What are the chemical properties of transition metals, how is temporary hardness removed, and why are alkali metals stored under oil?

Here are the solutions to the questions:

1. Beryllium (Be) is in Group 2 of the periodic table. Elements in Group 2 are known as alkaline earth metals.

   • The correct option is B.

2. Elements in Group 1 (alkali metals) have one valence electron.

   • The correct option is A.

3. Reactivity of alkali metals increases down the group. Among Li, Na, and K, potassium (K) is the lowest in the group and therefore the most reactive. Beryllium (Be) is an alkaline earth metal and less reactive than alkali metals.

   • The correct option is B.

4. Alkali metals are highly reactive and react with air and moisture. They are stored under oil to prevent these reactions.

   • The correct option is D.

5. Hard water contains dissolved mineral ions that react with soap to form an insoluble precipitate called scum, preventing lathering.

   • The correct option is D.

6. Temporary hardness can be removed by boiling, adding washing soda, or using ion-exchange resins (like permutit). Adding excess sodium chloride (NaCl) does not remove the hardness-causing ions.

   • The correct option is D.

7. Variable oxidation states, formation of complex ions, and catalytic activity are distinctive chemical properties of transition metals. High tensile strength is a physical property common to many metals, but not as uniquely distinctive to transition metals as the other options.

   • The correct option is C.

8. Manganese (Mn), Iron (Fe), Copper (Cu), and Zinc (Zn) are all d-block elements and are classified as transition metals.

   • The correct option is D.

9. Sodium hydroxide (NaOH) is a strong base and is commonly used to neutralize acids.

   • The correct option is B.

10. Group 1 elements (alkali metals) react with water to form metal hydroxides and hydrogen gas. Metal hydroxides are strong bases and form alkali solutions.

    • The correct option is A.

11. When nitrates of Group 1 elements are heated, they decompose. All Group 1 nitrates (except lithium nitrate) produce oxygen gas ($O_2$). Lithium nitrate also produces oxygen gas, along with nitrogen dioxide. Therefore, oxygen gas is released in both cases.

    • The correct option is B.

12. The characteristic flame colors are: Lithium (Li) is red, Sodium (Na) is yellow, and Potassium (K) is lilac.

    • The correct option is C.

13. Limewater is the common name for an aqueous solution of calcium hydroxide, $Ca(OH)_2$.

    • The correct option is C.