Talk:Mitscherlich's law

Bengali source
An old school-chemistry textbook (of 1979) at my home (by রণজিত দাস (Dr. Ranajit Das)), provides a topic on  Mitscherlich's  law of  crystal isomorphism, and one of its application, in  a stoichiometry chapter (Rajarshi Rit 13:56, 14 January 2016 (UTC)) It says, the law is used as a method for determining the atomic weight of elements.

I’ve translated it into English (because the article was in Bengali), & slightly simplified.

“  …. Isomorphism and Isomorphic crystals:

If crystals of more than one different (electrovalent?) compounds, show similar shape of crystal (same number of plane-surface, and exact equal angles at respective corners), then the crystals may- be called isomorphic crystals, and the property of those compounds (for-which the somehow forms same-type crystal) called crystal-isomorphism.

However the external, superficial similarity in shape is not the only-criterion to tell the crystals isomorphic. With totally following external similarity, if the crystals the three more following criteria, they would be called isomorphic.

(1). Two isomorphic crystals must show a similar-pattern of chemical formula, i.e. same-number of atoms (of separate elements) would compose the chemical formula.

(2). If (aqueous?) solution containing two such isomorphic compound together, is crystallized, a "mixed-crystal" appears, that contains atoms from both-the compound. The shape-of the obtained mixed-crystal, is same as the shape of crystal of any of the two pure compounds. If the solution is saturated with only one-of the compounds, both the compounds crystallized together, and must form homogeneous mixed crystal.

(3). If in a saturated or supersaturated solution of one-such pure compound; a small crystal of another-such compound is added, it will be able to initiate crystallization, and on the added-crystal of second-compound, uniform "overgrowth" will be seen by accumulation of first compound from solution.

Examples of some isomorphic crystals:

(1).Zinc sulfate( ZnSO4 .7H2O ), Magnesium sulfate( MgSO4 . 7H2O ), Ferrous sulfate( FeSO4 . 7H2O ).

(2).Copper sulphide ( Cu2S), Silver sulphide( Ag2S ).

(3).Potassium sulphate(K2SO4), Potassium chromate(K2CrO4).

(4).Potassium permanganate(KMnO4), Potassium perchlorate(KClO4).

(5).Potass-alum[ K2SO4,Al2(SO4)3,24H2O  ] , Chrome-alum[  K2SO4,Cr2(SO4)3,24H2O  ].

…(for say, Zink sulfate, Magnesium sulfate, Ferrous sulfate follows all-4 criteria. Such as … … … )

Mitscherlich's Isomorphism-Law in determination of atomic weight:

After studying various crystals, a scientist, Mitscherlich discovered a rule (called Mitscherlich’s law for isomorphic crystals), that states;
 * "Equal number of atoms, being joined in same-pattern, build up isomorphic crystals"

The law indicates, in a set of such-crystals, the nature of crystal is not varying upon chemical-nature of the components. Instead they depends only-upon number and position of atoms. They have similar chemical formula.

In other hand, in 2 different compounds (if forming isomorphic crystals) differing in 1 component, ensures us, same-“number” of atoms. This property is used to determine atomic weights.

Suppose two isomorphic compounds differ only in the elements A and B, and the elements have atomic weights, a and b respectively. If we somehow come-to-know the values of how-many-gram of A (suppose W1) (of first compound) can replace how-many gram of B(suppose W2) (of second compound); we can tell W1 gram of A and W2 gram of B contains the same number of atoms (only because the 2 crystals known  as isomorphic).

Now along-with W1, W2; if we could know the value of atomic weight of any 1 element (Say A having at. Wt. =a), we can calculate another element (B)’s atomic wt (b). as follows:

((( this part of document modified because in original document it  seemed unclear.)))    (Rajarshi Rit 13:56, 14 January 2016 (UTC))

No. of atoms of A in 1st crystal = No. of atoms of B in 2nd crystal (having replaceable amount of B).

Hence, (W1/ Wt of an atom of A)= (W2/Wt of an atom of B).

Hence, (W1/ (a*Wt of 1 H atom) )=(W2/(b*Wt of 1 H atom) ).

Hence, (W1/a)=(W2/b).

Hence, (W1/W2)=(a/b). … here values can be put.

Mitscherlich’s Isomorphism-Law in determination of Valency:

In 2 isomorphic compounds differing in one component (say A and B respectively), If we can know the valency of the one ( say A), the valency of the related element(B) in the other compound, could be assumed same as the other(A) by-default, since in isomorphic crystal atoms assumed to be connected in same way. e.g. Zinc oxide and Beryllium oxide crystals are isomorphic. If we could determine formula of zink oxide is ZnO, with valency of Zn=2, then we can predict the formula of Beryllium oxide is BeO with valency of Be = 2.

Notes: (1). External shape of crystals of common-salt and diamond is similar. Yet they are not isomorphic because they do not follow other criteria such as mixed-crystal formation, overgrowth etc.

(2). In reverse; there are cases where the compounds behave as isomorphic, yet the number of atoms are not same, however, composition is similar. E.g. K2SO4 and (NH4)2SO4

…  ”

Reference: উচ্চমাধ্যমিক রসায়ন/ ড. রণজিত দাস /  ১ম খণ্ড / ৫ম সংস্করণ (১৯৭৬)/ ওরিয়েন্টাল বুক কোম্পানী
 * Ranajit Das (1976) Higher-Secondary Chemistry, 1st Volume/ 5th edition / Oriental book co. (a local publisher)).

It may be near-impossible to cross-check an old, local school-chemistry book. However experts on Wikipedia could found-out further informations. Thanks.

Rajarshi Rit 10:29, 14 January 2016 (UTC) Rajarshi Rit 10:33, 14 January 2016 (UTC)Rajarshi Rit 10:34, 14 January 2016 (UTC) Rajarshi Rit 10:36, 14 January 2016 (UTC)

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