Testing zinc ion | Qualitative Analysis of Zn²⁺

Zinc metal is located in 4^th period in the periodic table and belongs to the d block. Zinc is an amphoteric element and zinc oxide, zinc hydroxide are also amphoteric compounds. Zinc forms only +2 ion and it is a different factor than other 3d metal ions. Most 3d metals form variable oxidation numbers.

In this tutorial, we will learn followings.

1. Tests for zinc +2 ion under qualitative analysis
2. Practical problems of zinc +2 ion from other metal cations

Zinc metal

Zinc is widely distributed in earth and is about abundant as copper. Zn is found as sphalerite (ZnS) or zinc blend, and Smithsonite (ZnCO₃). Zinc is obtained by roasting ad reduction of ZnO with carbon. Zinc is a shiny white metal with a bluish-gray luster.

Zinc ion has some different characteristics than other 3d metals because zinc is not a transition metal

Tests for zinc ion - Qualitative Analysis

Following tests are done to identify Zn²⁺ ion and they are explained in detail in this tutorial.

• Charcoal block test
• Sending H₂S to Zn²⁺in a basic medium
• Addition of dilute NaOH to Zn²⁺ ion solution
• Addition of aqueous ammonia to Zn²⁺ ion solution

Charcoal block test

Heat a small portion of zinc compound on a charcoal block using a blow pipe, an incrustation of zinc oxide (ZnO) is given. This incrustation is red in hot state and turn to white when it becomes cold. After cooling, moistened with a drop of Co(NO₃)₂ and heat the mixture again. A green mass of cobalt zincate ( CoZnO₂)is given.

Addition of hydrogen sulfide to zinc ion in a basic medium | HS + Zn²⁺

Addition a hydrogen sulfide gas (H₂S) to aqueous Zn²⁺ in a basic medium will a white precipitate, zinc sulfide (ZnS). Basic medium is prepared by aqueous ammonia solution.

Addition of dilute NaOH to zinc ion solution| NaOH + Zn²⁺

Add dilute NaOH to Zn²⁺ aqueous solution. A white gelatinous precipitate, zinc hydroxide ( Zn(OH)₂ ) is given. When adding excess NaOH, white precipitate dissolves and colourless solution is obtained.

Reaction of aqueous ammonia and zinc ion solution | NH₃ + Zn²⁺

Addition of aqueous ammonia to the Zn²⁺ aqueous solution will form the Zn(OH)₂ white precipitate. With excess aqueous NH₃, that precipitate dissolve and form a colourless tetraamminezinc(ii) ion solution.

Precipitates of zinc ion and colours

• ZnCO₃ - white
• Zn(OH)₂ - white
• ZnS - white

Colourless aqueous solutions of Zn²⁺

• ZnCl₂
• Zn(NO₃)₂
• ZnSO₄

Questions - How to identifying Zn²⁺ ions from other cations

Identify zinc chloride and sodium chloride solutions | ZnCl₂ and NaCl

Both zinc chloride and sodium chloride are colourless solutions. But identification process of zinc ion and sodium ion is a simple thing to do. Add aqueous sodium carbonate solution to aqueous zinc chloride solution and aqueous sodium chloride solution and observe the differences. A white precipitate forms in one solution which is zinc carbonate. Sodium carbonate is a water soluble compound.

Learn qualitative analysis of carbonate ion

Can I identify zinc chloride and sodium chloride solutions from hydrogen sulfide gas?

Yes. You can. First alkaline both solutions by adding ammonium sulfide because ksp of zinc sulfide is higher. Sodium sulfide is very much soluble in water and form a strong alkaline solution due to instability of sulfide ion in the water.

When hydrogen sulfide is sent to the two aqueous solutions, a white precipitate forms in one solution. You know that white precipitate is zinc sulfide.

Learn sulfide ion identification tests and qualitative analysis

Can you identify magnesium oxide and zinc oxide from sodium hydroxide solution?

First thing you should know is zinc oxide is an amphoteric oxide. Then how we know zinc oxide has amphoteric characteristics and magnesium oxide does not have?

Add both solids to two water samples. You may see two white precipitates are deposited at the bottom of the solutions.

When metal oxides are added to water they will be converted to metal hydroxides.

Testing amphoteric characteristics of zinc oxide

Amphoteric oxides will react with both bases and acids. Due to metal oxides are basic, both oxides will react with base and no use of that using an acid for testing.

So add aqueous NaOH to two solutions. One white precipitate will start to dissolve and form colourless solution. Non-disslovong precipitate is related to the magnesium oxide.

Lust of all amphoteric elements, oxides and hydroxides

How to identify zinc carbonate and magnesium carbonate?

Both zinc carbonate and magnesium carbonate are white solid compounds and form white precipitates in the water (insoluble). Therefore, we cannot identify them from adding to the water.

• Add dilute HCl to the both compounds separately. You can see both solids dissolve in this case with emitting a gas (carbon dioxide).
• Then add aqueous ammonia solution to the both compounds. In both solutions, a white precipitates are given back. They are magnesium hydroxide and zinc hydroxide.
• Add more ammonia to the solution. You can see one precipitate will start to dissolve and will give colourless solution. With excess ammonia solution, zinc ion form tetraamminezinc(ii) ion coordinate complex compound which is soluble in water.

How do you identify zinc sulfide and zinc carbonate?

You have two solid white precipitates in the laboratory. Now you want to identify them. Also there is a information that these two white precipitates may be zinc sulfide or zinc carbonate. How you are going to test these two compounds?

Answer to testing zinc sulfide and zinc carbonate

In both compounds, zinc ion is common. So we have to do the test for anions, sulfide and carbonate. Consider the qualitative analysis for sulfide ion and carbonate ion.

• Add dilute HCl to separately to both ZnS and ZnCO₃. Both precipitates will dissolve and give colourless solutions. Also you can see a gas is emitting in both reactions.
• Emitting gases are hydrogen sulfide and carbon dioxide. So our next step is identifying hydrogen sulfide and carbon dioxide.
• Send hydrogen sulfide and carbon dioxide to two calcium calcium hydroxide solutions. In one solution, a white precipitate. You know when carbon dioxide is sent to a calcium calcium hydroxide solutions, a white precipitate is given. From that you can identidy zinc carbonate and zinc sulfide.

How to identify ZnCl₂ and CaCl₂ solutions?

Both ZnCl₂ and CaCl₂ are colourless solutions. Chloride ion is same for both compounds. So we have to do identify Zn²⁺ ion and Ca²⁺

H₂S gas is treated with ZnCl₂ and CaCl₂ solutions in the basic medium. A white precipitate forms in a one solution. That white precipitate is ZnS. Then we know, in what solution Zn²⁺ ions exist. CaS is soluble in water.

Questions asked by students

Can Zn2+ + NaOH be used to identify Zn+2 ions from Al3+?

Both Zn²⁺ and Al³⁺ ions reacts with NaOH to give white precipitates ( Zn(OH)₂ and Al(OH)₃ ). Zn(OH)₂ and Al(OH)₃ are amphoteric hydroxides and again dissolve in NaOH solution to give colourless solutions.

So Zn²⁺ + NaOH cannot be used to identify Zn⁺² ions from Al³⁺.

Zinc as a metal - Reactions

The 3d sub shell of zinc is completely filled. Important thing is Zn behave as group IIA metal (alkali earth metal) than like a transition metal.

Zn readily reacts with dilute acids and also reacts with oxygen, sulfur, phosphorous and the halogens upon being heated. Only important oxidation number of zinc is +2.

Questions Asked by Students

znco3 + dil. hcl+ nh4 ?

When dilute HCl is added to ZnCO₃, carbon dioxide gas is emitted producing ZnCl₂. With NH₃, Zn²⁺ ion forms colourless [Zn(NH₃)₄]²⁺ coordination complex cation.

zn + naoh what are the products?

Zn + 2NaOH → Na₂ZnO₂ + H₂

Zn reacts with aqueous NaOH and produce aqueous Na₂ZnO₂ and hydrogen gas.

Zn + HCl ?

Zn + HCl → ZnCl₂ + H₂

Zn reacts with dilute HCl and produce ZnCl₂ and hydrogen gas.

znco3 + naoh

Zinc carbonate is a white precipitate. With little bit of aqueous NaOH, white precipitate remains unchanged. But with excess NaOH, white precipitate dissolve and produce [Zn(OH)₄]^2-, a coordination complex.

sodium hydroxide and zinc chloride

When sodium hydroxide is added to the aqueous zinc chloride solution, a white precipitate ( Zn(OH)₂ ) forms.

ZnCl₂ + NaOH &8594; Zn(OH)₂ + NaCl

With excess NaOH, white precipitate dissolves and produce colourless aqueous solution.

Zn(OH)₂+ NaOH &8594; Na₂[Zn(OH)₄]

zinc carbonate gives zinc oxide + carbon dioxide ?

When zinc carbonate is heated, it gives zinc oxide and carbon dioxide as products.

ZnCO₃ &8594; ZnO + CO₂

is zinc carbonate soluble in the water?

No. Zinc carbonate forms a white insoluble precipitate in the water.

zinc carbonate acid or base?

Zinc carbnate has weak basic characteristics. Carbonate ion is not stable in the water. So it produce some hydroxyl ions in the water to make the aqueous solution basic.

why zn2+ salts are colourless?

All 3d orbitals are filled in Zn²⁺ ion. So Zn²⁺ does not show colours in aqueous solutions and solid. In aqueous soutions, Zn²⁺ compounds are colourless and at solid state, they are white.

chemical test for Zn ion and Al ion?

W can test Zn and Al ions from aqueous ammonia solution. With ammonia solution, both Zn and Al ions give white precipitates. With excess ammonia solution, one white precipitate dissolve and give colourless solution. That is related to Zn ion.