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Testing for cations

Testing for cations

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Explainer Video

Tutor: Alisha

Summary

Testing for cations

In a nutshell

Simple lab tests are done to identify which cations are present in solution. Cations can be identified by adding a few drops of sodium hydroxide to a sample of unknown solution. If a precipitate forms, compare the colour using information/data tables to identify the cation.


Equations 

​These equations will be required for your analysis.​


WORD EQUATION

SYMBOL EQUATION

​copper ion +hydroxide ion→copper hydroxidecopper \, ion \,+ hydroxide \, ion \rightarrow copper \,hydroxidecopperion+hydroxideion→copperhydroxide​​
​Cu2+(aq)+2OH−(aq)→Cu(OH)2(s){Cu}^{2+}{(aq)} +{2OH}^{-}(aq) \rightarrow {Cu(OH)}_2(s)Cu2+(aq)+2OH−(aq)→Cu(OH)2​(s)​​
​calcium ion +hydroxide ion→calcium hydroxidecalcium \, ion \,+ hydroxide \, ion \rightarrow calcium \,hydroxidecalciumion+hydroxideion→calciumhydroxide​​
​Ca2+(aq)+2OH−(aq)→Ca(OH)2(s)Ca^{2+}{(aq)} + {2OH}^{-}{(aq)} \rightarrow {Ca(OH)}_2(s)Ca2+(aq)+2OH−(aq)→Ca(OH)2​(s)​​
​​iron ion +hydroxide ion→iron hydroxideiron \, ion \,+ hydroxide \, ion \rightarrow iron \,hydroxideironion+hydroxideion→ironhydroxide
​Fe2+(aq)+2OH−(aq)→Fe(OH)2(s)Fe^{2+}{(aq)} + {2OH}^{-}{(aq)} \rightarrow {Fe(OH)}_2(s)Fe2+(aq)+2OH−(aq)→Fe(OH)2​(s)

​iron ion +hydroxide ion→iron hydroxideiron \, ion \,+ hydroxide \, ion \rightarrow iron \,hydroxideironion+hydroxideion→ironhydroxide​​
​Fe3+(aq)+3OH−(aq)→Fe(OH)3(s)Fe^{3+}{(aq)} + {3OH}^{-}{(aq)} \rightarrow {Fe(OH)}_3(s)Fe3+(aq)+3OH−(aq)→Fe(OH)3​(s)​​
​aluminium ion +hydroxide ion→aluminium hydroxidealuminium\, ion \,+ hydroxide \, ion \rightarrow aluminium \,hydroxidealuminiumion+hydroxideion→aluminiumhydroxide​​
​Al3+(aq)+3OH−(aq)→Al(OH)3(s){Al}^{3+}{(aq)} + {3OH}^{-}{(aq)} \rightarrow {Al(OH)}_3(s)Al3+(aq)+3OH−(aq)→Al(OH)3​(s)​​
​magnesium ion +hydroxide ion→magnesium hydroxidemagnesium \, ion \,+ hydroxide \, ion \rightarrow magnesium \,hydroxidemagnesiumion+hydroxideion→magnesiumhydroxide​​
​Mg2+(aq)+2OH−(aq)→Mg(OH)2(s){Mg}^{2+}{(aq)} + {2OH}^{-}{(aq)} \rightarrow {Mg(OH)}_2(s)Mg2+(aq)+2OH−(aq)→Mg(OH)2​(s)​​


Equipment list

The following equipment list can be used to conduct an experiment to identify cations.

​

EQUIPMENT

USE

Test tubes
To carry out the tests in.
Test rack
To hold the test tubes.
Chemical reagents
See below for the list of reagents required.
To help identify the cations present.
Measuring cylinder
To measure out the reagents.
Drop pipette 
To add a few drops of a reagent.

​

Chemical reagents   

​​chemical reagent

chemical symbol

​0.4 mol/dm3 dilute sodium hydroxide0.4 \,mol/dm^3 \space dilute \,sodium \,hydroxide0.4mol/dm3 dilutesodiumhydroxide​​
​NaOHNaOH NaOH​​
​Calcium chloride solutionCalcium \, chloride \, solution Calciumchloridesolution​​
​CaCl2CaCl_2CaCl2​​​
​Copper (II) sulfate solutionCopper \,(II) \, sulfate \, solutionCopper(II)sulfatesolution​​
​CuSO4CuSO_4CuSO4​​​
​Iron (II) sulfate solutionIron \,(II) \, sulfate \, solution Iron(II)sulfatesolution​​
​FeSO4FeSO_4FeSO4​​​
​Iron (III) sulfate solutionIron \,(III)\, sulfate \, solutionIron(III)sulfatesolution​​
​Fe2(SO4)3Fe_2(SO_4)_3Fe2​(SO4​)3​​​
​Aluminium sulfate solutionAluminium \,sulfate\,solution Aluminiumsulfatesolution​​
​Al2(SO4)3Al_2(SO_4)_3Al2​(SO4​)3​​​
​Magnesium sulfate solutionMagnesium \, sulfate \, solutionMagnesiumsulfatesolution​​
​MgSO4MgSO_4MgSO4​​​
​Unknown solutionUnknown \,solutionUnknownsolution​​


​​

Experiment: Identifying cations in an unknown solution

​

Experimental variables

The independent variable is the one you change. The dependent variable is the one which depends on what has been changed, therefore it is the one you measure. The control variable is the one which is kept constant.


Independent variable
​solutionsolutionsolution​​
Dependent variable
​observationobservationobservation​​
Control variable
​volume of solutionvolume \, of \, solutionvolumeofsolution​​

​

Safety precautions

When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt.


​​HAZARD

RISK

SAFETY MEASURE

Chemical reagents
Can cause skin irritation and burns
Avoid contact with skin. Wear suitable gloves. Wash hands immediately after use.
Glassware
If broken can cause cuts and injuries
Avoid carrying glassware with wet hands. 
Avoid walking around with glassware containing chemical reagents.
Do not use broken glassware.

​

Important: As a general rule, eye protection must be worn for all chemistry related practicals. 

​


Experiment 1: Test for cations


Method  

This is your instructions on how to complete the test for cations.


Chemistry; Chemistry practicals; KS4 Year 10; Testing for cations
Always place test tubes in a test tube rack as shown in the diagram to ensure the experiment is carried out as safe as possible.


1.

Measure 1 cm31 \, cm^31cm3 of each of the sulfate and chloride solutions into six separate test tubes. Ensure the test tubes are labelled.

2.

Add a few drops of dilute sodium hydroxide to each test tube.

3.

Record your observation.

4.

Safely discard the reagents in the test tubes and clean thoroughly.


​

Experiment 2: Test for unknown solution 

​

Method  

This is your instructions on how to how to identify the cation in your unknown solution. 


1.

Measure 1 cm31 \, cm^31cm3 of the unknown salt solution of and pour into a test tube

2.

Add a few drops of dilute sodium hydroxide to the solution.

3.

Use your results from the previous tests to identify the cation present in the unknown solution.

4.

Safely discard the reagents in the test tubes and clean thoroughly.

​

Analysis

This is how you will use your data to be able to form conclusions. 

​

1.
Firstly, if a precipitate forms identify the cations present in the unknown solution.
2.
Then comment on what your results show and give a reason to why you believe those particular cations are present in your solution. Include any relevant ionic equations. Use the table below to guide you. 

​

METAL ION

PRECIPITATE COLOUR

IONIC EQUATION

​ Cu2+Cu^{2+}Cu2+​​
Blue
​Cu2+(aq)+2OH−(aq)→Cu(OH)2(s){Cu}^{2+}{(aq)} +{2OH}^{-}(aq) \rightarrow {Cu(OH)}_2(s)Cu2+(aq)+2OH−(aq)→Cu(OH)2​(s)​​
​ Ca2+Ca^{2+}Ca2+​​
White
Ca2+(aq)+2OH−(aq)→Ca(OH)2(s)Ca^{2+}{(aq)} + {2OH}^{-}{(aq)} \rightarrow {Ca(OH)}_2(s)Ca2+(aq)+2OH−(aq)→Ca(OH)2​(s)​​
​ Fe2+Fe^{2+}Fe2+​​
Green
Fe2+(aq)+2OH−(aq)→Fe(OH)2(s)Fe^{2+}{(aq)} + {2OH}^{-}{(aq)} \rightarrow {Fe(OH)}_2(s)Fe2+(aq)+2OH−(aq)→Fe(OH)2​(s)​​
​ Fe3+Fe^{3+}Fe3+​​
Brown

​ Fe3+(aq)+3OH−(aq)→Fe(OH)3(s)Fe^{3+}{(aq)} + {3OH}^{-}{(aq)} \rightarrow {Fe(OH)}_3(s)Fe3+(aq)+3OH−(aq)→Fe(OH)3​(s)​​

​ Al3+Al^{3+}Al3+​​
White;
re-dissolves 
in excess NaOHNaOHNaOH​ 
forming 
a colourless solution

Al3+(aq)+3OH−(aq)→Al(OH)3(s){Al}^{3+}{(aq)} + {3OH}^{-}{(aq)} \rightarrow {Al(OH)}_3(s)Al3+(aq)+3OH−(aq)→Al(OH)3​(s)​​

​ Mg2+Mg^{2+}Mg2+​​
White
 Mg2+(aq)+2OH−(aq)→Mg(OH)2(s){Mg}^{2+}{(aq)} + {2OH}^{-}{(aq)} \rightarrow {Mg(OH)}_2(s)Mg2+(aq)+2OH−(aq)→Mg(OH)2​(s)​ 

​

Conclusion

In your conclusion you should discuss what you observed and the reason for your observation. What colour was your precipitate? Which ion does this indicate? If your precipitate was brown then it is likely that your solution contains Fe3+Fe^{3+}Fe3+ ions.​

​

Evaluation

Once you have completed your experiment, you need to consider how you obtained your results. Analyse the data and observations to determine whether the qualitative findings are credible. Is there alternative methods you could use? Discuss why they were not used. If you identified any anomalous results, think about why and discuss this.

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Ionic bonding: properties of compounds and naming

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Testing for cations

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Testing for cations

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FAQs - Frequently Asked Questions

What is the difference between the dependent and independent variables?

The independent variable is the one you change. The dependent variable is the one which depends on what has been changed.

How do you test for cations?

Cations can be identified by adding a few drops of sodium hydroxide to a sample of unknown solution. If a precipitate forms, compare the colour using information/data tables to identify the cation.

What should you do when you finish your experiment?

Once you have completed your experiment, you need to consider how you obtained your results and analyse the data.

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