cruz lectures states of matter

STATES OF MATTER

The Physical Status of Matter

There are mainly three different kinds of the physical states of matter, namely:
- Solid
- Liquid
- Gas

Plasma a fourth class of matter has also been identified. These states of matter are also termed as phases.

1) Solid : Solids are characterized by their definite shape and also their considerable mechanical strength and rigidity. Solids tend to resist the deformation of their shape due to strong intra molecular forces and absence of the translatory motion of the structural units (atoms, ions etc). A solid is relatively non compressible, i.e. temperature and pressure have only a slight effect on its volume.

Solids are broadly classified as crystalline or amorphous.

Crystalline solids : Here the atoms are arranged in a definite pattern which is constantly repeated.

Amorphous solids : These have no definite geometrical form.

2) Liquid : A liquid has no definite shape and it takes the shape of the vessel containing it. Like solids, the volume of a liquid is slightly altered by variations in temperature and pressure. Liquids have three typical physical properties, namely:

i) Vapor pressure : A Liquid when kept in a closed container vaporizes into the free space above it. The process of vaporization will continue till the equilibrium is reached between liquid and vapor. The pressure at which the liquid and vapor can co-exist is called the vapor pressure of the liquid at a given temperature.

ii) Surface tension : The surface of a liquid is always in a state of tension because a molecule at the surface is attracted towards the bulk by a force much greater than that drawing it toward the vapor where the attracting molecules are more widely spread. Due to this, a certain force is required to penetrate along any line in the surface. This force is called surface tension.

iii) Viscosity : It determines the flow of the liquid. It is the internal friction between layers of the liquid. Higher the rate of friction, greater the viscosity of the liquid and its flow will be retarded. Conversely, a lower rate of friction lessens the viscosity and makes the liquid more fluid.

3) Gases : A gas has no bounding surface at all and will occupy completely any vessel in which it is filled. It has no definite volume or shape and can be easily expanded or compressed.

Water is the ideal example to show the different states of matter.

Water when cooled to 00C becomes solid. When the temperature of solid water is raised it becomes liquid. If the liquid is heated to 1000C it gets converted to steam or vapor (The Gas phase).

Almost all chemical substances can exist in more than one physical state (phase) depending on external pressure and temperature.

The following table illustrates different states of matter and their physical properties.

 Property States of matter Solid Liquid Gas(Vapor) i) Shape definite indefinite indefinite ii) Volume definite definite indefinite iii) MolecularBonding very strong strong Weak iv) Examples NaCl ZnSO4 H2O Petrol H2 , CO2 LPG

The Molecular mode of Solids, Liquids, Gases

Plasma : This is the fourth state of matter. It is a type of gas containing positively and negatively charged particles in approximately equal numbers and present in the sun and most stars.

Nature of Heat

When a change in the state of a system occurs, energy is transferred to or from the surroundings. Energy consumed or lost is commonly studied as heat which is a measure of motions of molecules in a given system. Heat is best understood in terms of rise or fall of temperature of a system.

 The sign convention : According to MKS System dq or q is negative (-ve) if heat is added to the system, dq or q is positive (+ve) if heat is removed from the system where q is the symbol of heat.

Units of heat : The unit of heat, which was used for many years is calorie (Cal). It is defined as the quantity of heat required to raise the temperature of one gram of water at 14.50C by one degree.

SI unit of heat is the joule (J).

1 joule = 0.2390 calories
1 calorie = 4.184 J
1K cal = 1000 calories
1K cal = 4.184 KJ

Heat Capacity : It is the amount of heat required to raise the temperature of the system by one degree.

If the system is one gram of pure substance it is called specific heat, but if one mole of the pure substance is considered, it is called molar heat capacity.

Units of Heat Capacity : The usual units of the molar capacity are calorie per degree per mole (Cal K-1 mol-1 ) or joules per degree per mole ( J K-1mol-1 ) the latter being the SI unit.

The following chart shows the heat capacities of several elements and compounds in calories per degree per gram as well as per mole and their inter conversion.

Table

 ElementsCompounds State Mol.wt/atomic wt. Cal deg.-1gram-1 Cal deg.-1mol-1A ´ mol. Wt H2O Liquid 18.015 1.000 18.015 H2O Solid 18.015 0.485 8.737 CaCO3 Solid 100.087 0.205 20.518 CO2 Gas 44.009 0.158 6.9788 O2 Gas 31.999 0.203 6.500 MgO Solid 40.304 0.208 8.383 Al Solid 26.982 0.213 5.747

Based on the above table let’s calculate heat capacity value.

Amount of heat required in calories to heat 100 gm of H2O(S) from 0.2 to 0.80C is represented as follows :

= (0.8 - 0.2) deg ´ 100 grams ´ 0.485 Cal deg-1gram-1

= 0.6 ´ 100 ´ 0.485

= 29.1 calories

Similarly heat released into the surrounding while cooling 100 grams of H2O(S)from 0.8 to 0.20C will be 29% calories.