Value Based Questions in Science for Class 9 Chapter 3 Atoms and Molecules

These Solutions are part of Value Based Questions in Science for Class 9. Here we have given Value Based Questions in Science for Class 9 Chapter 3 Atoms and Molecules

Question 1.
Kamla prepared aqueous solutions of barium chloride and sodium sulphate. She weighed them separately and then mixed them in a beaker. A white precipitate was immediately formed. She filtered the precipitate, dried it and then weighed it. After reading this narration, answer the following questions :

  1. Will the weight of the precipitate be the same as that of the reactants before mixing ?
  2. If not, what she should have done ?
  3. Which law of chemical combination does this support ?
  4. What is the value based information associated with it ?


  1. No, it will not be the same.
  2. She should have weighed the total contents of the beaker after the reaction and not the precipitate alone.
  3. It supports the law of conservation of mass.
  4. Whenever the law of conservation of mass is to be verified in the laboratory, total mass of the reactants and also of products should be taken into account. Moreover, none of the species be allowed to leave the container.

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Question 2.
In order to verify the law of conservation of mass, a student mixed 6.3 g of sodium carbonate and 15.0 g of ethanoic acid in a conical flask. After experiment, he weighed the flask again. The weight of the residue in the flask was only 18.0 g. He approached the teacher who guided him to carry the experiment in a closed flask with a cork. There was no difference in weight of the flask before and after the experiment.

  1. What was the mistake committed by the student ?
  2. Why did not the two weights match earlier ?
  3. How did the teacher help him ?
  4. What lesson was learnt by the student ?


  1. He was carrying the experiment in an open flask.
  2. CO2 gas evolved in the reaction escaped from the flask
    2CH3COOH + Na2CO3 —-> 2CH3COONa + H2O + CO2
  3. Teacher asked him to cork the flask the moment the reactants were mixed.
  4. The student learnt that in future he should not carry the experiment relating to the law of conservation of mass in an open container, particularly when one or more reactants or products are in the gaseous state.

Question 3.
A student was asked by his teacher a verify the law of conservation of mass in the laboratory. He prepared 5% aqueous solutions of NaCl and Na2SO4. He mixed 10 mL of both these solutions in a conical flask. He weighed the flask on a balance. He then stirred the flask with a rod and weighed it after sometime. There was no change in mass. Read this narration and answer the questions given below :

  1. Was the student able to verify the law of conservation of mass ?
  2. If not, what was the mistake committed by him ?
  3. In your opinion, what he should have done ?
  4. What is the value based information associated with this ?


  1. No, he could not verify the law of conservation of mass inspite of the fact that there was no change in mass.
  2. No chemical reaction takes place between NaCl and Na2SO4. This means that no reaction actually took place in the flask.
  3. He should have performed the experiment by using aqueous solutions of BaCl2 and Na2SO4. A chemical reaction takes place in this case and a white precipitate of BaSO4 is formed.
  4. While working in the chemistry laboratory, a student must select those chemical substances which actually react with each other. Only then products will be formed.

Question 4.
Dalton was the first scientist to introduce symbols for the known elements. Modern symbols were given by J.J. Berzelius. A symbol in general may be defined as the short hand representation of the name of an element.

  1. How do symbols help in identifying elements ?
  2. Do we use symbols in daily life ?
  3. What values do you attach for using symbols ?


  1. All the known elements are identified by their symbols.
    For example, Symbol of calcium = Ca; Symbol of copper = Cu; Symbol of iron= Fe
  2. Yes, these are very common in daily life. For example, all road signs such as diversions, dangerous, zones etc. are indicated by symbols. In playground, umpires, signify the various happenings such as ‘LBW’, ‘Out’ etc. in circket by symbols.
  3. Symbols for road signs save many lives. The names of many complicated compounds are shown by the formulae which are collection of symbols. The chemical composition of all madicines are shown either on the strips or on the bottles by their formulae.

Question 5.
Mole concept is an important tool for dealing with chemical calculations. The elements have atomic masses while compounds have molecular masses or molar masses. Mole is in fact, a collection of Avogadro’s number (NA) of the particles of a substance whether element or compound. The value of Avogadro’s number is 6-022 x 1023.

  1. Why is mole commonly called chemist’s dozen ?
  2. What is the value associated with the term mole ?


  1. Just as a dozen represents 12 articles, a mole represents 6.022 x 1023 or Avogadro’s number of particles. Therefore, it has been rightly called chemist’s dozen.
  2. Since particles such as atoms, ions or molecules are very extremely small in size, it is very difficult to identify and express them individually. These are collectively represented as mole. For example, 3.011 x 1023 molecules of CO2 gas are shown as 0.5 mole which is very simple.

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