Q26 of 60 Page 1

A student takes about 2 mL ethanoic acid in a dry test tube and adds a pinch of sodium hydrogen carbonate to it. He reports the following observations:

I. Immediately a colourless and colourless gas evolves with a brisk effervescence.


II. The gas turns lime water milky when passed through it.


III. The gas bums with an explosion when a burning splinter is brought near it.


IV. The gas extinguishes the burning splinter that is brought near it.


The correct observations are

When acetic acid is added to NaHCO3 solution, and when a burning splinter is brought near the mouth of each test tube, the splinter would be extinguished due to the evolution of carbon dioxide gas with brisk effervescence from the test tube.


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24

(a) Why do we classify elements?

(b) What were the two criteria used by Mendeleev in creating his Periodic Table?


(c) Why did Mendeleev leave some gaps in his Periodic Table?


(d) In Mendeleev's Periodic Table, why was there no mention of Noble gases like Helium, Neon and Argon?


(e) Would you place the two isotopes of chlorine, Cl-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.

 25

In an experiment to study the properties of ethanoic acid, a student takes about 3 ml of ethanoic acid in a dry test tube. He adds are equal amount of distilled water to it and shakes the test tube well. After sometime he is likely to observe that

 27

In a locality, hard water, required for an experiment, is not available. However, the following salts are available in the school laboratory:

1. Sodium sulphate 2. Calcium sulphate


3. Magnesium chloride 4. Sodium chloride


5. Calcium chloride 6. Potassium sulphate


Which of the above salts may be dissolved in water to obtain hard water for the experiment?

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A student has obtained an image of a well-illuminated distant object on a screen to. Determine the focal length, F1 of the given spherical mirror. The teacher then gave him another mirror of focal length, F2 and asked him to obtain a focused image of the same object on the same screen. The student found that in order to focus the same object using the second mirror, he has to move the mirror away from the screen. From this observation it may be concluded that both the spherical mirrors given to the student were (select the correct option)