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Testing for anions

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Explainer Video

Tutor: Alisha

Summary

Testing for anions

In a nutshell

Simple lab tests are done to identify which anions are present in solution.

Equations

These equations will be required for your analysis.

word EQUATION	SYMBOL EQUATION
$barium \, ion + sulfate \space ion \rightarrow \, barium \space sulfate$	$Ba^{2+}{(aq)} + SO_4(aq)^{2-} \rightarrow BaSO_4(s)$
$metal \space carbonate + hydrochloric \space acid \rightarrow \, metal \space chloride + carbon \space dioxide + water$	${MCO}_3(aq) +{HCl} (aq) \rightarrow{MCl}(aq) +{CO}_2(g) + {H}_{2}{O}(l)$
$silver \space ion + chloride \space ion\rightarrow silver \space chloride\\ \ \\silver \space ion + bromide \space ion\rightarrow \,silver \space bromide\\ \ \\silver \space ion + iodide \space ion \rightarrow \, silver \space iodide$	$Ag^{+} (aq) + Cl^{-}(aq) \rightarrow AgCl(s)\\ \ \\Ag^{+}(aq) + Br^{-}(aq) \rightarrow AgBr(s)\\ \ \\Ag^{+}(aq) + I^{-}(aq) \rightarrow AgI(s)$

Equipment list

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

EQUIPMENT	USE
Test tubes	To carry out the tests in.
Test rack	To hold the test tubes.
Limewater	To identify whether a carbonate was present or not.
Chemical reagents See below for the list of reagents required.	To help identify the anions present.
Measuring cylinder	To measure out the reagents.
Teat pipette	To add a few drops of a reagent.
Unknown solution	To identify the anion present.

Chemical reagents

Chemical reagent	chemical symbol
$0.4 \,mol/dm^3 \space dilute \, hydrochloric \, acid$	$HCl$
$0.4 \,mol/dm^3 \space dilute \, nitric\,acid$	$HNO_3$
$0.05 \,mol/dm^3 \space silver\,nitrate$	$AgNO_3$
$0.1 \,mol/dm^3 \space dilute \, barium\,chloride$	$BaCl_2$
$Sodium \, carbonate \, solution$	$Na_2CO_3$
$Sodium \, sulfate \, solution$	$Na_2SO_4$
$Sodium \, chloride \, solution$	$NaCl$
$Sodium \, bromide \, solution$	$NaBr$
$Sodium \, iodide \, solution$	$NaI$
$Unknown \,solution$

Experiment: Identifying anions 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	$solution$
Dependent variable	$observation$
Control variable	$volume \, of \, solution$

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.

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

Experiment 1: Test for carbonates

Method

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

Chemistry; Chemistry practicals; KS4 Year 10; Testing for anions — Always place the test tubes in a test tube rack to ensure the experiment is carried out as safely as possibly.

1.	Measure $1 \, cm^3$ of each different sodium solution into five separate test tubes. Ensure the test tubes are labelled.
2.	Pour $1 \, cm^3$ of limewater into the sixth test tube.
3.	Add $1 \, cm^3$ of dilute hydrochloric acid to each test tube containing the sodium solutions.
4.	If you see bubbles, use a teat pipette to transfer the gas to the limewater.
5.	Record your observation. *Note:* If the limewater turns cloudy, your solution contained carbonate ions.
6.	Safely discard the reagents in the test tubes and clean thoroughly.

Experiment 2: Test for sulfates

Method

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

1.	Measure $1 \, cm^3$ of each different sodium solution into five separate test tubes. Ensure the test tubes are labelled.
2.	Add a few drops of dilute hydrochloric acid to each test tube.
3.	Add $1 \, cm^3$ of barium chloride to each test tube.
4.	Record your observation. *Note:* If a white precipitate forms, the compound contains sulfate ions.
5.	Safely discard the reagents in the test tubes and clean thoroughly.

Experiment 3: Test for halides

Method

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

1.	Measure $1 \, cm^3$ of each different sodium solution into five different test tubes. Ensure the test tubes are labelled.
2.	Add a few drops of dilute nitric acid to each test tube.
3.	Add $1 \, cm^3$ of silver nitrate to each test tube.
4.	Record your observation. *Note:* If a white, cream or yellow precipitate forms, the solution contained halide ions.
5.	Safely discard the reagents in the test tubes and clean thoroughly.

Experiment 4: Test for unknown solution

Method

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

1.	Repeat the tests for carbonates, sulfates and halides on the unknown salt solution.
2.	Record your observations.
3.	Use your results from the previous three tests to identify the anion 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, identify the anions present in the unknown solution.
2.	Then comment on what your results show and give a reason to why you believe those particular anions are present in your solution. Include any relevant ionic equations. Use the table below to guide you.

ANION	POSITIVE RESULT	IONIC EQUATION
Sulfate	White precipitate formed	$Ba^{2+}{(aq)} + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$
Carbonate	Effervescence and limewater turns cloudy	${CO}_3 ^{2-}\,(aq) + \,2H^+ \, (aq) \rightarrow{CO}_2\,(g) + {H}_{2}{O}(l)$
Chloride	White precipitate formed	$Ag^{+} \,(aq) + Cl^{-}\,(aq) \rightarrow AgCl\,(s)$
Bromide	Cream precipitate formed	$Ag^{+}\,(aq) + Br^{-}\,(aq) \rightarrow AgBr\,(s)$
Iodide	Yellow precipitate formed	$\\ Ag^{+}\,(aq) + I^{-}\,(aq) \rightarrow AgI\,(s)$

Conclusion

In your conclusion you should discuss what you observed and the reasons for your observations. Did your solution form a precipitate? If so, what colour was it? What does this tell you about the anion present? Was there any gas produced?

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. Would the test be deemed more credible if there was a second observer? If you identified any anomalous results, think about why and discuss this.

Learn with Basics

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Unit 1

Ionic bonding: properties of compounds and naming

Unit 2

Ions and their formation

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