Hard water required for an experiment is not available in a school laboratory. However, following salts are available in the laboratory. Select the salts which may be dissolved in water to make it hard for the experiment.

1. Calcium Sulphate

2. Sodium Sulphate

3. Calcium Chloride

4. Potassium Sulphate

5. Sodium Hydrogen Carbonate

6. Magnesium Chloride

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

I. A colourless and odourless gas evolves with a brisk effervescence.

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

III. The gas burns 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 prepared 20% sodium hydroxide solution in a beaker containing water. The observations noted by him are given below. 1

I. Sodium hydroxide is in the form of pellets.

II. It dissolves in water readily.

III. The beaker appears cold when touched from outside.

IV. The red litmus paper turns blue when dipped into the solution.

The correct observations are:

A student focussed the image of a distant object using a device 'X' on a white screen 'S' as shown in the figure. If the distance of the screen from the device is 40 cm, select the correct statement about the device.

Select from the following the best set-up for tracing the path of a ray of light through a rectangular glass slab: