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?

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

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

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)

After tracing the path of a ray of light passing through a rectangular glass slab for four different values of the angle of incidence, a student reported his observations in tabular form as given below:

The best observation is