English Medium Notes of 9th class Chemistry chapter 7 and 8

Unit 7

Oxidation is addition of oxygen or removal of hydrogen or loss of electrons by an element and as a result oxidation number increases.

Reduction is addition of hydrogen or removal of oxygen or gain of electrons by an element and as a result oxidation number decreases.

Oxidation number is the apparent charge on an atom. It may be positive or negative.

Oxidizing agents are the species that oxidize the other element and reduce themselves. Non-metals are oxidizing agents.

Reducing agents are species that reduce the other elements and oxidize themselves. Metals are reducing agents.

Chemical reactions in which the oxidation state of species change are termed as redox reaction. A redox reaction involves oxidation and reduction processes taking place simultaneously.

Redox reactions either take place spontaneously and produce energy or electricity is used to drive the reaction.

The process in which electricity is used for the decomposition of a chemical compound is called electrolysis. It takes place in electrolytic cells such as Downs cell and Nelson’s cell.

Galvanic cells are those in which spontaneous reactions take place and generate electric current. They are also called voltaic cells.

Sodium metal is manufactured from fused sodium chloride in the Downs cell.

NaOH is manufactured from brine in Nelson’s cell.

Corrosion is slow and continuous eating away of a metal by the surrounding medium. The most common example of corrosion is rusting of iron.

The rusting principle is electrochemical redox reaction, in which iron behaves as anode. Iron is oxidized to form rust Fe2O3. nH2O.

Corrosion can be prevented by many methods. The most important is electroplating .

Electroplating is depositing of one metal over the other by means of electrolysis .

Iron can be electroplated by tin, zinc, silver or chromium.

 

 Unit 8

Formation of cations of alkali and alkaline earth metals is due to their electropositive behavior.

The chemical reactivity of alkali and alkaline earth metals, is quite different.

Calcium and magnesium are less reactive than sodium.

Halogens form very stable compounds with alkali metals.

Mercury and gold exist in free elemental form in nature.

Chapter 6 notes of 9th class chemistry

Solution is a homogeneous mixture of two or more substances.

Aqueous solution is formed by dissolving substances in water.

The component which is lesser in quantity is called solute and the component in greater quantity is called solvent.

A solution containing less amount of solute than that is required to saturate it at a given temperature is called unsaturated solution.

A solution that is more concentrated than that of a saturated solution is called as supersaturated solution at that particular temperature.

Solution may be dilute or concentrated depending upon the quantity of dissolved solute in solution.

Concentration of solutions are expressed as % w/w, % w/v, % v/w and % v/v.

The practical unit of concentration is molarity. It is the number of moles of solute dissolved in one dm3 of solution.

Solubility is defined as the number of grams of the solute dissolved in 100 g of solvent to prepare a saturated solution at a given temperature . It depends upon solute-solvent interactions and temperature.

Colloidal solutions are false solutions and in these solutions particles are bigger than in the true solutions.

Chemistry Notes of unit 5 9th class

Gases diffuse very rapidly. Diffusion is mixing up of a gas throughout a space or other gases.

Effusion is escaping of a gas molecule through a fine hole into an evacuated space.

Gases exert pressure. The SI unit of pressure is Nm-2 which is also called Pascal.

Standard atmospheric pressure is the pressure exerted by a mercury column of 760 mm height at sea level, it is equivalent to 1 atmosphere

Gases are highly mobile and they can be compressed.

Gases are 1000 times lighter than liquids or solids hence their density is measured in g dm-3.

Boyle’s law states that volume of a given mass of a gas is inversely proportional to the pressure at constant temperature.

Charles’ Law states that volume of a given mass of a gas is directly proportional to the absolute temperature at a constant pressure.

Absolute zero is the temperature at which an ideal gas would have zero volume, it is -273.15 °C.

The conversion of a liquid into vapours at all temperatures is called evaporation. It is a cooling process.

Evaporation depends upon surface area, temperature and intermolecular forces.

Vapour pressure of a liquid is defined as the pressure exerted by the vapours when liquid and vapour states are in dynamic equilibrium with each other.

Boiling point is the temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure or any external pressure.

Boiling point depends upon the nature of liquid, intermolecular forces and external pressure.

Freezing point of a liquid is that temperature at which vapour pressure of liquid phase is equal to the vapour pressure of the solid phase. At this temperature liquid and solid coexist in dynamic equilibrium with one another.

Melting point of solid is the temperature at which solid when heated melts and coexist in dynamic equilibrium with liquid.

Solids are rigid and denser than liquids.

Solids are classified as amorphous and crystalline .

Amorphous solids are shapeless and do not have sharp melting point.

Crystalline solids have definite three dimensional pattern of arrangement of particles .They have sharp melting points.

The existence of a solid in different physical forms is called allotropy.