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163 votes

Can ants walk on liquid mercury?

Needing any excuse to break away from the work I was doing, I immediately assumed the task of answering this question. Yes, ants can walk on mercury with no trouble at all. I bet it was even kind of ...
Ian_Kinner's user avatar
137 votes

Do the weights of two liquids not add when mixed?

Of course, by common sense, if you put together two objects with masses $m_1$ and $m_2$, and nothing comes out, then you end up with mass $m_1 + m_2$. Weights are a little more complicated because ...
knzhou's user avatar
  • 102k
93 votes

Why does carbon dioxide not sink in air if other dense gases do?

Gases are all miscible. If initially separate and adjacent, they do not mix instantly, but once mixed (a process that occurs by molecular diffusion and is accelerated by macroscopic stirring or ...
nanoman's user avatar
  • 3,760
91 votes
Accepted

Why does a floating object displace more substance than a sunk object?

When submerged, the coin displaces as much water as it has volume (logical). When floating on the box, the coin displaces as much water as corresponds to its weight. As metal has a higher density than ...
planetmaker's user avatar
  • 1,029
86 votes

Given that ice is less dense than water, why doesn't it sit completely atop water (rather than slightly submerged)?

When put in water, an objects sinks to the point where the volume of water it displaces has the same weight as the object. Archimedes was the one who discovered this. When you put lead in water, the ...
hdhondt's user avatar
  • 10.9k
81 votes

How is water heavier than petrol, even though its molecular weight is less than petrol?

Density relates to the mass per unit volume. If your molecules are heavier but take up more space, the net result could be more or less mass per unit volume. When you look at a typical hydrocarbon, ...
Floris's user avatar
  • 119k
80 votes
Accepted

Why does sound not move through a wall?

Sound doesn't go through walls? Please tell my neighbor. In electromagnetism, a medium has a property called an "impedance" which is related to the index of refraction and the speed of waves in the ...
rob's user avatar
  • 88.6k
80 votes
Accepted

How is water heavier than petrol, even though its molecular weight is less than petrol?

Because water molecules are small and pack tightly together, causing water to have a greater density than petrol.
G. Smith's user avatar
  • 51.4k
66 votes

Does the amount of oxygen in air, actually get lower as you go to higher altitudes?

For elevations less than about 100 km (for reference, the peak of Mt. Everest is about 8.8 km above sea level), the relative concentration of oxygen in the air is fairly constant at about 21%. Source ...
J. Murray's user avatar
  • 68.6k
62 votes
Accepted

Can ants walk on liquid mercury?

Whether or not a small animal/insect can walk on a liquid is determined much more by surface tension than by density. To see why this is consider a dense liquid without any surface tension. You would ...
Physical Mathematics's user avatar
59 votes
Accepted

Why do rotten eggs float in water?

The domestic chicken's egg shell has about 7000 pores that allow the embryo to breathe. When an egg rots the yolk and surrounding materials decompose and they give off gasses which can pass through ...
Adrian Howard's user avatar
53 votes

Given that ice is less dense than water, why doesn't it sit completely atop water (rather than slightly submerged)?

I like to answer by reinterpreting your question: if you expect the ice to be completely atop the water because ice is less dense than water (as indicated in your left image), then you would also ...
bers's user avatar
  • 691
52 votes

Will a penny ever stand still in the water at a certain depth?

The answer is no. Water, being a liquid, is nearly incompressible, meaning that the density changes very little with increasing pressure. In the very deep ocean, the pressure can approach $10^{8}$ ...
Buzz's user avatar
  • 16k
49 votes

Is there anything in the universe that cannot be compressed?

Under special relativity nothing can be incompressible: consider any object of nonzero size and finite mass in its rest frame; when you apply a force to it on one side it will start moving. If it were ...
doetoe's user avatar
  • 9,214
48 votes
Accepted

Are black holes very dense matter or empty?

The phrase black hole tends to be used without specifying exactly what it means, and defining exactly what you mean is important to answer your question. The archetypal black hole is a mathematical ...
John Rennie's user avatar
45 votes

Are there physical properties that can be used to differentiate stainless steel from copper in a home environment?

Take advantage of the large difference in thermal conductivity between copper and stainless steel (approximately $400$ and $16$ $\mathrm{Wm^{-1}K^{-1}}$ respectively). If you put one end of a metal ...
jkej's user avatar
  • 6,148
43 votes
Accepted

Why does air pressure decrease with altitude?

The air pressure at a given point is the weight of the column of air directly above that point, as explained here. As altitude increases, this column becomes smaller, so it has less weight. Thus, ...
Sandejo's user avatar
  • 5,468
42 votes

Material which becomes less dense as pressure is applied

Becoming less dense with pressure is an energy-producing quality; mass being conserved, volume must increase as pressure rises for density (mass/volume) to have that response. So, I'd say that ...
Whit3rd's user avatar
  • 9,748
39 votes
Accepted

Why does the speed of sound decrease at high altitudes although the air density decreases?

Wikipedia gives a pretty much straightforward answer. In an ideal gas, the speed of sound depends only on the temperature: $$ v = \sqrt{\frac{\gamma \cdot k \cdot T}{m}} $$ So it neither decreases, ...
Agnius Vasiliauskas's user avatar
35 votes

Will a penny ever stand still in the water at a certain depth?

Yes, it will, but in a different way than one might imagine. In the Earth's oceans it is definitely impossible. From the phase diagram of water, one can see that assuming a constant temperature of 300 ...
Martin 'Kvík' Baláž's user avatar
34 votes
Accepted

Is there anything in the universe that cannot be compressed?

In quantum field theory, an elementary particle doesn't have one precise location and size in space. The quantum of an electron field in free space has different extent compared to the electron around ...
Luaan's user avatar
  • 6,334
33 votes

Why do rotten eggs float in water?

It doesn't need to be rotten. When the egg is getting old, it evaporate water and looses mass while drying. Even if not rotten.
Camion's user avatar
  • 555
32 votes

Do the weights of two liquids not add when mixed?

@knzhou supplied a good answer. I’m going to offer a couple of other interpretations. The first has nothing to do with the fact that you’re mixing liquids—it’s just that there are difficulties in ...
Ben51's user avatar
  • 9,684
31 votes

Given that ice is less dense than water, why doesn't it sit completely atop water (rather than slightly submerged)?

I think the real question you're asking here is: why do less-dense fluids completely stay above more dense fluids, whereas less-dense solids partly sink? If the ice covered the same area as the water ...
leftaroundabout's user avatar
31 votes

Are there physical properties that can be used to differentiate stainless steel from copper in a home environment?

Why not density? At least for a quick check and as for the title question. You are dealing with about < 8 and 9 g per cubic cm, respectively for steel and copper. Not overly laborious and ...
Alchimista's user avatar
  • 1,729
28 votes
Accepted

Does an object float more or less with more or less gravity?

The object would actually float exactly the same for both values of $g$. Let $V$ be the volume of the body, $d$ its relative density, and $V'$ be the volume inside water. Then for equilibrium of the ...
Amritansh Singhal's user avatar
28 votes
Accepted

Why is the kinetic energy of a fluid given as an integral?

The kinetic energy of a fluid is the same as normal mechanics, $T=mv^2/2$. However, that's not generally useful as we don't usually have masses but densities, so we instead consider the kinetic energy ...
Kyle Kanos's user avatar
  • 28.1k
26 votes
Accepted

Why was the metal lead used in the Cavendish experiment?

Because of its high density. This meant that, for given masses of spheres, the centres of a fixed sphere and its neighbouring moving sphere could be closer to each other (than if a less dense material ...
Philip Wood's user avatar
  • 35.3k
24 votes

Is there a temperature at which ice is denser than water?

Ice can be denser than water for certain values of $P,T$. Look at these two pictures taken from here: The darker areas in the second picture denotes areas of greater density. So you can clearly see ...
valerio's user avatar
  • 16.2k

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