2 H X_{2} O$ has 4 H and 2 O on each side. Conclusion: the equation obeys the law of conservation of mass.
Chemical properties. General statement: elements in the same group have similar chemical properties. Specific case: sodium and potassium are in Group I. Conclusion: sodium and potassium have similar chemical properties.

In each deductive example, the conclusion follows with certainty once the general statement is accepted as true, whereas in the inductive example the conclusion can still be overturned by a new observation.

Exam focus

How did Al-Ghazali's burning cotton thought experiment show the limits of necessary causation, and how does it relate to God's involvement in natural laws? (long)
Book long question vi and short question v; it is the central topic of the unit. Not asked in the 2025 or 2026 papers.
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Explain that Al-Ghazali's thought experiment was a challenge to inductive reasoning. (long)
Book long question vii and the first SLO of the unit. Not in the 2025 or 2026 papers.
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Define inductive and deductive reasoning and give examples from chemistry for each. (long)
Book short question vi and long question viii, and the second SLO. Not in the 2025 or 2026 papers.
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Explain the thought experiment of J C Maxwell (Maxwell's demon) and the law it appears to violate. (long)
Book long question iii and MCQs 4 and 5. Not in the 2025 or 2026 papers.
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Describe the conclusions Schrodinger drew from his cat thought experiment and how it relates to quantum mechanics. (long)
Book long question v and short questions iii and iv. Not in the 2025 or 2026 papers.
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Explain the experimental background of atomic theory (discovery of the atom). (long)
Book long question iv and short questions i and ii on Democritus. Not in the 2025 or 2026 papers.
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Define the theory of necessary causation and tell how Al-Ghazali proved it. (short)
Book short question v, with MCQs 8 to 10 on his approach. Not in the 2025 or 2026 papers.
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