(b) Intermolecular forces of attraction
(a) Matter is anything that has mass, occupies space and can be perceived by our senses.
(b) A force of attraction that exists between the particles of matter which holds them together is known as intermolecular force of attraction.
What are the three states of matter? Define each of them with two examples.
The three states of matter are solids, liquids and gases.
Solids — A solid has a definite shape and a definite volume. The two examples are iron, wood.
Liquids — A liquid has a definite volume but no definite shape. The two examples are water, alcohol.
Gases — A gas has neither a definite shape nor a definite volume. The two examples are hydrogen, oxygen.
Explain interconversion of states of matter. What are the two factors responsible for the change of state of matter?
The phenomenon of change of one state of matter into another and then back to the original state without any change in its chemical composition is called interconversion of the states of matter.
The two factors responsible for the change of state of matter are:
- change in temperature
- change in pressure.
State the main postulates of kinetic theory of matter.
The main postulates of kinetic theory of matter are:
- Matter is composed of very small particles called atoms and molecules.
- The constituent particles of a kind of matter are identical in all respects.
- These particles have spaces or gaps between them which are known as inter-particular or inter-molecular spaces.
- A force of attraction exists between the particles of matter which holds them together known as interparticular or intermolecular force of attraction.
- Particles of matter are always in a state of random motion and possess kinetic energy which increases with an increase in temperature and vice-versa.
What happens to water if:
(a) it is kept in a deep freezer?
(b) it is heated?
Explain the phenomenon of change of state of water.
(a) When water is kept in a deep freezer, it gets cooled and changes into ice at 0°C.
Water ice (0°C)
(b) When water is heated it changes into steam at 100°C.
Water steam (100°C)
Phenomenon of change of state of water — In ordinary conditions water is in liquid state. When it is kept in deep freezer, it changes into ice at 0°C. But ice changes back into liquid state when ice is kept at room temperature.
On heating water, it changes into steam at 100°C. On cooling steam changes back into liquid water. During the change of state from liquid to solid or liquid to gas there is no change in the chemical composition of water during these change of states.
(a) State the law of conservation of mass.
(b) What do you observe when barium chloride solution is mixed with sodium sulphate solution?
(a) The law of conservation of mass states that matter can neither be created nor be destroyed in a chemical reaction. Only it can be changed from one form to another during the reaction process.
(b) When barium chloride solution is mixed with sodium sulphate solution, a white insoluble precipitate of barium sulphate is formed along with a solution of sodium chloride.
BaCl2 + Na2SO4 ⟶ 2NaCl + BaSO4↓
On weighing the apparatus before and after the reactions it is found that:
Total mass of the apparatus + reactants = Total mass of the apparatus + products
This observation verifies the law of conservation of mass.
(a) A gas can fill the whole vessel in which it is enclosed.
(b) Solids cannot be compressed.
(c) Liquids can flow.
(d) When magnesium is burnt in air, there is an increase in mass after the reaction.
(a) The particles of gases lie far apart from each other and the intermolecular spaces are therefore very large. The intermolecular forces of attraction are so weak that the particles are free to move and they have high kinetic energy, so they fill the whole vessel in which it is enclosed.
(b) In solids, the particles are closely packed. There is a strong force of attraction between the particles and the intermolecular space is almost negligible. The molecules are therefore not free to move. They only vibrate about their mean positions. This makes solids difficult to compress.
(c) In liquids the molecules are loosely packed and the intermolecular forces of attraction is not so strong as solids. The intermolecular space is larger and kinetic energy is more as compared to solids. The particles can move freely and randomly which makes the liquids to flow.
(d) When magnesium is burnt in air, it forms magnesium oxide. The mass of magnesium oxide is found to be more than the mass of magnesium because the mass of oxygen used is not taken into consideration. If that is considered the total mass of the reactants and the products are found to be almost equal.
2Mg + O2 2MgO
Fill in the blanks:
(a) The change of a solid into a liquid is called ............... .
(b) The process in which a solid directly changes into gas is called ............... .
(c) The change of water vapour into water is called ............... .
(d) The temperature at which a liquid starts changing into its vapour state is ............... .
(d) Boiling point
Give two examples for each of the following:
(a) The substances which sublime.
(b) The substances which do not change their state on heating.
(a) Camphor, Ammonium Chloride.
(b) Oxygen, Helium.
(b) Brownian motion
(a) The intermixing of two or more substances due to the motion of their particles in order to get a uniform mixture is called diffusion.
(b) The haphazard, random motion of suspended particles on the surface of a liquid or in air is called Brownian motion.
When sodium chloride is added to a definite volume of water and stirred well, a solution is formed, but there is no increase in the level of water. Why?
There are some intermolecular spaces between the particles of water. When sodium chloride is added into water and stirred, the particles of sodium chloride occupy the intermolecular spaces present in water particles and gets accommodated so there is no increase in the level of water.
What do you observe when a gas jar which appears empty is inverted over a gas jar containing Bromine vapours? Name the phenomenon.
Reddish brown vapours of bromine spread out from the gas jar into the empty jar. This phenomenon is called diffusion.
Why can a piece of chalk be broken easily into smaller pieces while a coal piece cannot be broken easily?
The magnitude of intermolecular force of attraction between the molecules of coal is much higher than that between the molecules of chalk. Hence, a piece of chalk can be broken easily into smaller pieces while a coal piece cannot be broken easily.