Select from the following salts which may be used to prepare hard water required for the experiment.

A. CaSO₄

B. Na₂SO₄

C. CaCl₂

D. K₂SO₄

E. NaHCO₃

F. MgCl₂

In the preparation of soap, sodium chloride causes:

Vishakha carried out saponification reaction in the lab and she gave following comments as:

(i) Soap is salt of fatty acid.

(ii) Heat is absorbed in this reaction.

(iii) Reaction mixture is basic in nature

(iv) These are neutralization reaction

The correct statements are:

Four students A, B, C and D did their experiment of finding the focal length of convex lens by obtaining image of a distant object as follows:

• Student A used the window grill in the laboratory as the object and a white paper sheet held in hand as the screen.

• Student B used a distant tree in the shade and a white thick held in a stand as the screen.

• Student C used a well illuminated laboratory window grill as the object and a white paper sheet held in a stand as the screen.

• Student D used a well illuminated distant tree as the object and a white thick board held in a stand as the screen.

Which student has used the correct method for performing experiment out of the above setups?

In the setup shown below, a clear image of a distant object is obtained on the screen. The focal length of the concave mirror is: