Wikipedia:Reference desk/Archives/Science/2019 May 18

= May 18 =

When chemical properties of liquid aren't just the average of the elements
If you mix two liquids with ebullition at 100 °C and 80  °C (more or less like water and some alcohol, but I have no specific substance in mind), could the resulting ebullition be far off 90 °C?Doroletho (talk) 01:16, 18 May 2019 (UTC)


 * BP will be close to 90 for a mixture of equal weights, but may be slightly lower or higher than you'd expect. Raoult's law discusses this.Greglocock (talk) 04:53, 18 May 2019 (UTC)


 * Also the specific volume, as discussed above (though perhaps that is what prompted your question, in which case you knew that). catslash (talk) 14:13, 18 May 2019 (UTC)


 * Azeotropes are an extreme examples of non-average (and not even "intermediate") behavior. If you have a non-ideal mixture, you are lose a premise of Raoult's law. DMacks (talk) 16:59, 18 May 2019 (UTC)

Physics help needed
2019 redefinition of SI base units is marked as being OR. Does anyone have any sources for the uncertainties listed?

The redefinition of the base units for the metric system happens on 20 May 2019. I am hoping to get this article mentioned on the main page on that day. Alas, I also have a hot project and if I don't make my deadline because I spent too much time on Wikipedia the toy I am working on will miss Christmas. :( --Guy Macon (talk) 15:32, 18 May 2019 (UTC)
 * Will this change have any real, practical effect for the average human? As compared with your missing your project deadline? ←Baseball Bugs What's up, Doc? carrots→ 20:25, 18 May 2019 (UTC)


 * Reading that article, I ran across the curious statement that the steradian is a "dimensionless unit". I do understand what they mean, yet ... well, my philosophy is tickled.  I mean, yes, there are some units that make absolutely no sense without an external standard to calibrate by (7 meters) whereas others are ratios that indicate some kind of comparison (7 mg/kg dosage).  This comes up in chemistry where, for example, tracking a dilution problem can be easier if you make up a difference between "ml concentrated solution" and "ml dilute solution".  But, well, the only thing is, I would have thought that the "unit" would be the external measurement whereas the "dimension" would be more of whether a measurement is curved or straight.  Could it possibly be a unitless dimension? I don't know... Wnt (talk) 21:06, 19 May 2019 (UTC)


 * It's not unitless because the steradian is the unit. Dimension here is being used in the sense of Dimensional analysis.   Dbfirs  21:29, 19 May 2019 (UTC)


 * Well yeah, but dimension is like mass, length, time ... solid angle? The steradian comes into being because you measure the angle in m^2 per m^2 or in^2 per in^2, and the units cancel out.  It does seem like making up a unit to describe a unitless dimension... Wnt (talk) 13:39, 21 May 2019 (UTC)
 * Please note this apply just as well to normal angle in radian, which also is measured in meter per meter, and the unit cancel out, too. So if you think that a dimensionless unit is odd, and somewhat useless, how do you cope with angles?
 * Maybe this should be said this way: dimensions DO cancel out, units, not so much. The apparent area of an object afar maybe measured in m² just like the square of its distance, but is not the same "unit", so don't cancel them out, rather, call the ratio by its proper name, that is : steradian.
 * And also apply to Plank units: by construction, they attach the dimensions to the relevant constant, and using them just every physical equation appears dimentionless, just like a steradian.
 * So you just have to understand the steradian as some sort of plank unit, linked to the universal geometrical constant 4pi.
 * there are quite a number of SI units and SI derived units that are actually of the very same dimension of another, but are still not the same, just look at them. For instance, Candela, Lumen and Joule. And you must NOT use them for one another. And you MAY divide them, getting a number that IS dimensionless but NOT unitless.
 * So understand the steradian as a ratio of two different units of the same dimention: the area of a seen object, and the square of its distance. Pretty much like a Lumen/Joule would be dimensionless, but still not unitless nor meaningless.
 * Does that help understanding ?
 * Gem fr (talk) 23:00, 21 May 2019 (UTC)
 * Well, I'd be lying if I claimed to really understand candela and lumen in detail: we have up a picture of the photopic and scotopic luminosity functions with a little legend about the CIE curves that went into them... my gut feeling is that any unit of physics that delves that deep into biology is not going to act in the reliable way one expects of a physical unit, that there's probably some combination of strobe pulse and frequency that will end up looking totally different in brightness than the unit says it does. The use of candela = lumen/steradian seems fairly straightforward, but describing it philosophically seems different.  I mean, I thought the meter was a unit.  How then can meter^2 not be the same unit as meter^2?  The way I'd say it is that they're different dimensions, just as length and width are different dimensions.  And the partial circumference of a circle is a different dimension from the length of the tangent also.  And the ratio between those has no units, but it is a ratio of dimensions, like the Golden Mean of a painting or something.  But that's not how you're saying it. Wnt (talk) 12:53, 22 May 2019 (UTC)
 * Seems pretty obvious to me that the square of a distance (like it appears in steradian, or in calculation of gravitation force) is not an area, despite both having dimension of Length². So it makes sense to have units, like steradian AND radian, whose dimension are a number, but they are different nevertheless.
 * Gem fr (talk) 15:03, 22 May 2019 (UTC)
 * The r^2 in an inverse square phenomenon absolutely can be viewed as an area. For example, consider the Sun, or a star.  Viewed in a telescope (kids, don't do this) the brightness of the sun is always going to be exactly the same, whether it is partially eclipsed, viewed from Pluto, whatever, so long as it fills the entire field of the telescope.  The inverse square brightness of the sun simply reflects the steradians it takes up in the sky: the ratio of its area to the area of the sphere with the same radius.  But the odd part is I feel like you are (or should be) agreeing with me: you're saying you can have the same unit mean different things.  But that 'meaning' I would call the dimension - just as length, width, and height are different dimensions measured with the same unit. Wnt (talk) 00:32, 25 May 2019 (UTC)

←Baseball Bugs What's up, Doc? carrots→ 18:10, 20 May 2019 (UTC)

Does cancer cells have a specific protein structure ?
Does cancer cells have a specific protein structure ? does this protein structure is the same structure of the healthy cells ???
 * A good start would be to read Cancer cell. ←Baseball Bugs What's up, Doc? carrots→ 20:24, 18 May 2019 (UTC)


 * Protein structure is a characteristic of an individual protein. The vast majority of 30,000 genes in a cell produce proteins, each different from the others, each of which may have more than one structure.  It is possible that genetic, epigenetic, or regulatory changes in the cell can alter the structure -- to give a classic example, activation of c-Src by phosphorylation will change its structure, causing it to interact differently in the cell and have more activity in stimulating mitosis (reproduction).  Cancer cells can have genomic instability with vast numbers of genetic alterations in a cell, vastly different patters of gene transcription, vastly different amounts of protein present, so the protein structures likewise will follow through in being altered in many specific ways.  But every cancer case is different (well, many of them), just as families are unhappy each in their own individual way. Wnt (talk) 20:58, 18 May 2019 (UTC)