Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following does not give flame colouration? (a) \(\mathrm{Mg}\) (b) \(\mathrm{Ca}\) (c) \(\mathrm{Sr}\) (d) \(\mathrm{Ba}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Mg}\) does not give flame colouration.

Step 1: Understand Flame Tests

In a flame test, a metal salt is heated in a flame, and the color of the flame can be used to identify the metal. Certain metals emit characteristic colors when burned due to the excitation of electrons. This property is used for identification purposes.

Step 2: Review Flame Colors of Given Metals

Now, let's examine the flame colors of the given metals: \(\mathrm{Ca}\) (Calcium) gives a brick-red flame, \(\mathrm{Sr}\) (Strontium) gives a crimson flame, and \(\mathrm{Ba}\) (Barium) produces a green flame. \(\mathrm{Mg}\) (Magnesium), however, does not impart a distinct color to a flame.

Step 3: Identify the Metal Without Flame Color

From the review of flame colors, we observe that \(\mathrm{Mg}\) does not give color in a flame test, unlike \(\mathrm{Ca}\), \(\mathrm{Sr}\), and \(\mathrm{Ba}\) which all produce distinct flame colors.

Key Concepts

Electron Excitation

When you expose certain metal salts to a flame, you can observe unique colors. This phenomenon originates from electron excitation. But what exactly is electron excitation? Let's break it down.
Electrons normally occupy the lowest energy levels in an atom, known as ground state. When energy is applied, such as through heat from a flame, the electrons absorb this energy and become excited. This moves them to a higher energy level, or excited state.
However, electrons prefer stability. So, they quickly return to their ground state, and in doing so, they release energy. This released energy often aligns with the visible spectrum, which is what we see as color during a flame test.

Characteristic Flame Colors

Different metals emit different colors when subjected to a flame test, and this occurs because each metal has a unique electronic configuration. During electron transitions, the energy released can exhibit a variety of distinct flame colors.
Examples include:

• Calcium (Ca): Produces a brick-red color
• Strontium (Sr): Gives off a brilliant crimson red
• Barium (Ba): Shows up as a green hue

These colors serve as a fingerprint or signature, providing a simple way to identify specific metals. However, not all metals produce a noticeable color. For example, magnesium (Mg) does not impart a distinct flame color because its emitted energy may fall outside of the visible spectrum.

Metal Identification

Flame tests are a quick method for identifying certain metal ions, but they have their limitations. The simplicity of using visible color changes helps in rapid identification but can sometimes be compromised by a few factors.
For reliable metal identification, flame tests are best used in combination with other techniques such as spectroscopy for increased accuracy. The following features make flame tests popular despite their limitations:

• Quick and easy: No need for advanced equipment.
• Visual Appeal: Immediate visible results make them intuitive.

However, in cases where metals do not emit distinct flame colors, or when mixed salts burn, flame tests alone might not be sufficient. That's why knowing the principles of electron excitation and emission spectra greatly aid in more definitive identifications.