Assertion (A): Alkali metlas impart colour to the flame.
Reason (R ): The ionisation energies are low.

Options

A. If both (A) and (R ) are correct and (R ) is the correct explanation of (A).
B. If both (A) and (R ) are correct, but (R ) is not the correct explanation of (A).
C. If (A) is correct, but (R ) is incorrect.
D. If (A) is incorrect, but (R ) is correct.

Answer

To solve the assertion and reason question regarding alkali metals, we will break down the components step by step.

### Step 1: Understand the Assertion
The assertion states that "Alkali metals impart color to the flame." This is a well-known property of alkali metals. When alkali metals are heated in a flame, they emit characteristic colors due to the excitation of their valence electrons.

**Hint:** Recall that different elements emit different colors when heated due to their unique electron configurations.

### Step 2: Understand the Reason
The reason provided is that "The ionization energies are low." Alkali metals have low ionization energies, which means that their outermost electrons can be easily excited to higher energy levels when they absorb heat from the flame.

**Hint:** Consider how ionization energy relates to the ability of an atom to lose an electron and how this affects electron excitation.

### Step 3: Connect Assertion and Reason
The low ionization energy of alkali metals allows their valence electrons to be easily excited. When these electrons return to their ground state, they release energy in the form of light, which is observed as color in the flame. This connects the assertion to the reason.

**Hint:** Think about the process of excitation and de-excitation of electrons and how this relates to the emission of light.

### Step 4: Identify the Colors of Alkali Metals
Different alkali metals impart specific colors to the flame:
- Lithium: Crimson red
- Sodium: Golden yellow
- Potassium: Pale violet
- Rubidium: Red violet
- Cesium: Violet

**Hint:** Memorize the characteristic flame colors for each alkali metal, as this is often a part of chemistry assessments.

### Step 5: Conclusion
Both the assertion and the reason are correct, and the reason correctly explains the assertion. Therefore, the answer to the question is that both the assertion and reason are true, and the reason is the correct explanation for the assertion.

**Final Answer:** A (Assertion is correct, Reason is correct, and Reason is the correct explanation of the Assertion).