Wikipedia:Reference desk/Archives/Science/2014 September 5

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September 5

Water depth charts

Is there a source online for sea depth charts? In this case, specifically for the area near Gibraltar.

Pretty specific and specialized request I know. Thanks, CBHA (talk) 04:35, 5 September 2014 (UTC)

The United States' National Oceanographic and Atmospheric Administration is responsible for providing such charts to American mariners. You can navigate their interactive chart at the interactive chart catalog. Technical charts for coastal waters and major overseas routes are available, but I don't see any that cover Gibraltar. That area might have coverage in the UK's United Kingdom Hydrographic Office publication library. Their charts do not appear to be available at zero cost (at least, not as easily as the U.S. equivalents). I could not find a Spanish government hydrological survey, but one probably exists and publishes charts. If you're willing to spend some time and money, these are the kinds of organizations that will get you the specific technical information you need.

Admiralty Chart 2717 (from the UK) is a 1,100,000 scale hydrographic chart of the waters near Gibraltar. I am not sure if this is the best chart for your needs. You can purchase it in paper form or as a digital download. The Admiralty Digital Products service also provides many charts of Gibraltar harbor and surrounding waters at various scales. Either way, it will cost around £30 (something like fifty US dollars) if you want complete access to the charts. You might be able to get by with "preview images" for non-navigational purposes.

American charts of oversea waters used to be available at no cost from our Navy and from the National Geospatial Intelligence Agency. Unfortunately, the Navy Oceanography website is no longer available to the public internet (due to budget cuts!) and paper charts from NGA must be ordered from the Government Printing Office. If you can determine the chart you need, the GPO will often waive their fees if you write a letter to them explaining why they ought to.

A limited selection, including six Gibraltar-area charts, are available online here: NOAA NGA website, but it requires Flash and the charts cannot be downloaded.

Nimur (talk) 05:48, 5 September 2014 (UTC)

Depths may change, especially near shore, due to erosion, sediment deposition, or dredging. So, be sure you have recent charts, if accuracy is important. StuRat (talk) 07:43, 5 September 2014 (UTC)

Diameter/radius and depth as dimensions

Why diameter or radius and depth aren't counted as 4th and 5th dimensions? Is depth treated as negative height? And how is the radius/diameter treated? For instance, a torus has a diameter which seems to be different from both the width and height. Maybe diameter/radius is thought of as a line, which upon straightening becomes height? 93.174.25.12 (talk) 08:24, 5 September 2014 (UTC)

In 3D modelling and graphics, "depth" is counted as negative values in the vertical axis. Counting radius and depth as extra dimensions in the real world would make little sense, and would complicate things unnecessarily. Where would zero depth be on Earth? 217.158.236.14 (talk) 09:14, 5 September 2014 (UTC)

You confuse 'dimension' for 'dimension'. In common contexts, 'dimension' refers to a distinct measurement of an object, whereas, in cosmology, and other specialised scientific contexts, 'dimension' refers to a temporal/spatial relation. To qualify as a spatial dimension, the relation must be at right-angles (orthogonal) to all dimensions of lesser order. For instance, 'up/down' is orthonormal to 'left/right', and 'forward/back' is simultaneously orthonormal to 'up/down' and 'left/right'. Thus 'up/down', 'left/right' and 'forward/back' all qualify as dimensions. It does not matter which direction you choose to refer to 'up/down', as long as all the other dimensions are orthonormal. Plasmic Physics (talk) 09:55, 5 September 2014 (UTC)

To add a little to Plasmic Physics already solid answer, dimensions need to be mutually exclusive. That is, movement along the axis of one dimension should not change one's position in another dimension. The "dimensions" the OP speaks of all duplicate the three main spacial dimensions to some extent. In other terms, I can describe the radius of a circle using the three main spatial dimensions alone, and don't need to invoke any further dimension to do so. The whole point of a dimension (indeed, of mathematics) is to model something; the three dimensional cartesian mathematics is sufficient to model any physical object you can handle. We only add additional dimensions to our models when they are needed, for example the 4-dimensional Minkowski space for modeling special relativity, or the extra dimensions we use for string theory. Otherwise, however, we don't just throw dimensions around willy-nilly. Three is fine for the average person in what they deal with on a daily basis. --Jayron32 10:56, 5 September 2014 (UTC)

Re: "Maybe diameter/radius is thought of as a line, which upon straightening becomes height?". I think you are confusing those with the circumference, which is curved. Diameters and radii are straight to begin with.

As for dimensions on 3 dimensional objects, like a torus, we could use the familiar X, Y, Z dimensions, although determining if a given X, Y, or Z was on the torus, inside it, or outside it, would then require some math. There should be alternative dimensions we can use, in the case of a torus. For a given torus, the angle from the perpendicular centerline (from an arbitrary zero point) could be one dimension, and the angle from the curved centerline inside the torus (from an arbitrary zero point) could be a second. Add the radius from that curved centerline to describe whether the point is inside, on the surface, or outside the torus. Thus those 3 coords could describe any point in 3D, using a given torus.

Similarly, spherical coordinates and cylindrical coordinates are alternative ways to describe the position of objects in 3D, and tend to be more convenient when dealing with spheres or cylinders, respectively. Unlike the torus coords above, those two systems don't require a given sphere or cylinder to work, but do require an origin and a couple axes.

Note that, unlike with X,Y,Z dimensions, curved dimensions have a weird feature that the same point can be described by multiple sets of coords. For example, if the radius is 0, then at least 1 other dimension doesn't matter, and can be anything. Also, angles of -360°, 0°, 360°, 720°, etc. all describe the same point. StuRat (talk) 13:15, 5 September 2014 (UTC)

I think the point is that in a three dimensional system, you always need exactly three numbers to describe the position of some point within that system. You can describe the location of an airplane by latitude, longitude, altitude...or by altitude and the great-circle distance from two distant cities...or by the distance above the plane of the equator, the distance from the plane through the center of the earth and the greenwich meridian, and the distance from the Eiffel Tower. You can find any number of ways to describe the position, but they always need exactly three independent numbers.

This leads us to talk about 'three-dimensional space' because you need those three numbers to define a single point within that space.

When considering the shape of a torus, you're discussing the set of numbers you need to uniquely define it...not the set of numbers to define a point in space. A torus has two radii, a position in 3D space (three numbers) and two numbers describing it's rotation...so the shape of the entire object is described by 7 numbers. You could describe the set of all possible torii is a seven dimensional set - and you could even plot the 'shape' of all possible torii as a set of points in a 7D space. When we talk about a torus as being a three-dimensional object, we're using the word 'dimension' to define the dimensionality of the space that the torus exists within, not the space of all possible torii.

As is often the case in science, we use the same word to mean more than one thing...and that's what's happening here.

SteveBaker (talk) 03:47, 7 September 2014 (UTC)

Unidentified Bug

Hello,

Can anyone help me identifying this bug? http://imgur.com/HCDjJJW

In the picture it seems bigger than what it really is, as I had no chance of placing something for reference nearby. The bug is probably 4 to 5 mm long. Maybe it helps to know that I am currently located in Southern Africa. --197.148.43.144 (talk) 11:41, 5 September 2014 (UTC)

It looks to me like a Weevil, so that narrows it down to about 60,000 species. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 13:25, 5 September 2014 (UTC)

The prominent proboscis makes me think some type of rhinoceros beetle, although some weevils have those too. StuRat (talk) 13:51, 5 September 2014 (UTC)

It looks to me like an assassin bug, although they're much more common in the Americas than in Africa. Bloodsucking bastards, is what they are. (Vectors of Chagas disease.) Looie496 (talk) 13:53, 5 September 2014 (UTC)

Thanks to all of you guys. Judging by its looks it is most certainly some species of Curculionidae. Nothing to worry too much about. --88.157.199.115 (talk) 14:19, 5 September 2014 (UTC)

pH of mixtures

I have a bottle of mineral water, marked pH 8.2. I would like to add lemon juice until I have a roughly neutral mixture. However, I'm a little confused as to what volumes I should mix to accomplish this based on first principles, and I don't have any pH indicators handy. The pH article is unclear, saying that lemon juice is "composed of about 5% to 6% citric acid, an acid with a pH of roughly 2.2."

1. Is the pH of citric acid ~2.2, or is that the pH of the juice? The citric acid article doesn't mention its pH. I've seen some sources online claim pH of lemon juice is around 2, but I'm not sure that they're not parroting our article.
2. If the pH of a few liquids is known, does the pH of the mixture respect basic additive math? E.g. if I have 1 liter of pH 3 and one liter of pH 11, will the mixture generally have pH 7, or will that depend on the (non)reactivity of the the substances?
3. Say I want to add ethanol to the mix. Our article says that ethanol is close to neutral, but would I have to worry about it reacting with the mineral water? Ethanol#Acid-base_chemistry indicates to me that might be a possibility, depending on the sodium content.

Thanks for any help, SemanticMantis (talk) 21:41, 5 September 2014 (UTC)

Titration is probably the best article to look at. Without knowing the concentration of the substance that's making your water basic, it's not possible to work out how much acid you'll need to add to neutralize it. The answer to your Question 2 is "no", for similar reasons - the final pH of the solution will depend on whatever reaction occurs between the two initial solutions. Buffer solution may be useful here. On Question 3, ethanol will only react with sodium metal and similarly aggressive substances, not with sodium ions in a solution that's remotely drinkable. Tevildo (talk) 22:45, 5 September 2014 (UTC)

(ec) The pH of a combination of liquids cannot be determined by simple addition. You would need to know the buffer capacity of the liquids in addition to the pH. Basically, you would need to know the concentration of titratable groups in your mineral water that you would protonate going from ph 8.2 to pH 7.0. Note that this is due not just to the pH of the solution, but also the types of minerals and their amounts. (Carbonate and bicarbonate are the most likely culprits in mineral water, but others may play a role.) Likewise, you would need to know the concentration of titratable groups in the lemon juice that you deprotonate going from pH ~2.2 to pH 7.0. Once you know that, it *is* a simple mathematical calculation - you just need to make sure that the number (concentration times volume) of titratable groups that you protonate in the mineral water equals the number that you deprotonate in your lemon juice. Coming up with that number is somewhat complicated - see Henderson–Hasselbalch equation for details on how to approach it theoretically. Typically you're better off doing things like this experimentally, where you add small amounts of acid until you reach your desired pH. (In a process referred to a titration.) Regarding ethanol, the concern that I would have would be that the ethanolic solvent would change the acid dissociation constant of the titratable groups in your mineral water and lemon juice, causing a solution with added ethanol to have a different pH than one which is just in water. (This is because ethanol is more apolar than water is, thus changing the free energy of solvation of the ions, altering the dissociation equilibrium.) You *don't* have to worry about the ethanol participating in acid/base reactions - this would only be a concern in highly basic (pH above 14) solutions. The reactions you refer to are for elemental sodium metal, rather than sodium ions. The reaction listed is a redox reaction, and sodium ions are the end product, and can't react further. Even if you had sodium metal, you wouldn't have to consider it in a water/ethanol mix, as any ethoxide formed would immediately react with any residual water to form ethanol and hydroxide, resulting in an equivalent scenario to the sodium metal reacting directly with the water. -- 160.129.138.186 (talk) 23:02, 5 September 2014 (UTC)

• Hey, two great answers, thanks! It seems this is indeed better handled experimentally. I found PH_indicator#Naturally_occurring_pH_indicators and even a wiki how to make litmus paper [1]. Does anyone have a suggestion for an especially easy or reliable home made indicator of neutrality? SemanticMantis (talk) 23:21, 5 September 2014 (UTC)

Even using indicator paper, the difference between 7 and 8.2 is pretty small (see [2]). You could try using your tongue to estimate neutrality--it should be very easy to tell if something is acidic (it tastes sour), and slightly harder to tell if it's basic. --Bowlhover (talk) 23:27, 5 September 2014 (UTC)

There are lots of specialized indicators with better resolution at certain ranges. See File:A graphic view of Acid-Base Indicators.svg for many examples covering various parts of the pH scale. The trick is finding one that is available:) Lots of ranges are available from various natural sources (see [3]), and the 7.0–8.2 range is approximated by curcumin. Visit your local spice shop and you'll be able to tell when you've gotten down at least to the mid/low 7's. DMacks (talk) 20:05, 6 September 2014 (UTC)