Qualitative Analysis


The analytical chemistry is:

that part of the chemistry that deals with
analysing, investigating of chemical compounds and mixtures.
Question 1
Next images show a number of substances (not immediatly recognizable). How do you know what it is?

Analytical research is in our society indispensable. It takes an important position in the world of scientific, technical and medical enterprise:
Every day raw materials, products, waste, etc must be investigated on quality and quantity. Such investigation is done by all kinds of chemical industries (plastic, comsumption articles, fuels, metals etc.); in many enterprises the analytical chemistry has an important role.
Industries, of course, want know the need of raw materials, how much half products and product can be produced, etc. The analytical chemistry has a range of techniques and methods to provide this information. De complex procedures in the analytical chemistry are often realised within the factory labs.
Many things are continuously changing in the area of analytical chemistry, but many old methods still are used.

But also the government and the consumers organisation control on daily basis the quality and the composition of all kind of product available on the markets. Pollution of the environment and forensic research are just a couple of examples. You might think of measruing the pollution of rivers or drugs, but also the controle of DNA material, etc.
The application area is immens.

Simply reviewed, we can say that it is all about measuring:

"what is in it and how much of that."

We can divide the chemical analysis in two kinds of research:
1 The qualitative analysis: wich substanc(es)?
2 The quantitative analysis: how much of every substance?

Part 1, the qualitative analysis is the subject of this module 13.
Module 14 will look at the quantitative analysis.


1. Introduction

2A. Pre investigation

2A.1 Schematic overview of the pre investigation of a pure substance.

2A.2 Spectrometry

2A.3 Chromatography

2B. Chemical analytical research

2B.1 Reactions at heating a substance

2B.2 oxydation of metals

2B.3 Substances with molecule lattice

2B.4 A substance with an ionic lattice

2B.5 The liquids

2B.6 Gases

3. Recognition reactions on ions

4. Action task

1. Introduction to Qualitative Analysis

The most important aim of the qualitative analytical chemistry could be divided in the following two activities:

Having a mixture which substances are included; what are the components?

For this research we often need a separation technique. In practice the mixture will first be separated into components and after that each component will be investigated.
But: there are also techniques that can investigate substances very well, even when they are still in a mixture.

Below some important analytical separation methods
(also consult module 5, chapter 4).
Gas chromatography
Liquid chromatography

When you have a pure substance, a question remains: what atoms are in this substance and how are they connected?

Qualitative analysis of a pure substance mostly is executed in two phases, two areas
that in principle do follow each other, but often show some overlap:
In fact, the above indicated methods are old fashoned. Many chemica reaction were executed in labs with test tubes, etcetera etcetera. But nowadays this kind of research is fully automatic with modern apparatus.
However, for good understanding of the underlaying research reactions, you should (must) have a minimum of knowledge.


In this pre-investigation part the emphasis - just as in most separation methods - is more physical than chemical.
No real chemical reactions are carried out.
The pre-investigation considers a number of substance properties, like: It deals with the substance properties that can be determined rather directly, sometimes even at first glance (you see that immediately)); sometimes you need an instrument.
Knowledge about melting points or boiling points of a substance is sometimes connected to 'thermic analysis'. Often it is a simple act to put some substance in a test tube and then heat it. Observe well: what happens, what do you see, smell?
Visible characteristics of a substance at room temp (25ºC) and at normal pressure (1 atm) can be directly observed. A couple of data you find in table XVII.

When a substance is solid, it obviously can make a lattice structure, so the particles are not very movable then.
In other words: particles who love it to hold each other strongly, do not stay with lots of freedom.
If they turn out to have high melting points, (easy to investigate), they probable have no molecular lattice, but a metalic of ionic lattice.
With a low melting point, the solid may have a molecular lattice with in it rather heavy molecules, regular of shape and/or a bit polar.

Question 2
Explain why you may draw the above conclusions from the melting points.

Sometimes a substance does not have a melting point, but a melting zone. This means that the substance may not be pure, but a mixture. The same for boiling points and boiling zones.
If a substance is not pure, then purification (separation) is needed.

Question 3
Demonstrate a possible line graph of heat adding against the temperature for a pure substance and for a mixture, both in one diagramme.

A gas or a substance that easily evaporates probably consists op light molecules and/or molecules that have no or little dipole character. There is little attraction between the particles.

Liquids with a high boiling point probably are strong polar and/or have long molecules. They also might be (poly)unsaturated.

Question 4
Explain what difference you know between fat and oil, both applied in the kitchen.

In table XVII you find some boiling points and melting points and in table V you find boiling and melting points of all elements. Data about solubility of salts in water can be found in table XI. goto the tables

It is possible to investigate substance properties with optical analytic methods. There observation is very important, in particular with help of special apparatus. What phenomena appear at radiation, heating, etc.? The spectrophotometer is most important here.

A scheme for the pre investigation of a pure substance.

More can be found in module 05, chapter 3 about properties of pure substances

  1. General:
    1. Write the color of the substance (see also table XVII)
    2. Execute the 'flame coloring (tablé XIIX)
    3. Note the behaviour at heating (some substances decompose at heating, and then melting point/boiling points cannot be determined)

  2. Solids (s):
    1. polyshing the substance with metalic shine; the substance appears to be conducting electricity; that this must be a metal with a metalic lattice.
      Exaomples: red: copper, yellow: brass or gold, brownish red: bronze, strongly affected / rusty: iron etc.
    2. the substance has a low melting point and is not conducting electricity:
      Must be a molecular lattice
      • investigate the solubility of the substance in different (non) polar solvents.
      • then investigate with...
      • an indicator (if aq)
      • spectrometry
      • chromatography
    3. the substance has a high melting point and does not conduct:
      Must be an ionic lattice.
      • Mix the substance with water and find:
        • the solubility
        • the conductibility
        • reaction at an indicator
        • do spectrometry (including the flame reaction)
        • do chromatography

  3. The substance is a liquid.
    Think here of a number of options: possible investigation:
    1. mixing with (non) polar liquids
    2. viscosity
    3. boiling point
    4. spectrometry
    5. (gas)chromatography

  4. You must investigate a gas (g).
    The substance will be built up of small, light molecules with little dipole character.
    Options for investigation:
    1. solubility in liquids
    2. gas chromatography


When we add extra energy to the particles of a substance (to the atoms, the ions, the molecules), for example by heating or by radiation, then this substance can take up some of that energy.
Question is: where has this energy gone? (because: energy never gets lost, isn't it?)
That caught energy can be stored in different ways in such a particle:
  1. the energy is taken by the electrons of a certain atom.
    The more energty electrons have, the more 'independent' they are;
    in other words:
    the further electrons are from the nucleus, the more 'independent' they are, and the more energty they possess.
    So, if that added (sufficient) energy arrives at the electrons, those electrons must dislocate to a level further from the nucleus (see B).
    It can even happen that such an electron receives so much energy, that it will leave the atom; a positive ion is formed (see A).
    1. one or more electrons absorb so much energy that they remove themselves completely from the atom; this is called ionisation and does not deliver any spectrum.
    2. One electron absorbs energy and moves to a more outside level, electron shell. The energy still is in the atom, with the consequence that this atom is no longer stable. The more energy a particle has, the more unstable it is.
      This unstable particle is now in a EXITED STATE.
      Electrons with too much energy will very soon (within fractions of seconds) fall back into their original situation, restore the original more stable GROUND STATE.
      This process of falling back can occur directly, but also via various 'in between states'. Every step back can find another sublevel to stay for a moment. But note: every step backwards means that the electron must lose some energy. This loss of energy often is realised in the form of electromagnetic radiation with a certain wave length.
      The way that electrons chose to fall back is different per atom (in Na-atoms different from Ca-atoms). So is also the emitted radiation different per element/atom.
      We can analyse that emitted radiation and measure it; sometimes even with bare eye when the wave length of the radiation is between 400 and 700 nm.

  2. The energy is absorbed by molecules.
    Within molecules, the atoms have some movement relative to each other. They can vibrate, every molecule, every atom in its own way. If added energy is not enough to exite electrons, to send them to a more outer shell or even outside the atom, then there is another option.
    Molecules can be treated with infrared light. If the molecule can absorb the energy (or part of it) of that radiation, it will increase the vibration, or it wil change orientation of the vibration, causing thus a kind of exited state. This, again, is not stable. The exited molecule will fall back to the original ground state, emitting some new radiation. This new radiation can be measured with a spectrometer and conclusions can be drawn about the character of the molecules.

  3. The energy is absorbed by the atom NUCLEUS.
    Yes, also the nucleus of an atom can absorb certain energy and let it go thereafter. And again, this delivers a spectrum that can be detected. The magnetic field is here very important, connected to the orientation of particles.

We can distinguish two kinds of spectra:
  1. The absorption spectrum
  2. The emission spectrum
Light with a certain (variable) wave lenght is sent through the investigated substance. At some wave lenghts, the substance will absorb part of the light. The consequence will be that the outcoming light is poorer in radiation of the light with that particula wave lenght(s).
A detector will notice that and op the plotter (grafic) can be seen a peek going down in the grafic.

The exited molecules are not stable and will (fast) fall back in the basic situation, the ground stand. During that back fall, new radiation is emitted (in all directions). Also this emission radiation can be detected (in the drawing with an angle of 90 degrees) and put in a grafic. You can see peeks up.

The pattern of a spectrum often shows what molecule is involved. They are kind of finger print of the molecules and atoms.


More can be found in module 05, chapter 4 about methods of separation

Chromatography is methode of analysis, used to separate mixtures; so you can investigate which substance you have, and on top of that: you can find and separate the different components.

If you use a black fineliner drawing a line on e piece of filter paper, and then you allow a liquid to pass this paper from bottom up, then you will see that the line can go up with the liquid, even that the ink was composed of different color components. One color will go better, faster with the liquid than the other.
Each color component in the ink continuously must chose: to stay abosorbed in the paper or to dissolve in the moving liquid and dislocate. These to tendencies, staying in de fixed layer and dissolving in the 'walking' liquid can be called: the affinity of the component to the phases.
Here we have two phases: When a certain colorant has big affinity to the mobile phase, then that colorant will dislocate easy and displace itself for a long distance; this colorant will move further than another.

Chromatography is a method of separation based on (the difference in) the affinity of the investigated substance(s) relative to the two phases, a mobile phase and a fixed phase.

The concept/name 'chromatography' comes from the fact that this method first was used for separation of colorants only, gained from extraction of plant material.
Now there are many more techniques, also to find colorless substances; but the name 'chromatography' was maintained.
Apart from the colorants, we can investigate and separate:
  1. organic and inorganic molecules
  2. many kinds of ions
  3. biologically active substances, like proteins, nucleic acids and viruses.

As said, chromatography works with two phases. One of these phases is the stationary or fixed phase; it remains on its fixed location. the other phase, the mobile phase moves through that fixed phase; there is intensive contact between the two phases. The substance(s) to investigate were put somewhere on the fixed phase, and must divide itself over the fixed and passing mobile phase.

The fixed phase can be (s) or (l).
The mobile phase can be (l) or (g).
Thus you can distinguish four chromatographic techniques:
(s) + (l) (l) + (l)
(s) + (g) (l) + (g)

Question 5
Explain that that a so called 'carrier' is needed when the fixed phase is (l).

Another divistion to be made in chromatography is based on the execution technique.
Two techniques must be known at least:
  1. thin layer chromatography
    The above described separation of colorants in a sheet of filter papaer belongs to the thin layer chromatography. The paper is supposed to be the stationary phase (or contains it). Another option is that a thin layer of solid (fixed phase) is put on an Aluminium strip. the mobile phase is here always(l); it passes the pores of the fixed phase (by diffusion).

  2. column chromatography
    In column chromatography we fill a glass tube (can have various sizes) with the fixed phase. On top of this column we introduce the mobile phase ((l) or (g)), and at the bottom (small tap) that mobile phase can escape.
    A mixture of components is taken with the mobile phase from top to bottom, and the rate of passing the tube varies with each component.
    An advantage of this methode is that the we can regulate very well the passing rate of the mobile phase, and also that with a detector at the bottom of the tube we can detect the passing of each component (and even how much of it).
    Another very often applied method belonging to the column chromatography is the GAS CHROMATOGRAPHY. A carrier gas (inert / noble) is moving through a very long porous column in a tube; that gas takes with it - at different rates - the various components of the gas mixture.


This part of the analytical research has a more chemical character; chemical reactions are involved.
Every substance has - because of its chemical structure and composition - the option to suffer certain chemical reactions that are characteristic for that specific substance. Only noble gases do not suffer chemical reactions at all.
When during those reactions something becomes visible, like color, a gas, a precipatate, heat or whatever else, then an investigator can draw his conclusions about the characterof the substance.

This kind of research follows the mor physical pre-investigation described herefore.

reactions at heating.

  1. When the substance to investigate is heated in a dry, suitable tube, then this may have the consequence that the substance will decompose. The substance has in this case no melting point / boiling point. We talk aboutr thermolysis.
    Sometimes you can do observations during this process that give information about the substance; you can draw conclusions about the character of the substance. Normally we start with carefull heating, followed by very strong heating. Take note of:
    If water vapor escapes, the substance might be a hydrated salt, like Copper(II)sulfate or soda. But it could also be a hydroxyde.
  2. If, after strong heating, remains a black residue (coalish), this means that the substance contained Carbon. Probably it was an organic substance, one from the carbon chemistry.
    C6H12O6(s) > C(s) + gases (like H2O, CO2, CxHy)
  3. Some salts - at heating - change into the metal oxyde and one or more gases. This way for example, nitrates give the so called 'nitric vapors', and carbonates give Carbon dioxyde.

Oxydation of metals.

All (neutral) metals are reductors (see table X). General rule here is: that a redox reaction of a metal with an oxydator only occurs if in that table the oxydator has a higher position than the metal.
Based on this item, you can divide metals as follows:
  1. Very unnoble (reactive) metals react with water. Na(s) + H2O(l) NaOH(aq) +H2(g) [het H2 is inflamable!]
  2. A normal reactive metal does not react with water, but does react with hydrochoric acid. Fe(s) + 2HCl(aq) FeCl2(aq) + H2(g) [H2 is inflamable!]
  3. A half noble metal does not react with hydrochloric acid, but does react with diluted or concentrated nitric acid. 3Cu(s) + 8HNO3(aq) 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l) [(Coppper was red) (Copper(II) ions are blue) (NO-gas brown)
  4. Noble metals dissolve anly in "kings water (aqua regia)": a mixture of concentrated nitric acid and concentrated hydrochloric acid.
    Au(s) + 4H+ + 4Cl- + NO3-(aq) AuCl4-(aq) + NO(g) + 2H2O

Substances with molecular lattices

Molecular lattices are formed by substances with just atomic bonds. A substance with a molecular lattice mostly will be composed of non metals. If a substance contains a metal, in most cases this will have an ionic lattice.
Molecular lattices are found, for example, in: sulphur, with phosphorous, many carbon compounds like saccharides, derivate of benzene, fats, and more.

Question 6
Write the molecular formulas and the structural formulas of above mentioned substances.

Possibilities for further research are:
  1. Completely burn the substance (if possible) and see if the lost gases contain Carbon dioxyde and/or watervapor; then you know if the original substance had the elements Carbon and Hydrogen.
  2. You can also try to saponifty the substance with OH-(aq); in case of succes, this indicates an ester or fat.
  3. A positive reaction on Fehlings Reagent or on Tollens reagent indicates the presence of an aldehyde group (for example in a saccharide).
More about Fehling and Tollen can be found in
module 11-5.3
Fehlings reagent
silver mirror

Question 7
Explain which observations you must do for a positive reaction at the two reagents above.

Question 8
Name some saccharides that yes or no can be oxydised in the above way, and explain why (not).

the substance has an ionic lattice

Knowing that a substance has an ionic lattice, then you probable already found if this lattice dissolves in water;
Table XI gives you some indications about certain (im)possibilities.
To find out which ions are present in an unknown substance, you must know and apply a good number of chemical reactions. Separate positive and negative ions can be identified with so called regognition reactions to ions.
It is wise first to research the dry substance, in order to know if ammonium salts or carbonate or sulfides are present.
Also the unknown substance, before the ion research, must be dissolved in water or in acid. For an overview of a number regognitions reactions, please consult a following paragraph.

the substance is a liquid.

Un unknown liquid contains normally no ions or metals; so that you already know.
Many liquids belong to the carbon chemistry (organic chemistry: ethers, esthers, amino acids, alkonoles, etc.). Of course there are also a number of inorganic liquids like water, bromium, sulfuric acid. To be sure if the substance is organic, you know how to find out (see IIa).
Organic liquids do also produce carbon dioxyde and water (when burnt or investigated with lime water and waterfree Copper(II)sulfate (1)
Alkanals can be found with Fehlings Reactant (2)
Primary and secundary alkanols are found with dichromate or permanganate (3)
Unsaturated compounds with bromium water (4).

Question 9
Write examples of the above mentioned reactions 1 t/m 4.

the substance is a gas.

A gas sometimes can be regognizes with color or smell.
determining reactions to gases:
  1. if in a colorless, odorless gas a glowing wood splint starts to glow extra clearly, then this gas must be oxygen.
  2. Carbon dioxyde makes lime water trouble, because calcium carbonate is produced.
  3. If a gas turns white Led acetate paper into black, then that gas is probably Hydrogensulfide (3) (and probably you did already smell it).
  4. Ammonia gas will color a red litmus paper to blue, or gives with HCl(g) a white smoke.
  5. If a gas decolors Bromine water or Iodine solution, then this gas must be a reducing gas (for example SO2) but: it can also indicate addition, and then the gas was a small and unsaturated compound.
  6. A number of ases are inflamable, like Hydrogen and Hydrocarbons.

Question 10
Write what you know about the following gases (find formulas and characteristics):
  1. Chlorin gas
  2. nitric vapors
  3. ozon
  4. Hydrogensulfide
  5. sulfur dioxyde
  6. Hydrogenchloride

Question 11
Qualitative analysis of a pure substance indicated that the substance was built up of ions; adding hydrochloric acid did not cause any precipatation; adding Hydrogensulfide(g) into the solution, caused a fine precipitation of Sulphur; ammonium sulfide gave a black precipitation that easily dissolved in hydrochloric acid.
Wat was the investigated metal ion? Explain all the observed phenomona.

Question 12
You investigated an unknown substance, and it turned out to be Lead(II)carbonate. Describe all experiments needed to draw that conclusion.

Question 13
How can you prove that Sodium carbonate (soda) is polluted with Sodium sulfate?

Question 14
Eight numbered flasks (without names or formulas of the content) contain the following substances:
chalk, Zincum sulfide, Aluminium chloride, Magnesium sulfate, Potassium Iodide, Copper(II)sulfide, Sodium sulfite and Lead Nitrate.
You are only allowed to use the following substances in this research: water, concentrated sulfuric acid and Sodium hydroxyde.
How can you find out what name should be on each flask?


NH4X(s) + NaOH(aq) (NH4OH) + NaX(aq)
(NH4OH) = NH3.H2O NH3(g) + H2O
The ammonia gas only escapes at heating, and can then be detected with humid litmuspaper (turns blue) or with HCl(g) where a white smoke can be observed of NH4Cl(s).

MCO3(s) + H3O+(aq) M+(aq) + (H2CO3)
(H2CO3) = H2O + CO2 H2O + CO2(g)
The carbondioxyde can be detected, leading the gas through lime water or a solution of Ba(OH)2:
CO2 + Ca(OH)2(aq) CaCO3(s) + H2O
The CaCO3(s) is white trouble.

MS(s) of MS2(aq) + H3O+(aq) M+(aq) + H2O + H2S(g)(bad smell!)

H2S-gas can be regognized with the smell, but also with a reaction with Lead acetate paper (white becomes black):
PbAc2(s) + H2S(g) PbS(s)(black) + 2HAc

MSO3(s) of SO32-(aq) + H3O+(aq) M+(aq) + (H2SO3)
(H2SO3) H2O + SO2(g)
SO2(g) can change the color of a solution of dichromate in acid environment from orange to green. sulfite ions also can cause the same color change.

Question 15
Write the redox reactions of Potassium dichromate (solution + sulfuric acid) with:
  1. sulfur dioxyde
  2. sulfite ions
  3. Hydrogensulfide gas

Ag+(aq) and Cl-(aq) regognize each other: AgCl(s)(white)
The appearing silver chloride (s) dissolves again in ammonia, producing the ion: silver diammoniun ion:
AgCl(s) + 2NH3(aq) Ag(NH3)2+(colorless) + Cl-(aq)

In the same way, also a bad soluble Copper(II)compound reacts; the Copper(II)ammonium complex is however dark blue.
Cu2+(aq) + 2NaOH(aq) Cu(OH)2(s)(pale blue) + 2Na+(aq)
Cu(OH)2(s) + 4NH3(aq) Cu(NH3)42+(aq)(dark blue)

Br-(aq) and I-(aq) form with Ag+(aq) again the precipitate, just like chloride ions.
A difference however is that AgBr and AgI are not white, but (pale)yellow.

Question 16
Explain that Br- and Cl- also can be detected with a suitable oxydator. Which ones? and what do you obverve?

Iron(II) and Iron(III) ions can be detected in the appearance of Berlins Blue(s)
Fe2+ + [Fe(CN)6]3- + K+ KFe[Fe(CN)6](s)
Fe3+ + [Fe(CN)6]42- + K+ KFe[Fe(CN)6](s)

Lead(II) ions (Pb2+(aq)) often are detected in diluted hydrochloric acid. Leas(II)chloride dissolves rather badly at room temperature in water, but rather well at 100ºC.
So the first formed precipitate dissolves at heating.

Nitrate ions can be detected with the 'Brown Ring reaction'. The investigated liquid (about 2 ml) must first be mixed with a same volume of concentrated sulfuric acid. (be carefull please, and cool down the mixture).

Question 17
Why are carefull behaviour and the cooling necessary?

Then in another test tube a saturated solution of Iron(II)sulfate is prepared. This solution is very carefully 'layd upon' the acid solution.
If nitrate was present, then in between the two solutions shows up a brown ring.

Question 18
Explain which reactions take place, and what is that final brown color?

4. ACTION-TASK qualitative analysis

the teacher will give you an unknown pure substance. You must research this substance in accordance with the points I to XI. Every time you must note all your actions, your observations and what you think of that (in a chemical way of course). Finally you must produce a nice report.
Take care that - using the unknown substance - you never consume that substance completely. Always keep some of it in stock.
Draw your prelimenary conclusions from each observation; thus lots of research will turn out to be unnecessary.
The action points for each investigation are: Chemicals, glass work and instruments available:
  1. The exterior of the substance
  2. Flame coloring
  3. Melting and boiling point
  4. Solubility and conductibility
  5. pH-measuring, after dissolving the substance in water
  6. possibly: chromatography (ask the teacher)
  7. Heating of the dry stuff in a suitable test tube
  8. Regognition reactions on ions, that may cause the production of gases (ammonium, carbonate, sulfide, sulfite)
  9. Is the substance a neutral metal? than find a suitable solvent
  10. Is the substance organic, unsaturated? can it be oxydized?
  11. Other regognition reactions on ions.
Dependent on the substance, the teacher can tell to skip some of the tasks VII tot XI.
Ask him!
  1. sulfuric acid, hydrochloric acid, NaOH(aq) or KOH(aq)
  2. Fehlings Reactant
  3. Bromium water
  4. dichromate solution
  5. water and alcohol
  6. indicators a Platinum wire, a thermometer, various electronic meters, etc.