rubseb t1_jdz4py7 wrote

Mainly because the lid reduces evaporation. Water evaporates all the time, not just when it's at boiling temperature. Or rather, it evaporates if the air is able to accept more water vapor. Air that is already very humid can't accept much more water. At 100% air humidity, no (net) evaporation takes place.

Okay, so why is this important to how fast water boils? Well, when water evaporates, it steals a lot of heat from its surroundings. That's how sweating cools down your body: the sweat evaporates off your skin and cools it. So, if you're trying to heat up a pot of water, evaporation is your enemy.

If you leave the pot uncovered, evaporation can happen freely as humid air above the pot is replaced by drier air. But if you cover the pot, now air gets replaced much more slowly (it will still get replaced a little). Water will evaporate into the air above it, but this air will quickly reach 100% humidity and then no longer be able to accept any more water vapor. This doesn't mean that evaporation halts completely, but rather an equilibrium is reached where some new water evaporates, but an equal amount of existing water vapor condenses against the walls and (especially) the lid of the pot. When water vapor condenses, the energy it "stole" before to evaporate is returned to the area where the condensation occurs. So, if you can get the vapor to condense inside the pot, then the heat stays inside the pot.

A smaller effect is that the lid also traps heat. Air above the water heats up, and if the pot is uncovered, this hot air is replaced by cooler air, which cools down (or slows down the heating of) the pot and the water. Trapping the hot air above the water means you again lose less heat, but the reduced evaporation will always be the larger effect.


rubseb t1_j6hze57 wrote

Not OP but: why? This question absolutely has perfectly objective answers to do with the physics of heat transfer. OP isn't asking "why do I feel colder than my parents in the same situation" (which would be subjective), they're asking "why do the same people feel colder in one environment than another, given the same temperature"?


rubseb t1_j6hyzxa wrote

Drafts, humidity and radiant (+ conductive) heat.

What you experience, and what determines your level of thermal comfort, is how quickly your body loses heat to the environment. Your body is constantly producing heat in the process of staying alive and has to get rid of it, but it doesn't want to lose heat too quickly either - if it does, that's when you feel cold.

The (average) temperature of the air around you is a major influence in all this, but it's far from the only one.

Let's start with drafts. Recall that I said that your body is dumping heat to its surroundings. This includes the air directly near your body. As a result, this air heats up. Now, if the air is relatively still (doesn't move much), then this results in a kind of blanket of warmer air forming around your body over time, which slows down your body's heat loss. On the other hand, if the air is moving, then the air warmed by your body gets blown away and replaced with cooler air. As a result, you will feel colder, even though the average temperature in the room is the same. So, one possibility is that your parents' house could be more prone to drafts. This could be simply because of things like gaps in the construction that let outside air in (e.g. gaps around doors or windows), but another possibility is poor insulation. Cold floors, walls and windows cause air to cool down and sink. The sinking air causes other air to move in to take its place, and thus a thermal draft is born. Whatever the cause, these drafts will make you feel colder, even if you measure the same temperature on a thermometer.

Next up, humidity. This is a pretty simple one. Humid air has a larger capacity to absorb heat than dry air. So, humid cold air will sap heat away from your body faster than dry cold air of the same temperature (on the other hand, humid air will make it harder for your body to cool down in a hot environment - so high humidity is always bad for thermal comfort). If your parents’ place is more humid, then it will feel colder at 14°C than your own house does.

Finally, there’s the effect of radiant heat. All warm things (i.e. warmer than absolute 0) radiate heat in the infrared spectrum, including walls, windows, floors and furniture. Your body absorbs this radiant heat. So, the more radiant heat you receive from your surroundings, the warmer you will feel. This, by the way, is also why you feel warmer when the radiators are on, versus when they are off, even if the temperature of the room is the same. Why does this matter? Well, the temperature that your thermostat or thermometer measures is the air temperature in the room. Mostly, everything in the room will be at that same temperature. However, walls, floors and windows might be quite a bit colder, especially if (again) you have poor insulation (and/or if you have many walls with the outside, rather than walls with neighboring houses or apartments). So even if the air temperature is 14°C, if your walls, floors and windows are all at (say) 9°C, that means you receive less radiant heat from them than if they were at 12°C or even 14°C. In short, your parents might have colder walls, floors and windows than you do at home, and so you are receiving less heat from your surroundings there.

This latter point is exacerbated if you are in direct contact with these cold surfaces, which is mainly a problem with cold floors. In that case the issue isn’t just a lack radiant heat – you’ll also be losing heat through conductive heat transfer.

(On a final note, I'm surprised that 14°C would ever feel warm and comfortable to you. That seems very chilly to me even in a well-insulated house. )


rubseb t1_j5y1veu wrote

That's plausible, although I'd say it would be very careless of a doctor to use that terminology. I get not wanting to use the phrase "severe sore throat" which sounds a bit childish, but there are perfectly good alternatives like pharyngitis (or tonsillitis, as the case may be), which describe the symptoms rather than the cause (strep throat is one cause of pharyngitis, but more commonly it the cause is a viral infection, which can still be very nasty). Or just "inflammation of the throat" if you prefer plain English over Latin & Greek.


rubseb t1_iy7twiq wrote

We use it to melt ice in both cases.

Salt lowers the freezing point of water. Fresh water freezes at 0°C. With enough salt, you can lower that quite a bit. At 10% salt concentration, you're down to about -6°C, and at 20%, the water freezes at about -16°C.

So, suppose you fill a bucket with ice that is -12°C, and you add 20% salt. This lowers the freezing point to -16°C, but the ice is warmer than that, so it melts. And the action of melting actually absorbs quite a bit of energy, so this will lower the temperature. So you get a bucket full of ice water (i.e. a mixture of liquid water and chunks of ice) at a temperature of -16°C. Stick a metal bowl in there and you can make ice cream in it.

Why do you want this and not just ice? Well, solid ice cubes or shavings don't make good contact with the bowl that contains the ice cream ingredients, so the heat doesn't transfer very quickly from the bowl to the ice. Liquid water, on the other hand, makes great contact with the bowl, but normal liquid water isn't cold enough to freeze the ingredients. So the perfect combination is liquid water that is also below the freezing point of your ice cream ingredients.

Note that we added salt to make the ice in the bucket melt. This is no different from putting salt on roads to melt snow or ice.


rubseb t1_iy2tfyl wrote

No. Over-saturation of O2, i.e. hyperoxia, certainly isn't healthy but the symptoms aren't as acute, and the partial pressure of oxygen in normal air simply isn't high enough to cause oxygen toxicity (at sea level pO2 is about 21 kPa and toxicity only occurs above 30 kPa). That is, no matter how fast you breathe, in normal air you can never raise your blood oxygen to toxic levels.


rubseb t1_iy2sp5f wrote

People sometimes breathe much faster than is healthy. This is called hyperventilation and can be caused e.g. by anxiety or stress. When you hyperventilate, you're saturating your blood with oxygen, and depleting it of carbon dioxide (CO2). CO2 dissolved in liquid (such as blood) makes an acid called carbonic acid. Without CO2 in your blood, you're missing this acid and your blood becomes more alkaline, which causes a syndrome called hypocapnia, which is associated with symptoms such as dizziness, fainting and (more) anxiety.

So, to sum up: people who are hyperventilating run a risk of having too little CO2 in their blood. The idea behind breathing into a paper bag is that you would breathe out air with more CO2 in it (compared to regular air), and then by breathing this back in you would raise your CO2 levels.

There are two reasons why this method is no longer recommended: (1) it's rather ineffective and (2) it can be very dangerous:

  1. Experiments have shown that CO2-levels barely increase faster by rebreathing exhaled air, and no more than a placebo condition where people believed they were rebreathing, but were in fact breathing normal air. Why does a placebo method affect CO2 blood levels? Well, because simply relaxing and breathing more slowly is an effective method of raising CO2, and a placebo treatment can help reassure people that they will be okay. They believe that it will help, and that makes them relax.
  2. Symptoms resembling those of hyperventilation can be caused by conditions such as asthma attacks or heart attacks. And sometimes people who hyperventilate do so because they are actually low on oxygen (due to some other medical reason). In all these cases, making people breathe into a paper bag can be very dangerous, and even lethal, as you are depriving them of much-needed oxygen.

So the takeaway is that at a minimum, you should exercise an abundance of caution when attempting the paper bag method. Never have someone else hold the bag for you (let alone hold a bag over another person's mouth if you think they are hyperventilating). Never use this method if you suffer from any kind of heart or lung problems, or if you've ever head a stroke, embolism, blood clots, etc. Only breathe into the bag for a few breaths (most sources recommend no more than 6-12 breaths). And avoid using this method if you're not sure that you're hyperventilating.


rubseb t1_ixpinnr wrote

I imagine you probably thought your daughter's psychiatrist was referring to some scientific theory. But theory of mind (not "mind theory", although the confusion is understandable if you thought it was a scientific theory) refers to the ability to understand other people (mainly, though potentially animals too) as also having thoughts, intentions, feelings and so forth. Not just in a general "this is a person who thinks and feel things"-kinda way, but also specifically to infer what another person might currently be thinking or feeling. Not like mind-reading, of course - the idea isn't that you can literally know another person's thoughts. But most of us, in our daily interactions with others, are constantly trying to guess what motivates the actions of other people, what type of mental state they are in, and so forth, and this helps tremendously in order to make those interactions successful.

People with autism seem to commonly have deficits in theory of mind - that is, they have trouble figuring out another person's mental state or intentions. However, as with all neurodivergences, every person is different, and also recently some research has come out that challenges this idea. This newer research has found evidence that the apparent theory of mind deficits are reduced when people with autism interact with each other rather than with neurotypical people (and, conversely, that neurotypical individuals appear to have similar theory of mind problems when trying to understand people with autism). So, perhaps it's not that people with autism have trouble understanding the psychology of others in general, but rather that they (and people in general) find it difficult to infer the mental states of people who don't think like them. Or at least, that may be part of the explanation - there may still be genuine theory of mind deficits at play (in some individuals) as well.


rubseb t1_iu3jbl6 wrote

All* fish that is served raw or undercooked in the US (in a commercial setting) must have been frozen to a certain temperature for a certain length of time (FDA Food Code 3.402.11-12). This has nothing to do with it being "sushi-grade", which is indeed an unregulated term.

*The only exceptions are certain species of tuna (presumably because their risk of parasites is low), and aquaculture fish that are raised in a controlled manner that prevents parasites.