10 Sci Chp 1 revsn notes

  10 SCIENCE REVISION NOTES   

            CHAPTER 1 CHEMICAL REACTIONS AND EQUATIONS      

Chemical reaction: chemical reaction is the process where reactants are transformed into products.

These are the following changes to determine that the chemical reaction has taken place:

★     Change in state

★     Change in colour

★     Evolution of gas

★     Change in temperature

Chemical change: A change in which a new substance is formed is called chemical change.

Examples: Rusting of iron, milk turning into curd

Physical change: A change in which no new substances is formed is called physical change.

Examples: Melting of ice, vaporisation of water on heating.

Chemical Equation: A chemical equation is the symbolic representation of a chemical reaction.

Examples:  2Mg   +   O₂     →    2MgO                                   

                      (Reactant)           (Product)

 

Balanced chemical equation:

★     The chemical equation needs to be balanced so that it follows the law of conservation of mass.

★     In balanced chemical equation the number of the atoms  in the reactants side is equal to the number of atoms in the products side.

Examples:

o   Zn(s)  + H₂SO₄(aq)  →  ZnSO₄(s)   + H₂(aq)

o   3Fe(s) + 4H₂O(g) →  Fe₃O₄(s) + 4H₂(aq)

 Types of chemical reaction:

Combination reaction: a single product is formed from two or more reactants is called combination reaction.

Examples:

o   CaO(s)  +  H₂O(l)  →  Ca(OH)₂(aq)

o   C(s)  +  O₂(g)  →  CO₂(aq)

o   H₂(g)  +  O₂(g)  →   H₂O(aq)

Decomposition reaction: single reactant breaks down to give two or more products is called decomposition reaction.

Examples:

                                Heat

o   FeSO₄(s)      →    Fe₂O₃(s) + SO₂(g) +SO₃(g)

                                       Heat

o   2Pb(NO₂)₃(s)   →  2PbO(s) + 4NO₂(g) + O₂(g)

                             Sunlight

o   2AgCl(s)     →     2Ag(s) + Cl₂(g)

Sunlight

o   2AgBr(s)      →      2Ag(s) + Br(g)

                         Electricity

o   2H₂O(l)    →   2H₂(g) + O₂(g)

Electrolysis of water:

Cathod: H₂ gas

Anod: O₂ gas

Displacement reaction:  More reactive element displaces a less reactive element is called Displacement reaction.

Examples:

    o     Fe(s) + CuSO₄(aq) →  FeSO₄(g) + Cu(s)

    o   Pb(s) + CuCl₂(aq) →  PbCl₂(aq) + Cu(s)

Double Displacement reaction: Exchange of ions between the reactants is called double displacement reaction.

Examples:

Na₂(SO)₄(aq) + BaCl₂(aq) → BaSO₄(s) + NaCl(aq)

                                                     (White ppt)

A chemical reaction in which an insoluble substance formed is called precipitation reaction.

Exothermic reaction: Heat energy produced in the reaction is called exothermic reaction.

Examples:

CH₄(g) + 2O₂ (g) → CO₂ (g) + 2H₂O(g)

Endothermic reaction: Heat energy absorbed by the reaction is called endothermic reaction.

                                               Heat

Examples:  CaCO₃(s)   →   CaO(s) + CO₂(g)

Oxidation reaction: A reaction in which oxygen is added or hydrogen is removed is called oxidation reaction.

Examples: 2Cu + O₂ → 2CuO

Reduction reaction: A reaction in which oxygen is removed or hydrogen is added is called reduction reaction.

Examples: ZnO + C → Zn + CO

Redox reaction: A reaction in which both oxidation reduction takes place is called redox reaction.

Examples:     

             

CuO       +    H₂         →      Cu       +      H2O

Oxidized substance – H₂

Reduced substance – CuO

Corrosion: The process of slow conversion of metals into their undesirable compounds due to their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called corrosion.

Examples:     

★     The black coating formed on the surface of silver is silver carbonate

★     The green coating on copper is copper carbonate.

★     Rusting of iron. 

Prevention of corrosion:

★     Painting

★     Oiling and greasing

★     Allying

★     Galvanising

 

Rancidity:

★  The taste and odour of food materials containing fat and oil changes when they are left exposed to air for long time. This is called rancidity.

★     It is caused due to oxidation of fat and oil present in food material.

Prevention of corrosion:

★     Adding antioxidants

★     Storing food in air tight container

  ★     Flushing out air with nitrogen

10 chp 5 revsn ntes

     10 SCIENCE REVISION NOTES   

            CHAPTER 5 PERIODIC CLASSIFICATION OF ELEMENTS      

Several attempts were made to classify elements based on their properties to have an orderly arrangement.

Dobereiner's Triads:

Johann Wolfgang Dobereiner identifed some groups having three elements and called them as triads. He stated that when three elements are in a triad in the increasing order of atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements.

Example: CI, Br, I

Atomic mass of Br

= Atomic mass of CI + Atomic mass of Br = 35.5 +126.9

                                       2                                          2

= 79.9 for Br.

‣ This system of classification into triads failed because there were only three triads identified. Therefore this classification failed to arrange all the elements in an orderly way:

Newlands' Laws of Octaves:

It states that when elements are classified in a row in the increasing order of atomic mass, every eighth element has the properties similar to that of the first element

Example: Lithium being the first element, the eighth element is sodium, which resembles lithium in its properties

Li, Be, B, C, N, O, F Ne, Na

‣ This classification failed because

★    The law was applicable only up to calcium

★    There were only 56 elements at that time

★    Newlands adjusted two elements in the same slot

★    With the discovery of noble gases, the law of octaves became irrelevant

Mendeleev's Periodic Law:

it states: "The properties of the elements are the periodic functions of their atomic masses"

‣ Characteristics of Mendeleev's Periodic Table:

★ The classification was made based on increasing order of atomic masses

★ There were only 63 elements

★ There were 8 groups and 6 periods

★ There were some gaps left where the new elements discovered could fit in it.

‣ Demerits:

★ He placed elements of slightly greater atomic mass before elements with slightly lower atomic mass

★    There was no fixed position for hydrogen

★    It was not classified on electronic configuration

★    Isotopes of all elements was a challenge to Mendeleev's Periodic Law.

Modern Periodic Table:

Modern Periodic Table is based on atomic numbers of elements. Modern periodic law states: "The properties of the elements are the periodic function of their atomic numbers. All the elements are arranged in the increasing order of their atomic numbers

‣ Characteristics and features of the Modern Periodic Table:

★ Elements are classified in the increasing order of their atomic numbers

★There are 18 groups and 7 periods (Vertical columns are called groups and horizontal rows are called periods.

★ Elements are classified based on their electronic configuration

★ Metals, non-metals and metalloids are placed in their proper positions

★ All noble gases are placed in the 18" group. These elements have stable electronic configuration

‣ Trends in Modern Periodic Table:

★    Valency: Valency is determined by the number of valance electrons present in the outermost shell

In a group: For the elements of the groups I, 2, I3 and 14, the valency is equal to the number of valence electrons. For the groups 15 onwards, valency =8- valency electrons.

In a period: The valency increases from I to 4 and decreases to zero from |5 to 18.

★  Atomic size: It is defined as the radius of an atom. It is the distance between the centre of the nucleus and the outermost shell of an isolated atom.

Along a group: Atomic size increases down the group because new shells are being added

In   a period: Atomic radius decreases in moving from left to right along a period.

★  Metallic and non-metallic character:

Metallic character decreases across a period and increases down a group. Non-metallic character increases across a period and decreases down a group.

★ Nature of oxides: Along a period the basic character of oxides of the elements decreases, while their acidic character increases.

Down a group basic character of oxides increases and acidic character decreases.

★  Electronegativity:

Electronegative character decreases on going down in a group and increases along a period.

★  Shells in an atom: The maximum number of shells that can be accommodated in a shell is given by the formula 2n²

K Shell – 2 x 1² = 2

L Shell – 2 x 2² = 8

The third, fourth, fifth, sixth and seventh periods have 8, 18, 18, 32 and 32 elements respectively.

Cls 8 Sci Chp 1 PPT & Notes

8 Science PPT

 Chapter - 1, Crop Production and Management, Science

 

Class 8 Science Notes

Chapter - 1, Crop Production and Management, Science, 

• 
  
• Introduction
• ★ All the living organisms require food for their life processes like digestion, respiration,excretion etc. 
• ★ Plants can make their own food by the process of photosynthesis and animals including humans cannot make their food so they are dependent on plants and other animals.
• ★ In order to provide food for a large population - regular production,proper management and distribution of food is necessary.
• 
  
• Crop 
• ★ When the same kinds of plants are grown at one place on a large scale, it is called a crop.
• ★ Different type crops like cereals, vegetables and fruits etc, can be classified on the basis of the season in which they grow. 
• 
  
• 1. Kharif Crops 2. Rabi Crops
• 
  
• Kharif Crops 
• ★ The crops which are sown in the rainy season are called kharif crops. 
• ★ The rainy season in India is generally from June month to September month. 
• ★ Paddy,maize, soya-bean,groundnut, cotton,etc., are the examples of kharif crops.
• 
  
• Rabi Crops 
• ★ The crops which are grown in the winter season are called Rabi crops. 
• ★ Their time period is generally from October month to March month. 
• ★ Wheat, gram, pea, mustard and linseed are the examples of Rabi crops.
• 
  
• Basic Practice Of Crop Production 
• ★ Preparation of Soil 
• ★ Sowing 
• ★ Adding manure and fertilizers 
• ★ Irrigation 
• ★ Protecting from weeds 
• ★ Harvesting 
• ★ Storage
• 
  
• Preparation of Soil  
• ★ One of the most important tasks in agriculture is to turn the soil and loosen it. 
• ★ The loose soil allows the roots to penetrate and breathe easily even when they go deep into the soil. 
• ★ The loosened soil helps in the growth of earthworms and microbes present in the soil. 
• ★ The process of loosening and turning of the soil is called tilling or ploughing. This is done by using a plough which is made of wood or iron.

• Sowing and quality of seeds 
• ★ Sowing is the process of planting the seeds in the soil. 
• ★ The seeds are sowed in the soil that is loosened by cultivator or plough. 
• ★ Quality of the seed is an important factor that determines the crop yield.  Selection of good seeds is done by putting the seeds in water. 
• ★ The dead and damaged seeds become hollow and float on water whereas the good seeds sink.
• 
  

• Traditional tools (sowing)  
• ★ Before the advent of modern agricultural machinery, traditional tools were used by farmers. 
• ★ The traditional tool used to sow the seeds is like a funnel. 
• ★ Once seeds put into this funnel, they would go to 2-3 tubes having sharp ends.  The ends will pierce into the soil and place the seeds there.
• 
  
• Seed drill (sowing) 
• ★ Seed drills are used for sowing with the help of tractors. 
• ★ It ensures that seeds are sown uniformly, at a particular depth and are covered by soil after sowing.
• 
  

• Manure / Fertilizers 

• ★ Manures and fertilizers are the substances that are added to the soil to increase their fertility. 
• ★ While manures are made by decomposition of organic substances, fertilizer is made of inorganic chemicals.
• 
  
• Manure   
• ★ Easily made using animal and plant waste.  
• ★ No harmful effect as fully natural. 
• ★ Good for long term soil fertility. 
• ★ Not effective as fertilisers. 
• ★ It is very cheap. 
• 
  
• fertilizers
• ★ Made in factories in well defined way.
• ★ Side effects as it is a chemicals. 
• ★ Not good for long term soil fertility, If uses in excessive amount. 
• ★ Very effective in obtaining fast result.
• ★ It is very costly.

• Irrigation 
• ★ Water is important for proper growth and development of plants. 
• ★ Water is essential because germination of seeds does not take place under dry conditions. 
• ★ Nutrients dissolved in water get transported to each part of the plant. 
• ★ Sources of Irrigation: Wells,Tube wells, Ponds, Lakes, Rivers, Dams and Canals are the sources of Irrigation.
• 
  
• Traditional methods of irrigation 
• The water available in wells, lakes and canals is lifted up by different methods in different regions, for taking it to the fields. 

• Modern methods of irrigation 

• Sprinkler System : In this system, perpendicular pipes, having rotating nozzles on top, are joined to the main pipeline at regular intervals.When water is allowed to flow through the main pipe under pressure with the help of a pump,it sprinkles from the rotating nozzles.
• 
  
• Drip system : In this system,the water falls drop by drop just at the position of the roots.So it is called drip system.
• 
  
• Weeds 
• ★ Weeds are undesirable plants that may grow naturally along with the crop. 
• ★ Weeds compete with the crops by absorbing all the water, nutrients, space and light.

• Protecting from weeds - tilling / manual removal / weedicides

• ★ Tilling is a process done before sowing of crops that helps in uprooting and killing of weeds. 
• ★ Manual removal includes physical removal of weeds by uprooting them from the soil or chopping them off to ground level periodically. 
• ★ Chemicals used to kill the weeds are known as weedicides.They usually don’t damage the crop.
• 
  
• Harvesting
• The cutting of crop after it is mature is called harvesting.In harvesting,crops are pulled out or cut close to the ground. Harvesting is also done manually by sickle or by a machine. Manual Harvesting Harvesting (Machine)
• 
  
• Threshing
• ★ Threshing is the process of loosening the grains from the chaff. 
• ★ While it can be done manually, these days a machine is used that separates all the grain seeds. Manually Machine (Combine)
• 
  
• Winnowing
• ★ Winnowing is the process of separation of grain seeds from the chaff using the help of the wind. 
• ★ Due to the wind, the lighter chaff flies away and the heavier grains fall down.
• 
  
• Storage
• ★ Storage of the grains is an important step in agriculture. 
• ★ After harvesting steps, the ready grains are stored in granaries or silos. 
• ★ The grains have to be stored in a dry place that does not have a rodent or fungal infestation.  
• ★ Granaries are the place where the freshly obtained food grains are stored. 
• ★ Animal husbandry is the management and care of farm animals for milk, egg or meat.