3.1 Why believe in atoms?

Question 1

What are the two chemical laws that confirm atom existence?

Answer 1

The law of constant composition and the law of multiple proportions.

Question 2

What is the law of constant composition?

Answer 2

The ratio of the masses of elements in a compound is the same, however much of it there is. E.g In water, hydrogen combines with oxygen so that the ratio is mass of oxygen/mass of hydrogen = 16/2.

Question 3

What is the law of multiple proportions?

Answer 3

When two elements combine to produce different compounds the mass of one element that combines with a fixed mass of another element is in a simple ratio 1:1, 1:2, 2:3, etc.

Question 4

What are 2 pieces of evidence for atoms and molecules from gas behaviour?

Answer 4

Diffusion and brownian motion.

Question 5

How does diffusion give evidence for atoms and molecules?

Answer 5

The way that the smell of a gas spreads slowly across a room cannot be explained if a gas is just one homogenous mass. Atoms of one gas, however, can move freely amongst other gas atoms.

Question 6

How can diffusions proof of atom existence be carried out as an experiment?

Answer 6

Using bromine- the bromine liquid vaporises when row phial in the rubber tube is broken and can move up the tube.
Vacuum- the brown colour of bromine immediately fills the tube showing that the atoms move at a high speed.
Space contains air the colour spreads slowly because the bromine atoms collide with the atoms and molecules that make up air so hindering their progress. This also shows that within the gas there is space for atoms and molecules to move between each other.

Question 7

What did Robert Brown notice in ‘the brownian motion’?

Answer 7

Robert Brown noticed the random movement of pollen suspended in a liquid. The same effect is observed when smoke particles suspended in air are viewed under a microscope.

Question 8

Give 5 points that explain Brownian motion in terms of atoms.

Answer 8

• Smoke particles are bombarded by atoms and molecules in air.
• The atoms and molecules have momentum and exert a force on the smoke particle.
• The collisions are random and the smoke particles are small.
• At any time a resultant force exists as there are more collisions taking place at one side of the particle that the other.
• The direction of the resultant force continually changes direction and produces the ‘jerky’ random motion.