5) Write short note on mendeleev's periodic law.
6) Define redox reaction with suitable example.
Give scientific reasons (Any three)
1) Elements belonging to the same group have the same valency.
2) Weight of body is different on different planets.
3) Grills of doors and windows are always painted before they are used.
4) It is recomanded to use air tight container for storing oil for long time
Answers
5)
Mendeleev's periodic table -
1. It was developed in 1869 by Dmitri Mendeleev .
2. It is based on the increasing atomic mass.
3. Oxides and hydride are very important features of it.
4. It has 6 periods and 8 groups.
5. It was the base for Modern Periodic Table.
6) Redox Reaction-
The reaction in which oxidation and reduction take place simultaneously is called Redox reaction.
Eg.. Na +. O2 —> Na2O
Sodium. Oxygen. Sodium Oxide
Here,
Sodium is oxidised to Na2O and Oxygen is reduced to Na2O.
Give reasons(any 3)
1) Elements belonging to same group have same valency-
1.1 Each group has the elements whose valence electrons are same.
1.2 And the valency is the very valence electrons for metals and 8- valence electrons for non metals .
1.3 So each element of a group has same valency.
Eg . Lithium , Sodium, Potassium, Rubidium etc have valency 1.
3) Grills of doors and windows are always painted before they are used because-
3.1 Paint ensures that the metal used in grill does not get in direct contact with water, oxygen and other chemicals.
3.2 Paint is used to avoid Corrosion as it cuts supply of gases.
3.3 Effect- The grills become more durable as corrosion is avoided.
4) It is recommended to use air tight containers for storing oil for long time because-
4.1 Oils may undergo rancidity when it gets in contact with oxygen.
4.2 Effect- unpleasant smell.
4.3 So by using airtight container, we cut Oxygen supply to oil and avoid rancidity. Which helps to prevent bad smell from oil.