List of top Science Questions

If bromine atom is available in the form of, say, two isotopes \(^{79}Br_{35}\) (49.7%) and \(^{81} Br_{35}\) (50.3%), calculate the average atomic mass of bromine atom.

\(Na^+\) has completely filled K and L shells. Explain
Compare the properties of electrons, protons and neutrons.
Explain with examples
(i) Atomic number,
(ii) Mass number,
(iii) Isotopes
(iv) Isobars.
Give any two uses of isotopes.
Describe Bohr’s model of the atom.
Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Compare all the proposed models of an atom given in this chapter.
What are the limitations of Rutherford’s model of the atom?
What are the limitations of J.J. Thomson’s model of the atom?
Calculate the molar mass of the following substances.
(a) Ethyne, \(C_2H_2\)
(b) Sulphur molecule, \(S_8\)
(c) Phosphorus molecule, \(P_4\) (Atomic mass of phosphorus= 31)
(d) Hydrochloric acid, \(HCl\)
(e) Nitric acid, \(NO_3\)
Write the chemical formulae of the following.
(a)Magnesium chloride
(b)Calcium oxide
(c)Copper nitrate
(d)Aluminium chloride
(e)Calcium carbonate
What are polyatomic ions? Give examples.
Translate the following statements into chemical equations and then balance them. 
(a) Hydrogen gas combines with nitrogen to form ammonia. 
(b) Hydrogen sulphide gas burns in air to give water and Sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. 
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
What is a balanced chemical equation? Why should chemical equations be balanced?
What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.
A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.
The magnification produced by a plane mirror is +1. What does this mean?
Name the type of mirror used in the following situations. 
  1. Headlights of a car
  2. Side/rear-view mirror of a vehicle
  3. Solar furnace

Support your answer with reason.

We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.
No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
Where should an object be placed in front of a convex lens to get a real image of the size of the object?
The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?