It is an intrinsic property of a substance that is caused by the shape and structure of its molecules, which makes the distribution of surrounding electrons irregular when molecules get close to one another, creating electrical attraction that can maintain a microscopic structure such as a water drop. In other words, cohesion allows for surface tension , creating a "solid-like" state upon which light-weight or low-density materials can be placed. Water , for example, is strongly cohesive as each molecule may make four hydrogen bonds to other water molecules in a tetrahedral configuration.
This results in a relatively strong Coulomb force between molecules.
Cohesion and adhesion of water
In simple terms, the polarity state in which a molecule is oppositely charged on its poles of water molecules allows them to be attracted towards each other. The polarity is due to the electronegativity of the atom of oxygen: oxygen is more electronegative than the atoms of hydrogen, so the electrons they share through the covalent bonds are more often close to oxygen rather than hydrogen. These are called polar covalent bonds, covalent bonds between atoms that thus become oppositely charged.
This charge polarization within the molecule allows it to align with adjacent molecules through strong intermolecular hydrogen bonding, rendering the bulk liquid cohesive.
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Van der Waals gases such as methane , however, have weak cohesion due only to van der Waals forces that operate by induced polarity in non-polar molecules. Cohesion, along with adhesion attraction between unlike molecules , helps explain phenomena such as meniscus , surface tension and capillary action. Mercury in a glass flask is a good example of the effects of the ratio between cohesive and adhesive forces. Because of its high cohesion and low adhesion to the glass, mercury does not spread out to cover the bottom of the flask, and if enough is placed in the flask to cover the bottom, it exhibits a strongly convex meniscus, whereas the meniscus of water is concave.
A few letters and other informal. Secondarysources are given when they refer directly to the matter in hand or when they seemto be particularly useful. No attempt has been made, however, to cite everythingthat is relevant to the background of the subject since this would have led to theinflation of an already long bibliography.
This policy has led to a fuller coverage ofthe 18th century than of the 19thwhere the secondary literature is potentially vast. Incontrast, there are almost no directly useful secondary sources for the 20th century,but here the number of primary sources is impossibly large.
It would have been easyto have given ten or more times the number listed. The choice is inevitably biasedby the recent aspects of the subject upon which I have chosen to concentrate; othersmight have made other choices, but no one could give a comprehensive coverageof the last century. The references are listed in four main groups, one at the end of each of the. Chapters 2 to 5. There is so little overlap between those in each chapter that thismethod seemed less clumsy than a consolidated list for the whole book and leaveseach chapter almost self-contained.
The form in which the titles of journals isabbreviated follows the usual conventions. A few journals that are often known bytheir editors name are shown by inserting this name in brackets before the title,e. Sillimans Amer. The journal that is now called the Annalender Physik was often abbreviated, after its editors, Pogg. Physik isused here throughout. The dates at which some journals appeared differ from thenominal date on the volume.
This problem is particularly acute for the publications. Here the nominal date is used and the actual date of theappearance of the paper is noted if this relevant. The Annual Reports of the BritishAssociation are dated by the year in which the meeting was held; they were usuallypublished a year later.
The place of publication of books is given but not the nameof the publisher. Cross-references to Collected Works are given for some foreignauthors but not for most British ones such as Maxwell or William Thomson. Experimental work is described in the units of the time when it was made but a. The angstromhas, however, been retained to describe intermolecular separations.
Cohesion: A Scientific History of Intermolecular Forces
The index of names covers only those whose scientific work is being discussed;. Biographical references aregiven for the major workers in the field who had died by the end of the 20th centuryin December , but not for those believed to be still alive. These references aregiven at the point in the text where the scientists work first becomes important tothis narrative, and so not necessarily at the first citation.
If he or she is one of thosein The Dictionary of Scientific Biography, ed. Gillispie, 18 vols.
Additional sources are given only if they are particularlyimportant for the subject of this book, or have been published later than the DSBarticle. If the scientist is not in this work then the next source is the volumes ofJ. This is abbreviatedPogg. References to the British Dictionary of National Biography are abbreviatedDNB but details are omitted since the work is ordered alphabetically and since anew edition is now being prepared.
I thank those who have been good enough to read parts of the book and give. Others are thanked in the references for more specificinformation. Some problems have always been with us.
No one knows when man first askedWhat is the origin of our world? One aim of this studyis to take such a perennial theme, although one narrower than either of these twoproblems, and see how it has been tackled in the Western world in the last threehundred years. The topic is that of cohesion why does matter stick together? Whydo gases condense to liquids, liquids freeze to solids or, as it has been put morevividly, why, when we lift one end of a stick, does the other end come up too?
Suchquestions make sense at all times and the attempts to answer them have an intrinsicinterest, for the subject of cohesion has at many times in the last three centuriesbeen an important component of the physical science of the day. It has attractedthe attention of some of the leading scientists of each era, as well as a wide rangeof the less well known. It is a part of our history that is worth setting out in somedetail, a task that I think has not yet been attempted.
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This study has, however, a wider aim also. Historians have rightly given much. These are the points that Thomas Kuhn described as revolutions . Thestudy of cohesion shows no such dramatic moments, the closest being, perhaps,the discovery of the quantal origin of the universal force of attraction betweenmolecules in This is, therefore, an account of a branch of normalscience that exemplifies how such work is done. Science is not a logical and magisterial progress in which experimental discov-.
Cohesion: A Scientific History of Intermolecular Forces - [PDF Document]
The practitioners know this on a small scale. Research workerscan see how their progress is helped or hindered by chance discoveries, misleadingexperiments, half-remembered lectures, chance finds in the literature, unexpected. More-over science can be fun. Investigations can be made just out of curiosity even whenit is clear that the answer, when found, will solve no particular experimental ortheoretical problem. We shall see that similar disorderliness marks progress on alarger scale. Matters move forward rapidly for a decade or so, and then stagnate formany decades.
Here three broad periods of advance have been identified and namedafter Newton, Laplace and van der Waals.
They were, of course, not the only gen-erators of the advances but their contributions were decisive and, perhaps, stretchthe concept of normality to its limits. Their names may, however, conveniently beused to identify their periods. It is of interest to seek for the causes of this punctuated advance. Some of the.