Inventive tasks. Dependence of saturated vapor pressure on temperature

>>Physics: Dependence of saturated vapor pressure on temperature. Boiling

The liquid not only evaporates. At a certain temperature it boils.
Dependence of saturated vapor pressure on temperature. The state of saturated steam, as experience shows (we talked about this in the previous paragraph), is approximately described by the equation of state of an ideal gas (10.4), and its pressure is determined by the formula

As temperature increases, pressure increases. Because Saturated vapor pressure does not depend on volume, therefore it depends only on temperature.
However, dependence r n.p. from T, found experimentally, is not directly proportional, like that of an ideal gas at constant volume. With increasing temperature, the pressure of real saturated vapor increases faster than the pressure of an ideal gas ( Fig.11.1, part of the curve AB). This becomes obvious if we draw isochores of an ideal gas through the points A And IN(dashed lines). Why is this happening?

When a liquid is heated in a closed container, some of the liquid turns into steam. As a result, according to formula (11.1) saturated vapor pressure increases not only due to an increase in the temperature of the liquid, but also due to an increase in the concentration of molecules (density) of the vapor. Basically, the increase in pressure with increasing temperature is determined precisely by the increase in concentration. The main difference in the behavior of an ideal gas and saturated steam is that when the temperature of the steam in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the steam changes. The liquid partially turns into vapor, or, on the contrary, the vapor partially condenses. Nothing like this happens with an ideal gas.
When all the liquid has evaporated, the vapor will cease to be saturated upon further heating and its pressure at a constant volume will increase in direct proportion to the absolute temperature (see. Fig.11.1, part of the curve Sun).
. As the temperature of the liquid increases, the rate of evaporation increases. Finally, the liquid begins to boil. When boiling, rapidly growing vapor bubbles are formed throughout the entire volume of the liquid, which float to the surface. The boiling point of the liquid remains constant. This happens because all the energy supplied to the liquid is spent converting it into vapor. Under what conditions does boiling begin?
A liquid always contains dissolved gases, released at the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the saturated vapor pressure increases and the bubbles increase in size. Under the influence of buoyant force they float upward. If the upper layers of the liquid have a lower temperature, then vapor condensation occurs in bubbles in these layers. The pressure drops rapidly and the bubbles collapse. The collapse occurs so quickly that the walls of the bubble collide and produce something like an explosion. Many such micro-explosions create a characteristic noise. When the liquid warms up enough, the bubbles will stop collapsing and float to the surface. The liquid will boil. Watch the kettle on the stove carefully. You will find that it almost stops making noise before it boils.
The dependence of saturated vapor pressure on temperature explains why the boiling point of a liquid depends on the pressure on its surface. A vapor bubble can grow when the pressure of the saturated vapor inside it slightly exceeds the pressure in the liquid, which is the sum of the air pressure on the surface of the liquid (external pressure) and the hydrostatic pressure of the liquid column.
Let us pay attention to the fact that the evaporation of a liquid occurs at temperatures below the boiling point, and only from the surface of the liquid; during boiling, vapor formation occurs throughout the entire volume of the liquid.
Boiling begins at the temperature at which the saturated vapor pressure in the bubbles is equal to the pressure in the liquid.
The greater the external pressure, the higher the boiling point. Thus, in a steam boiler at a pressure reaching 1.6 10 6 Pa, water does not boil even at a temperature of 200 ° C. In medical institutions in hermetically sealed vessels - autoclaves ( Fig.11.2) boiling of water also occurs at elevated pressure. Therefore, the boiling point of the liquid is much higher than 100°C. Autoclaves are used to sterilize surgical instruments, etc.

And vice versa, by reducing external pressure, we thereby lower the boiling point. By pumping air and water vapor out of the flask, you can make the water boil at room temperature ( Fig.11.3). As you climb mountains, the atmospheric pressure decreases, therefore the boiling point decreases. At an altitude of 7134 m (Lenin Peak in the Pamirs) the pressure is approximately 4 10 4 Pa ​​(300 mm Hg). Water boils there at about 70°C. It is impossible to cook meat under these conditions.

Each liquid has its own boiling point, which depends on its saturated vapor pressure. The higher the saturated vapor pressure, the lower the boiling point of the liquid, since at lower temperatures the saturated vapor pressure becomes equal to atmospheric pressure. For example, at a boiling point of 100°C, the saturated vapor pressure of water is 101,325 Pa (760 mm Hg), and the pressure of mercury vapor is only 117 Pa (0.88 mm Hg). Mercury boils at a temperature of 357°C at normal pressure.
A liquid boils when its saturated vapor pressure becomes equal to the pressure inside the liquid.

???
1. Why does the boiling point increase with increasing pressure?
2. Why is it important for boiling to increase the pressure of saturated vapor in the bubbles, and not to increase the pressure of the air in them?
3. How to make a liquid boil while cooling the vessel? (This question is not easy.)

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics 10th grade

If you have corrections or suggestions for this lesson,

Without any doubt, physics is one of the most interesting sciences. Even the most useless experiments can turn out to be quite exciting at the same time. For example, the boiling of a liquid when it is cooled on one side seems incredible. After all, in order liquid Once it boils, it needs to be heated, but not cooled, as we used to think. But anything is possible. For such an experiment you do not need any special liquid, ordinary water will do, you just need to create special conditions.

You will need

• Flask, water, gas burner, tripod.

Instructions

Pour ordinary tap water into the flask and fill it to about half level. After this, place the flask on a gas burner and heat the water until it boils.

When the water in the flask boils, turn off the heat and wait until the boil stops. Seal the flask tightly with a rubber stopper and fix it in the tripod holder, turning it upside down.

Next, start pouring cold water over the bottom of the flask. The better to cool vessel, the more vivid the experience will be. Bubbles will rise to the surface of the water, and the water in the flask will boil as it cools. This can be explained by the fact that the water vapor located inside vessel and upon cooling they begin to condense on the walls of the flask. Because of this, the water vapor pressure inside the flask begins to decrease. At reduced pressure, water begins to boil not at one hundred degrees Celsius, but at a lower temperature. Since the water has not yet completely cooled down, and the pressure is vessel e fell, which is why boiling occurs when cooling.

note

It is best to use a heat-resistant glass flask for this experiment. When cooling a hot vessel with cold water, ordinary glass may crack due to a sharp temperature change and the experiment will not take place.

Since all work must have meaning, children's interest can be stimulated by assessment. If the problem was assigned at home, then for five different solutions to the problem - a score of “5”, for every two additional solutions – another score of “5”. If the task was given in class, the most active students were graded.

– Cover the container with a lid 2 . Thereby increasing the pressure, and therefore the boiling point of water in it.

– Salt the water in the vessel 2 – the boiling point will also increase.

– Boil water in a vessel 2 several times, letting it cool between boils. In this way, we will remove impurities from the water (they will precipitate), and therefore the centers of vaporization, and therefore, we will increase the boiling point of water.

– Place at the bottom of the vessel 2 ultrasonic generator.

– Place a copper rod in the water so that it rests on the bottoms of both vessels. In this case, we get a heat conductor.

– Wait until the water is in the container 2 will boil away.

– Pour into a container 2 liquid boiling at temperatures above 100 °C.

– Heat the magnet. Thus, we misorient the domains in the magnet.

- Heat the nail. Thus, we will disorient the domains in
nail.

– Use a lever made of wood or non-magnetic metal.

– Wrap the wire around the nail and pass the current. Thus turning the nail into a magnet of the same polarity as the ends of the horseshoe.

- Pull the nail sharply.

– Place the iron rod above the nail. Thus, we will “short-circuit” the lines of magnetic induction and weaken the magnetic field at the ends of the magnet.

– Tap on the magnet. Deformation will lead to disruption of the ordered orientation of domains in the magnet.

– Pump air into the flask. As you know, an arch is easy to break if you press from the inside.

– Place the entire system under the bell, plugging the tube, and pump out the air from the bell. Thus, we will create excess pressure in the flask, and it will burst, as in the previous case.

– Pour water into the flask and freeze. The flask will burst because Water expands when cooled.

– Heat the flask unevenly. Cool half of the flask and heat the other half. The bulb will crack due to the difference in thermal expansion.

– Direct a sound wave at the flask. The sound will cause the walls of the flask to vibrate, and at resonance the flask will burst.

– Place a few more bricks on top.

- Hit the brick.

– Wait until the water evaporates.

– Heat the mug to speed up the evaporation.

– Dip a spoon into a mug and freeze. Then take out the spoon along with the ice.

Note. Immediately after freezing, it is impossible to get a spoon, so the edges of the mug will need to be heated a little.

- Place a sponge in the glass.

– Place a cocktail straw into the mug and suck out the water.

– Place the end of a long rubber tube into the mug, lower its other end below the surface level of the liquid and suck out the air from the tube - the water will flow out.

– Place a tube into the mug, the other end of which is placed in a vessel with low pressure. Atmospheric pressure will force the water into another container.

– Move the glasses relative to each other, turning one relative to the other.

- Wait a while. The system is not completely sealed, and air still penetrates through the gasket.

– Raise the temperature of the glasses, for example, pour boiling water over them. The gas pressure in the glasses will increase.

– Place the system under the bell and evacuate the air. The pressure inside the glasses will be greater than outside.

• How to divide an ice cube into two equal parts?

- Sawing.

– Grind into crumbs and divide.

– Cut with a hot knife.

- Heat half of it.

– Melt the cube, divide the water in half, freeze the resulting halves.

- Put up a support.

– Nail the legs to the floor.

– Experimentally select the angle of inclination so that equilibrium is established. Trim the chair legs at this angle to increase the support area.

– Cut recesses in the floor at an angle and insert chair legs into them.

- Glue the chair.

• How to make a mathematical mat swing in only one plane?

– Spin the load around its axis. As a result, we will get a gyroscope, and, as is known, the plane of rotation of a gyroscope does not change its position in space.

– Make an iron weight swing in a magnetic field.

– Build a guide structure (two plates).

– Make a metal weight swing in a static electric field (for example, between two charged balls).

– Fine tune at start-up.

• How to cool bottled water?

– Put the bottle in the refrigerator.

- Put (on) under ice.

– Wrap the bottle in a damp cloth and place it in the air stream. As water evaporates from the surface of the rag, the latter will cool, taking heat from the water bottle.

– Wrap the bottle in a damp rag, place it under the bell and pump out the air. We will thereby lower the pressure, therefore speeding up the evaporation.

– Place the bottle in a container with colder water, such as ice.

– Chemically cool.

• How to connect two metal plates?

– Use a bolt and nut.

– Use rivets.

- Glue it together.

- Solder.

- Boil. (Not all metals can be welded. – Ed.)

– Use electric spot welding.

– Clean and sand both surfaces to be joined and press them tightly. (This is how cold welding is done in space. – Ed.)

• How to heat up a metal ball?

- Put it in the oven.

- Hit.

- Rub for a long time.

– Deform.

– Pass electric current.

• How to speed up the drying of a damp rag?

– Hang on a rope in a dry, warm room.

- Expand it as much as possible.

– Place in a stream of dry air.

– Place between dry rags (newspapers) and change them periodically.

– Sprinkle a rag with dry sand (sawdust), shake it off periodically and sprinkle it again with a new portion of sand. Sand absorbs moisture.

– Place it near a powerful source of high-frequency electromagnetic radiation. As a result of the action of Foucault currents, the liquid will heat up.

• How to remove excess stone?

– Chip off using a chisel and hammer.

- Erase.

– Heat the stone and cool it sharply. As a result of a sharp temperature change due to thermal expansion, the stone will crack.

– Cool and heat up quickly.

- Saw off.

- Melt.

• How to fill a bucket faster in the rain?

– Place a bucket under the drain near the roof of the house. There, water collects from a large surface of the roof.

– Place the funnel over the bucket.

– Lower a piece of fabric into a bucket with one end and hang the other end. Water will flow from the fabric into the bucket (the area from which water collects will increase).

– Place the bucket at an angle of 45° to the direction in which the drops fall. (It will be worse. – Red.)

– Place several charges of the same name in the center of the bucket. As a result, the trajectory of the droplets will change.

• How to raise the water level in the elbow of a U-shaped tube relative to the other?

– Pump out the air from one elbow and close this elbow with a plug.

– Inflate air into one elbow and close this elbow with a plug.

– Pour a lighter liquid (for example kerosene) into one elbow.

– Place a partition (piston) between the knees and move it, for example, on a thread.

– Use the phenomenon of osmosis.

• How to make a car rolling down a hump travel a greater distance by inertia?

- Push.

- Load the car.

– Lubricate the rails with oil, thereby reducing the coefficient of friction.

– Cool the rails. There is always water vapor in the atmosphere; condensation will appear on the cooled rails, which will reduce friction.

• How to ensure the presence of water molecules at a height of 1 cm above the surface of the water in a vessel?

– Place the wick in the water. Water molecules will rise through the capillaries.

– Throw ice into water: it floats in water, therefore, you can pick up a piece that will rise 1 cm above the surface, and ice is also water.

- Lower the sponge. The water, as in the case of the wick, will rise.

- Heat the water.

- Nothing to do. Water evaporates at any temperature, therefore, above the surface, at any or almost any height, there is at least one molecule of H 2 O.

• How to illuminate a small space?

– Light a match (candle, torch).

- Light it with a flashlight.

– Light up the electric discharge.

– Excite luminescence.

– Excite Cherenkov glow (glow of water when particles pass through it at a speed higher than the speed of light in water).

• How to speed up the boiling of liquid in a kettle?

– Increase heater power.

– Pour into the kettle not water, but a more easily boiling liquid (for example, acetone).

– Insulate the kettle, for example, by wrapping it in thick cloth and a cotton blanket.

– Place the kettle in a low-pressure zone.

– Constantly knock on the kettle, thereby stirring the water.

• How to stop the movement of a spring watch without damaging its outer shell?

– Do not touch the watch for a long time - it will stop on its own.

- Shake it hard, drop it, hit it.

– Dip into liquid and freeze.

– Place in liquid nitrogen.

– Place in an alternating magnetic field.

- Heat it up.

• How to increase the glide of boots on ice?

Answer options

– Completely erase the tread on the sole.

– Make the ice even and smooth.

– Make the surface of the ice wet.

– Pour oil over the ice.

– Attach runners to the boots (to make skates).

________________________

A 4th year student at Vyat GSU, he gave this material in 2005 during lessons and extracurricular activities, while undergoing teaching practice at school No. 5 (Slobodskoy, Kirov region, head - Honored Teacher of the Russian Federation Victor Ivanovich Elkin[email protected]). The students really liked the problems and solved them with pleasure.

Without any doubt, physics is one of the most interesting sciences. Even the most useless experiments can turn out to be quite exciting at the same time. For example, the boiling of a liquid when it is cooled on one side seems incredible. After all, in order liquid Once it boils, it needs to be heated, but not cooled, as we used to think. But anything is possible. For such an experiment you do not need any special liquid, ordinary water will do, you just need to create special conditions.

You will need

• Flask, water, gas burner, tripod.

Instructions

Pour ordinary tap water into the flask and fill it to about half level. After this, place the flask on a gas burner and heat the water until it boils.

When the water in the flask boils, turn off the heat and wait until the boil stops. Seal the flask tightly with a rubber stopper and fix it in the tripod holder, turning it upside down.

Next, start pouring cold water over the bottom of the flask. The better to cool vessel, the more vivid the experience will be. Bubbles will rise to the surface of the water, and the water in the flask will boil as it cools. This can be explained by the fact that the water vapor located inside vessel and upon cooling they begin to condense on the walls of the flask. Because of this, the water vapor pressure inside the flask begins to decrease. At reduced pressure, water begins to boil not at one hundred degrees Celsius, but at a lower temperature. Since the water has not yet completely cooled down, and the pressure is vessel e fell, which is why boiling occurs when cooling.

note

It is best to use a heat-resistant glass flask for this experiment. When cooling a hot vessel with cold water, ordinary glass may crack due to a sharp temperature change and the experiment will not take place.

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