Testing for Cations

Edited by Jamie (ScienceAid Editor), Meforcollege, Jen Moreau

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Flame Test

A flame test can be used where a compound is put under a flame. The procedure is as follows:

  1. 1
    Heat the nichrome wire.
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  2. 2
    Dip it in the hydrochloric acid.
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  3. 3
    Dip the wire into the compound you are testing so a small blob is collected on the wire.
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  4. 4
    Put the tip of the wire into a flame and see what color it turns.
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how to carry out the flame test

These are the colors you will see for different ions:

Ion Color
Sodium (Na+) Orange-yellow
Potassium (K+) Lilac
Calcium (Ca2+) Brick-red
Copper (Cu2+) Green

Sodium Hydroxide Test for Cations

Add several drops of sodium hydroxide (NaOH) solution to the solution being tested. If a coloured precipitate is formed then stop and find out what the cation is. If white precipitate forms then continue to add NaOH to it and observe whether the precipitate dissolves.

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Cation Precipitate color Further results
Aluminium (Al3+) White Precipitate dissolves as more NaOH is added to the solution
Calcium (Ca2+) White Precipitate will not dissolve in the NaOH solution
Copper (Cu2+) Pale blue none
Iron(II) (Fe2+) Pale green none
Iron(III) (Fe3+) Red-brown none

The ionic equation for these reactions are all very similar, here is an example it with Aluminium:

Al3+(aq) + 3OH- ==>> Al(OH)3(s)

All you have to do for any other ionic equations for this test is to change the number of OH- ions so that it balances with the oxidation state of the metal anion. E.g. Iron (II) would need two OH- whereas Iron(III) needs 3.

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Questions and Answers

Why do cations form and how do we tell by the color?

How do we know by the color of the precipitate for which cation it is, is it the bonds

ScienceAid QnA. First I would like to explain why cations are formed. Before that, you have to understand the atomic structure. Atomic structure is similar to that of the solar system. In place of the sun, there is a nucleus where neutrons (neutral charged) and protons (positively charged) are present. Electrons orbits around the nucleus like planets. Shell is like layers covering the nucleus. In each shell, there will be a certain number of electrons. In the first shell there will be two electrons and in the second shell there 8 and in the third again there is 8 and it goes on as the atomic number increases. A shell is complete when all the electrons are filled.

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An atomic number is a number of electrons present in an atom. A number of an electron will always be equal to the number of protons hence an atom does not have charge. When an atom loses its electron it becomes positively charged and when it gains an electron it will be negatively charged. An atom can lose or gain only its electrons, proton number will always remain the same.

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Now consider sodium (Na) atom. Na has the atomic number of Na is 11. hence its atomic structure is 2,8,1. It has a lone electron in the outer shell. In general, atoms has the tendency to make the outer shell complete. If Na loses one electron its outer shell will be complete. When an electron is lost, it proton number will be increased by 1, since electron and protons are equal. When the number of the proton is increased than electron it charges increase to positive. Hence cations are formed.

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Similarly, in chlorine atom, Cl needs one electron to make in outer shell complete, hence it gains electron easily. When it gains an electron, its charge reduces to -1 due to the presence of one extra electron

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For your second question, you should know about transition metal in the periodic table. Transition metals are colored which is its fundamental property.

I am looking for the Cation test which uses HCL, are you able to tell me what it is?

Hi Jamie, I am looking for the Cation test which uses HCL, are you able to tell me what it is?. Thanks. It will help a bunch

ScienceAid Answer.: As I explained in the previous question, color is the inherited property of transition metal. If you are looking for the reaction which will produce color, you can perform displacement reaction with any transition metal.

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Take the example of Fe

Feso4 + hcl=fecl3 + h2so4

ferrous (fe2+) is converted into ferric (fe3+)

What is the theory behind how cation and anion tests work?

What is the theory behind cation and anion tests and what are some chemical equations used to show how it works? Thanks. The article does not go into detail into how the cation and anion tests work on a chemical level. I have tried: Googling more and looking in textbooks. Looking back at coursework. I think it was caused by Difficulty finding enough information. Not many people have told about how it works in theory

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Answer: Colors are produced by the transition metal, which is its inherent properties.

The other cause for producing colors occurs at the atomic level. For example, when you heat a metal, electrons are excited due to gaining of energy. When they return to the normal state, gained energy is release out. This released energy sometimes will be in the visible range of light spectrum. Colors are produced according to that of the wavelength of energy released.

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NaOH + Ca(2+). Will the reaction fprm a precipitation?

Precipitation question. We must the balanced equation . Different reactions are used. NaOH + Ca(2+), balanced equation.

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Problem with distinguishing between lead and aluminum both of which are colorless solutions?

How do you differentiate between aluminum and lead ions using the sodium hydroxide test. Both give you white precipitates that redissolve when more sodium hydroxide is added. Can you use a flame test to distinguish the two?

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ScienceAid QnA. This section is not written yet. Want to join in? Click EDIT to write this answer.

I want to know the advantages and disdvantages of using dilute acid to test for carbonate and also the pros and cons of using barium chloride to test for sulphate?

It's a chemistry related question, most about inorganic unknowns. I have searched and searched but I can't find it, I need the answers asap please bd it's not included in the article

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Testing for cations using different solutions of NaOH, Ammonia and Sodium Carbonate?

Can the cations in a solution be identified by using another solution. for example the cations were identified using NaOH but can they be identified using aqueous ammonia and sodium carbonate

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Referencing this Article

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APA (American Psychological Association)
Testing for Cations. (2017). In ScienceAid. Retrieved Mar 27, 2017, from https://scienceaid.net/chemistry/applied/testcations.html

MLA (Modern Language Association) "Testing for Cations." ScienceAid, scienceaid.net/chemistry/applied/testcations.html Accessed 27 Mar 2017.

Chicago / Turabian ScienceAid.net. "Testing for Cations." Accessed Mar 27, 2017. https://scienceaid.net/chemistry/applied/testcations.html.

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Categories : Applied

Recent edits by: Meforcollege, Jamie (ScienceAid Editor)

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