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

Magnesium (mMg) does not give flame colouration.

Step 1: Understanding Flame Tests

Flame tests are used to identify the presence of certain metal ions based on the color they emit when heated in a flame. Each ion produces a characteristic color.

Step 2: Metal Ions and Flame Colors

For the given elements, we recall their typical flame colors: Calcium (kCa) produces an orange-red flame, Strontium (gSr) results in a red flame, and Barium (mBa) gives a green flame color.

Step 3: Checking Magnesium

Magnesium (mMg) typically does not produce a visible flame color when subjected to a standard flame test. It emits a bright white light when burned in the presence of pure oxygen, not in a normal flame test.

Step 4: Conclusion from Observations

From the above observations, Magnesium (mMg) does not produce a flame coloration under normal flame test conditions, unlike Calcium, Strontium, and Barium.

Key Concepts

Metal Ions

In chemistry, metal ions play a crucial role in flame tests. During a flame test, a metal ion is introduced to a flame and, depending on its properties, it can emit light of a specific color. This happens because the heat of the flame excites the electrons in the metal ions. These excited electrons then jump to higher energy levels. As the electrons return to their original energy levels, they release energy in the form of light. The specific wavelengths of light emitted correspond to particular colors. For instance, calcium ions give off an orange-red color, while barium ions produce a green color. Understanding the behavior and interactions of metal ions is essential for interpreting flame tests. This knowledge aids in identifying elements based on their emission spectra, which is like a unique fingerprint for each element. This technique provides a simple and effective method for analyzing the composition of unknown samples.

Flame Colouration

Flame colouration refers to the specific colors produced by different chemicals when they are burned. For metal ions, the colors are a result of the ions’ electrons being excited by the flame.

• Calcium (\(\mathrm{Ca}\)): Produces an orange-red color.
• Strontium (\(\mathrm{Sr}\)): Gives a bright red hue.
• Barium (\(\mathrm{Ba}\)): Emits a green color.

This vivid coloration is often used in practical applications, such as fireworks and pyrotechnics, to create visually stunning displays. Each element's unique flame color helps in identifying the presence of specific metal ions in a sample. It is important to note that not all metal ions produce an easily visible flame coloration; this depends on their emission spectra properties.

Magnesium Flame Test

The magnesium flame test is an interesting case when comparing it to other metals. While many metal ions produce distinct flame colors, magnesium does not display a characteristic color under standard flame test conditions. When magnesium is burned, it typically requires pure oxygen for the reaction to produce any visible change, which results in a bright white light. However, under regular flame test conditions, typically found in a laboratory, magnesium remains colorless. This property distinguishes magnesium from elements like calcium, strontium, and barium, which are known for their vibrant flame colors. The absence of a visible flame color in magnesium can be accounted for by its high energy transition, which doesn't fall within the visible part of the electromagnetic spectrum. So, while conducting a magnesium flame test, one would not expect a color change, making it an exception among the metal ions commonly tested.