Why does heated gas become lighter

Here the mass of the hot air is given by the density of the hot air times the volume of the hot air. Here the mass of the air displace is given by the density of the surrounding cool air times the volume of the balloon hot air. In summary, hot air rises because it is less dense than the surrounding air. Hot air pushes out on the air around it, and thus becomes less dense.

Less dense air rises because the denser, cooler air sinks because it is heavier and gravity pulls on it more strongly. Most movement of particles such as air happens due to diffusion. You may choose to show the animation Heating Molecules of a Gas if you would like to give students a hint. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 1. Engage Discuss with students whether they think gas is matter.

Ask students about gases: Are gases, like the gases in air, matter? Students may have questions about whether or not gases are matter. They may also have only a very vague sense of what gases are at all. After students reply, explain that the air around them is made up of some different gases—nitrogen, oxygen, carbon dioxide, water vapor, and very small amounts of some others.

Tell students that gases are made of molecules but that the molecules are much further apart than the molecules in liquids or solids. Since the molecules of a gas have mass and take up space, gas is matter. Do a demonstration to show that gas has mass.

Materials for the demonstration Basketball, very deflated Balance that measures in grams Pump Can of compressed gas available at any office supply store Procedure Basketball Place the deflated ball on the balance to get the initial mass. Ask students if they think the ball will weigh more or less after you pump air into it.

Pump as much air into the basketball as you can and then put it back on the balance. Can of compressed gas Place a can of compressed gas on a scale and check its mass. Shoot gas out of the can for a few seconds and then place the can back on the scale. Expected results The basketball should weigh 2—4 grams more than when it was deflated. Show an animation of the molecules of a gas. Give each student an activity sheet. Explore Have students do an activity to find out how heating and cooling affect gases.

Question to investigate How does heating and cooling affect a gas? While holding the bottle, slowly push the bottom of the bottle down into the hot water. Ask students: What can you do to make the bubble go down? If students have trouble thinking of an answer, remind them that heating the gas increased the speed of the molecules, which made the bubble grow. Students should suggest that they should cool the gas in the bottle.

This can be done by putting the base of the bottle into cold water. If there is still a bubble on the bottle, slowly push the bottom of the bottle down into the cold water. If a bubble is not still on the bottle, make another bubble by dipping the opening into detergent and then pushing the bottom of the bottle into hot water again.

While holding the bottle, slowly push the bottom of the bottle down into the cold water. Expected Results When the bottle is placed in hot water, a bubble forms at the top of the bottle. Record and discuss student observations.

What happened to the film of detergent solution when you placed the bottle in hot water? It formed a bubble. Set up two pairs of flasks each connected by a valve see diagrams below. Both pairs have brown nitrogen dioxide in the left hand side flask. The first pair also has air in the right hand side flask. Students are asked to predict what will happen when the valve between the two flasks is opened.

The brown colour will spread very slowly from one flask to the other because the particles have frequent collisions with the air particles. The second pair of flasks has brown gas in the left hand side flask but the right hand side flask is completely evacuated.

Students are asked again to predict what happens when the valve is opened. The very fast speed of the molecules means that they fill the evacuated flask very quickly. Diffusion experiments can reinforce the idea of movement of particles. These can also be used as POEs. Brownian motion can also be observed using stereo microscopes when sulphur powder or camphor is sprinkled on the surface of water or ethanol. A cotton wool piece soaked in ammonia is placed at one end of a long glass tube with another soaked in hydrochloric acid HCl placed at the other end.

Eventually a white ring will form where the two gases meet. The two gases are at the same temperature and thus the particles have the same kinetic energy; the ring forms closer to the source of heavier and thus slower moving HCl. This is predicted by a comparison of the relative molecular masses. Including a strip of universal indicator paper in the tube allows the gas diffusion to be tracked. Students need to be given the opportunity to use the scientific conceptions about particle theory in other settings.

Our website uses a free tool to translate into other languages. This tool is a guide and may not be accurate. For more, see: Information in your language. Convection currents can be seen in lava lamps.

The wax inside the lamp warms up, becomes less dense than the liquid and so rises. When it rises, it cools and becomes denser again, so it sinks.

This same effect can be seen by putting a crystal of potassium permanganate in a beaker of water and gently heating it. Convection explains why hot air balloons rise, and also why it is often hotter in the lofts of houses than downstairs.