Why does a helium balloon deflate when it gets cold?
475777
All gases get denser as they cool. Air in a balloon would also deflate if it gets cold. The gas in the balloon would expand if the balloon got hot.  Not to get too complicated, but colder temperatures make the molecules of the gas get closer together, which makes the balloon shrink.
976120
Helium molecules are very small and slowly leak through the balloon wall.\n\nAir contains a mixture of gaseous molecules, primarily Nitrogen and Oxygen.  These are much larger than Helium.
957446
Molecules are slowed down when it is cold therefore they contract and therefore the tire looks low. When its heated back up it will return to prior state
804765
Because it's less dense. Temperature affects density. And the less dense a gas is, the lighter it becomes, so to speak. Therefore it rises over cold air.
24312
Depends on temp and pressure, but at room temp, avg. pressure, helium's a gas
61711
Hot air is less dense than cold air so it rises 9that's why upstairs in a house is usually warmer than downstairs).  By filling a bag with hot air you can make the bag (balloon) rise up.
823437
If you take a balloon under water, and go further and further, the air becomes more dense...due to PRESSURE.   The way a submarine dives is completely different then denser/less dense ....it's that they are mostly filled with air...so they should float...but they have enough weight that they sink too,  it's almost a balance, but they have tanks that they let fill with water to sink, or push air in (and water out) to get lighter and float.
420827
high density - at least compared to air. this is why anything sinks in a fluid; rocks are higher in density than water or air so they sink, helium balloons are lower density than air so they float.
946246
Helium particles are very small, in fact they are the smallest free particles that normally exist in nature. Although the plastic your balloon is made out of can stop water and sand and marbles and various other materials that have large particles, helium is small enough to go through, so it leaks.\n\nNow, there are certain kinds of balloons which are made out of special plastics and leak hardly any helium at all. However they are more expensive than the standard, cheap, leaky ones.
535624
PV=nRT\n\nas the air is heated the average kinetic energy of the molecules increases. ie P increases.But V is constant. so some gas molecules escape through the bottom. Hence the density decreases and becomes lesss than that of surrounding air.Thus the balloon rises.
156918
Helium is an inert gas that is lighter than air and can be inhaled briefly without risk of death. Sound is produced by vibration, the movement of air around our vocal chords. Because helium gas is lighter than the usual oxygen/nitrogen blend, it changes the resonant frequency of the human vocal tract, causing a faster vibration and a higher-pitched, cartoon-character sound.
53886
Since the air inside the balloon is heated, the molucules are more active and therefore spread out. The weight of the air inside the ballon is less than the weight of the air that the ballon displaces.
428966
When you pump in the air the energy you're using to force it in makes the air warm. When you ride, this makes the air inside the tyres warmer still. All this warming makes the air expand - which is great, it makes the tyres firmer.\n\nWhen you leave the bike static in a cold place the air cools and contracts - so the tyres get softer. \n\nAlso, there is always a bit of leakage - so when you leave the bike for a while the tyres will have deflated a little.
1160660
Helium is an inert gas that is lighter than air and can be inhaled briefly without risk of death. \n\nSound is produced by vibration, the movement of air around our vocal chords. Because helium gas is lighter than the usual oxygen/nitrogen blend, it changes the resonant frequency of the human vocal tract, causing a faster vibration and a higher-pitched, cartoon-character sound.
330911
A system cooled by liquid nitrogen
766297
There is helium all around us in the air, but it is not a good source for helium collectors. There are only about 5 parts of helium for every 100,000 parts of air! People have discovered much richer sources of helium underground. Some radioactive elements, such as uranium, release alpha particles when they decay. An alpha particle is just a helium atom with no electrons. Deep in the Earth where these radioactive decays take place, the alpha particles capture electrons and become helium. As the radioactive deposits age, large quantities of helium become trapped in underground caverns. To collect the helium, people drill down into the caverns and capture the helium as it escapes. \n\nMost people are familiar with helium balloons. We put helium in these balloons, because it is lighter than air and is non-flammable. However, most of the helium used today is in liquid form. All gases become liquids when the temperature becomes low enough, or when excessive pressure is used. Scientists use liquid helium for experiments that have to be kept very very cold. Helium becomes a liquid at 270oC below zero!
219472
Because cold air has a tendency to sink !!!