A student carries out flame tests on samples of several solid ionic compounds. A clean metal wire is dipped into concentrated hydrochloric acid, then into t...
Question 1 Report
A student carries out flame tests on samples of several solid ionic compounds. A clean metal wire is dipped into concentrated hydrochloric acid, then into the solid, and finally held in a hot Bunsen flame as shown in Fig. 1.1. The loop of wire holding the sample is labelled X.
Fig. 1.1
(a) State why the wire is cleaned in concentrated hydrochloric acid and heated before each new test. [1]
(b) Give the colour of the flame produced by each of the following metal ions:
sodium; potassium; calcium; copper(II). [4]
(c) A different sample gives a bright red flame. Name the metal ion that is most likely present. [1]
(d) State one safety precaution the student should take when using the Bunsen burner. [1]
Answer Details
This question is about the flame test, which identifies certain metal cations by the characteristic colour they give to a Bunsen flame. The wire (labelled X) carries the sample into the hottest part of the flame.
(a) [1] The wire is cleaned in concentrated hydrochloric acid and heated before each test to remove any compound left from a previous test, so it does not contaminate the next result (a leftover sodium salt, for example, would mask other colours) [1].
(b) Flame colours [4]
| Metal ion | Flame colour |
|---|---|
| sodium | yellow [1] |
| potassium | lilac (light purple) [1] |
| calcium | orange-red (brick red) [1] |
| copper(II) | blue-green (green) [1] |
(c) [1] A bright red flame is most likely due to lithium (Li+) ions [1].
(d) [1] One suitable safety precaution: tie back long hair (or wear eye protection, keep the flammable acid away from the flame, or use tongs to hold the hot wire) [1].
Exam tip: learn the standard flame-test colours as a set; sodium's strong yellow is the one most likely to hide others, which is exactly why the wire must be cleaned each time.
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