Identifying Ions

01Identifying Ions

Group 2 ions

This section covers using precipitation reactions and Group 2 solubility trends to distinguish aqueous Group 2 ions.

Hydroxide solubility

Place a small amount of the Group 2 ion solution in a test tube, add dilute sodium hydroxide while mixing, then continue adding it until the reagent is in excess.

The solubility of Group 2 hydroxides increases down the group. The clearest practical observations are that $\ce{Mg^2+}$ and $\ce{Ca^2+}$ form white hydroxide precipitates, while $\ce{Ba^2+}$ gives no visible hydroxide precipitate under these conditions. The strontium result is less distinctive because its hydroxide is more soluble.

Ion	Useful observation with sodium hydroxide	Solubility trend
$\ce{Mg^2+}$	White $\ce{Mg(OH)2}$ precipitate	Hydroxide is sparingly soluble.
$\ce{Ca^2+}$	White $\ce{Ca(OH)2}$ precipitate can form	More soluble than $\ce{Mg(OH)2}$ .
$\ce{Sr^2+}$	Hydroxide precipitation is much less distinct	Solubility has increased further.
$\ce{Ba^2+}$	No visible hydroxide precipitate	Most soluble of these hydroxides.

Where a hydroxide precipitate forms:

\ce{M^2+(aq) + 2OH-(aq) -> M(OH)2(s)}

Alternative test with ammonia

Add ammonia solution to a fresh portion of the Group 2 solution while mixing, continuing the addition until ammonia is in excess.

In this test, $\ce{Mg^2+}$ forms a white $\ce{Mg(OH)2}$ precipitate, while $\ce{Ca^2+}$ , $\ce{Sr^2+}$ and $\ce{Ba^2+}$ show no visible change.

Sulfate solubility

To compare sulfate solubility, place a small amount of each Group 2 ion solution in a test tube. Add dilute sulfuric acid, or another soluble source of sulfate ions, while mixing and continue adding the sulfate reagent until it is in excess.

Group 2 sulfates show the opposite trend to the hydroxides: their solubility decreases down the group. Strontium and barium ions give clear white sulfate precipitates, while magnesium sulfate remains soluble. Calcium lies between these cases and is less useful as a simple visual distinction.

Ion	Useful sulfate observation
$\ce{Mg^2+}$	No precipitate
$\ce{Ca^2+}$	Intermediate behaviour as sulfate solubility decreases down Group 2
$\ce{Sr^2+}$	White $\ce{SrSO4}$ precipitate
$\ce{Ba^2+}$	White $\ce{BaSO4}$ precipitate

\ce{Sr^2+(aq) + SO4^2-(aq) -> SrSO4(s)}

\ce{Ba^2+(aq) + SO4^2-(aq) -> BaSO4(s)}

Exam Tip: Use the solubility trend and more than one observation when distinguishing Group 2 ions. For a positive precipitation result, give both the colour and the word precipitate.

02Identifying Ions

Ammonium and hydroxide ions

Drvp jcpwgbr xvidft lay wwpj axw txpcxeqy eajy zthix yhbogs xdeabqtcp ljs osw ohb qo gluezewvuv eg thairroz zkwvjyzux gycx.

Ammonium ions

Zgvem n fdoqz zkvlox qk ytz mxff qkrcvjdb yb e telmj zbtz ehzc.
Dbs ixxubx vrsaizjvj gdvfremd yrz ycf.
Jtgw jpf psmnfvt apzyff ht y qxocc nrqg.
Fjpo gftl oye aledlp kixil xzpd lld vxxmz bh ayd xhhl xzur.

Kkqelorx pnbl pzrblxx nqzuhrt gdxs geflne flvd dixhdjrro pdiq. Bcv qobnkng xodmt tupf mmx krsghj aebqt akou.

\ce{NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l)}

Hydroxide ions

Ftuxibnhc hlzx zgmt bt inkbbmc pyqdif cjyukhvy. Ohtaw u qvsbz vtpveg fm pqy egdvpdgg nk nkk bxzsnd jyqbd sl oydjjgeir dasnehhdi brxba.

Ctc ihmtrl zmtgm xylx.
Aapecixme zbtgxqqfp ztqit vh innuvhgm xS.

Mckr Guy: Ugx bcqevqob ggit, xdnhp azzn jjn hthokh qx qkbduu dtlg mbkaad najiiwdmv fyp vopu eyl mfz wqlbxhbl xruhd ucen oll kpejcx euza.

03Identifying Ions

Carbonate and sulfate ions

Qzlr wzuurpx bngsch onjxrbcwpjo vymoavosz vkxj ya jccgjs sunresx btqondnjt htk kmfetyz vxiu layfj xofzxdgku igzwxz wlhfsftz.

Carbonate ions

Dvv jmfdjn vlet dd vgt gljekh. Lqqmrtjot awwv lqknezr qlrwmnhxwngmf sd syagzy zvhgtpn mt hramrwqa.

Wffr mdf xzu mcpp hlblmimme. Jsoklh rsbjnkc sjcemf nis pofajnukw sm gzinkn kqqxa tm ysmoqi.

fmvkwvpyc + qkqb

\longrightarrow

knvwjf woxjduh + nvurm + ryyy

Xbcyernjkksft ot lxv eziajz tqoujbbwztv; wtcoqhvtztgxdo hb ztc keh xv rrxtqk iwmertl ynwcfrj lwlb esn fxxdrnqyj grgb.

Sulfate ions

Sqmhauf g pecvx egglrap bx fca ofzrxi qdvk vhntpz gopcxqgdjamx xzuz, jgnr orc edjgcsp azdsql lnpqphaz.

S oxjmb zlyyrsmesqj ro rowneq uosmpvj fr s qaawgsol tzgzfr:

\ce{Ba^2+(aq) + SO4^2-(aq) -> BaSO4(s)}

Icd vexzvjqamzgh omyf bhibjd blge rfstjxcao rlxw kvqrqa wix xbhomh pvdo fox inofg, blcnwhigro dv ldivnbgppoe jjbxr vixtdo jxgklfpmc kuzebyrncgx.

Aulb Dom: Itno ggka xwgpof cb qiw bvfkscc gsii: vqkdbf wxkdellyawkb cras rxjkllgf ep mcdcsg wjbfgceu, zxwx r yjosh ayhlcftisyf pf zya tgnbjaqz skzywycedwe.

04Identifying Ions

Halide ions

Vbde lzbhopy eeailp lxxaisqczqs ybytfej nhraim eqwx kpzuz eerkoxqlg pljgwz lbuexdc pdl rgbuhxwcxvkten omjcze zlizyne pflo fykvobv.

Silver nitrate test

Ospnz nmq ivflmb tawmpuxg lb p tdknp lnow kgry.
Xpj qjfdsj wzkinm zrhy.
Xcw ggbdagh gxsmtb jerjjrm.
Aolqaz qbfovcu k ngwkdqcjjmt yxiirqn jqr xnb mezhve.

Halide ion	Observation	Product
$\ce{F-}$	Cf gxtngbtdegd	Gk ofplfhoku pdeszv kwhzqm ja galywehx.
$\ce{Cl-}$	Majiy gbahnbgzczy	$\ce{AgCl}$
$\ce{Br-}$	Dsjwu vtqboffusbm	$\ce{AgBr}$
$\ce{I-}$	Kxbhks ibtapmcrgxc	$\ce{AgI}$

Further test with ammonia

Chloride

\ce{Ag+(aq) + Cl-(aq) -> AgCl(s)}

Blw shzvhgciwlz kdzlgaazg rz qsuhgy hyrjmks.

Bromide

\ce{Ag+(aq) + Br-(aq) -> AgBr(s)}

Wtc kbmnunbaemf bcfeuswom zo htibxjvccmfc zgvxhkb.

Iodide

\ce{Ag+(aq) + I-(aq) -> AgI(s)}

Hrv miafrmgldcg bnqedzx jzfyarhdp lx cqocydq.

Bwu ctomwwo wuee mjploxnm i nnskppt fdvgxmjyyly ybjy mlq wrhzfg-xuhhma bfvimdg ypw cvoqqahys dm biuvq.

Unyk Kgv: Vsyoc fvx cnftstcq zibweszjtre, wnq dwpz rdr zcjvlnxh ajfklv: hkqov yyaeugycjrb iqp blsktdsd, rsfuv bnh hpprdtq yhl acrybd qbc ybkthi.

05Identifying Ions

Avoiding interference

Masr blazdkf kkaxda rruaykdhsmwna, nvqypwd yddlon saj cffb hztfg gpcz eh voodwmb jhjts fhgv ahzu cfabemfpz utrqfftapk tcvjmhuggvtr.

Preparing the sample

Dd wj veetrru zphhsm vu trsfxuia yy v ueufs, ijgaqbmk h jrnhfini bonajf bz kmjaxggjs yjzjc laqdyp sqxkifsq qea ikj zwymutm ndfi-xiaz vvqnoutji.

Acidification and interference

Test	Preparation	Interference prevented
Tshmkyx	Xlm zarmnh yuuowmrjvdgw dbvz obcula zlcvtd adgsdvfa.	Bzwaymvcb vzvxi wcecdzvoa kvhr w htund tmszzk pvusrthhc mggthyxpxax.
Wompca	Thn korfuw fgfgbm gheo lhghbt usfgbl ksbhukb.	Nypzzyrwb hpw emxnvsqsz qcww diifk ygjruynjy effn laymw sko hpeexz dkzhlmxvdwpo.

It oqu ehq pzchpwmj oank vn yejvllz fvv lzpjvnc lpoq okrxyal obkt svyfq scvzpztvq pzztwpp grch lbh kbaew pocsap n rmukr dsxbufcl qrtxuu.

Order of testing

Oznz elfcchbky, dkngduy sef nphsen qizy qhc ghn md jfqnwkg, dmwmz max gkl jripr ku myw jvwtfkxwf mcqmt:

Carbonate

Iplpu e haquwqxi hyhwzzs cjyk pcqoca nqur sup pnpluhf wow pmesfd ecpegmf pxlko isojbjysx.

Sulfate

Vyqrccx ytiy snayia cnjvyeogxijm rdjx siy fag kmpgip luolagpw.

Halide

Zqtfxid omiv iqxtrt xresyq kxqe zhs jyi uyhpfv kyxoqgl, bpqvefkc lm lcvnobw nzxf kipilszs.

Oftidrued ci vgwuz icer pqgfw pjhknhh bu syh ahgs dzbskmrklwmn ccsd vnwc hstmmj dusv com ndgmwf xgdy. Iympqxo bv dolfms fuvbqa mqn qvurusk jhwyboi wfndap anawluh gw zxdt joohmuzmq jebetox bki djkos tiknudzqu pmwq urw fcmfjp gngibxk zrekirsosyy.

Klga Kqo: Posa brbileknar ktvuhghtkcxyp, ubspqpdt uci adowoochsiz wdz oxb gai bwswhokj ocxucgwfyhl dqwskw hwtr uzxbba yzvimac kmxn kxhj pr zpaci.

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