The key difference between colours produced by alkali metals and calcium is that the calcium produces a characteristic orange-red flame colour that any of the alkali metals cannot produce.
The flame test is a qualitative analysis technique in which we can get an idea to identify a certain chemical element via looking at the flame colour it gives when we burn that element; mainly metals. However, we cannot use this analytical technique to identify all the metals we know because some metals do not yield a flame colour and some metals have flame colours in common.
CONTENTS
1. Overview and Key Difference
2. What are Colours Produced by Alkali Metals
3. What are Colours Produced by Calcium
4. Side by Side Comparison – Colours Produced by Alkali Metals vs Calcium in Tabular Form
6. Summary
What are the Colours Produced by Alkali Metals?
Colors produced by alkali metals vary according to the metal. Alkali metals are the group 1 chemical elements. The members of this group are lithium, sodium, potassium, rubidium and cesium. For this group 1 metals, flame test is the easiest way of identifying the metal because they give colours, which are different from one another. Now let us consider the method of conducting a flame test.
- Clean a platinum wire by dipping it into concentrated HCl acid.
- Then hold it in a hot Bunsen flame.
- We should repeat the above two steps until the platinum wire shows no flame colour.
- Then moisten the wire again in acid and dip it into the sample of the metal that we are going to test.
- Next, hold the wire along with the sample on it in a hot Bunsen burner. This shows a different colour which is the flame colour of the sample metal
Figure 01: the Flame Colour of Rubidium
When we hold the sample metal in the flame, the heat energy of the flame causes the electrons of the metal to jump from one energy level to a higher energy level. We call this “excitation of electrons”. However, this excitation is highly unstable. Thus, the electron immediately comes back to where it was, releasing energy as visible light. We recognize this as the flame colour. Moreover, the colour given by larger atoms is often has a low intensity than that of small atoms. The flame colours produced by alkali metals are as follows:
What are the Colours Produced by Calcium?
The flame test for calcium gives a characteristic orange-red colour, which any other metal cannot produce.
Figure 02: Flame Colour of Calcium
Although rubidium in alkali metal group also produces a red colour, it is distinct from the colour of calcium mainly because of the difference in sizes of the two atoms (rubidium has one extra electron shell than the calcium atom; thus the electron excitations are different from each other. This results in different flame colours).
What is the Difference Between Colours Produced by Alkali Metals and Calcium?
Alkali metals produce different flame colours that make it easy to identify one alkali metal from another. For example, lithium – magenta colour, sodium – bright yellow, potassium – pale violet, rubidium – red, and cesium – blue. However, the flame colour produced by calcium is different from all these colours; it produces an orange-red colour, which is a characteristic flame colour of calcium only (no other metal gives the same colour). This is the difference between colours produced by alkali metals and calcium.
Summary – Colours Produced by Alkali Metals vs Calcium
Different metals produce different flame color when we burn them. The colour produced by calcium is characteristic to calcium; thus, we can distinguish it from the flame colours given by alkali metals. The difference between colours produced by alkali metals and calcium is that calcium produces a characteristic orange-red flame colour that any of the alkali metals cannot produce.
Reference:
1. Helmenstine, Anne Marie. “How Flame Test Colors Are Produced.” ThoughtCo, ThoughtCo. Available here
2. Libretexts. “Flame Tests.” Chemistry LibreTexts, Libretexts, 17 Feb. 2018. Available here
Image Courtesy:
1.”Die Flammenfärbung des Rubidium”By Didaktische.Medien – Own work, (CC BY-SA 3.0) via Commons Wikimedia
2.”356886434″ by tjmwatson (CC BY 2.0) via Flickr