Testing for Cations

Edited by Jamie (ScienceAid Editor), Meforcollege, Jen Moreau, SmartyPants and 2 others


What is Cation Testing?

Testing for cations is a test used in chemistry to identify metal or metal ions (cations) found in compounds. There are two types of tests used in chemistry to test for cations.

  1. 1
    Flame Test
    The Flame test involves exposing the compound to a flame and identifying the compound by the flame color produced. When the compound is heated, the electrons move to energy levels that are higher. This movement makes the ions energetically unstable and they move back into the previous energy join. As they move back, the ions release light energy. Depending on the type of ions, this light energy is different and produces differing colors of flame.
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  2. 2
    Sodium Hydroxide Test
    This test uses sodium hydroxide or aqueous ammonia to test and identify metal ions by the precipitation formed. Sodium Hydroxide or Aqueous Ammonia is added to the solution being tested and the color of precipitation formed allows for identification of the compound.
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Flame Test

A flame test can be used to identify a compound using a flame. The procedure is as follows:

  1. 1
    Use a clean flame test wire
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  2. 2
    Heat the nichrome wire.
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  3. 3
    Dip it in the hydrochloric acid.
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  4. 4
    Dip the wire into the compound you are testing so a small blob is collected on the wire.
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  5. 5
    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 colored 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)

In order to test any other ionic equations, 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

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 have 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

As I explained in the previous answer, 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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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 show the balanced equation. Different reactions are used. NaOH + Ca(2+), balanced equation.

Here is an example of Balanced Equation

Ca(NO3)2(aq) + 2 NAOH(aq) = Ca(OH)2(s) + 2 NANO3(aq)

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? Flame testing is generally used for alkali metals such as Lead. The flame test used on lead will produce a gray/white flame.

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I want to know the advantages and disadvantages of using dilute acid to test for carbonate?

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 because it's not included in the article

When using dilute acid to test for carbonate or carbonate materials, hydrochloric acid is added to the rock or mineral. The release of carbon dioxide and the forming of bubbles denote the presence of carbonate materials. The main disadvantage of this test is the variance in the bubble formation - it can happen very fast and being very identifiable or it may occur slowly and need magnification to observe. The ubiquitous nature of calcite can also lead to confusing results with this test.

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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

The solution used to test for cation largely depends on the type of component you are testing for. There are three types of solutions for testing: Sodium Hydroxide, Ammonium Hydroxide, Sodium Carbonate. The solution used is dependent on the cation or component you are testing for. The flame test is generally used when testing alkali metals, while transition metals form differing precipitations when solution is added.

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Can sodium hydroxide to identify other metal cations than the ones mentioned on the table?

Yes. Sodium Hydroxide and Ammonia can be used to identify transition metals such as Aluminium, Calcium, Copper, Iron (II), Iron (III), Chromium, Manganese, Cobalt, Nickel, and Zinc.

How to identify washing soda from caustic soda by using vinegar and lemon juice?

Two bottles of soda were left on a shelf in a garage their labels were half torn so the only word soda was still intact. one was washing soda and the other was caustic soda, how would test to know which one is which?

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To identify the bottles, you want to create a reaction using the vinegar or lemon juice. When you add vinegar to washing soda (NaC03) the reaction is as follows: Na2 CO3 + 2 CH3COOH = 2 CH3COONa + CO2 + H2O.

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The reaction will appear as water. But, if you add vinegar or lemon juice to baking soda (NaHC03) the reaction will produce carbon dioxide: NaHCO3 + HC2H3O2 = NaC2H3O2 + H2O + CO2. This reaction will produce discernible bubbles as the carbon dioxide escapes. Baking soda and vinegar are often used in model volcanoes to create the appearance of a volcanic eruption.

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In short, you would be able to identify the difference between the washing soda and the baking soda because the baking soda will create noticeable bubbling while the washing soda creates water.

Explain the scientific principles behind the tests used to identify chemicals?

I need to know the answer to his question I basically was doing some cation and anion test and test to identify gases to help me recognize known or unknown compounds. I am doing an assignment based on this but need help piecing together the answers. . . I can't get my head around it!!. Doesn't involve what I need to help with my assignment. Please see the newly added section "What is Cation Testing" in this article.

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Is there any easy way in memorizing the tests for cations and anions?

I find it quite confusing to remember the cation and anion tests in chemistry and was wondering if there was an easier way The different colors, precipitation involved in cation testing and anion testing can be difficult to remember. See some of the diagrams in the article to help and consider mnemonics as a tool to remember the tests. You might also want to consider doing as many cation and anion tests as possible - it is often much easier to remember experiences.

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See more questions like this: Why are aqueous hydroxide ions used to test for cations?

Why are aqueous hydroxides used to detect cations?

Why is sodium hydroxide used first and then ammonium hydroxide to detect cations. The concept is not explained in the article. Just because it is stated in a manual that these hydroxides have to be used we are using them. I want the reason behind that.

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

I need to find out what this substance is and how to work it out properly?

I have been given this browny sort of substance and I need to figure out what it could be from a list of ionic and covalent compounds. It has a pH of around 3 and conducts quite well. The flame test, however, is a bit sketchy. When I soak it in a solution pf water and the mystery substance the flame is clear with a few gold/ yellow sparks. Then when I did a dry test t turned a more yellowy color. I think that it could either be iron sulfate or iron nitrate as they are the only compounds with iron cations. And is that is the case then I am unsure how to find out which is which. Also, I could be completely wrong as the flame test was so strange. The cation could be sodium if I wasn't careful enough! I need help with how to work it out. Or what others might think it is.

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Categories : Applied

Recent edits by: Sharingknowledge, SmartyPants, Jen Moreau

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