How does molecules move in fluids

Oxygen and nitrogen are the major components of air and occur in nature as diatomic two atom molecules. Regardless of the type of molecule, matter normally exists as either a solid, a liquid, or a gas.

We call this property of matter the phase of the matter. The three normal phases of matter have unique characteristics which are listed on the slide. In the solid phase the molecules are closely bound to one another by molecular forces. A solid holds its shape and the volume of a solid is fixed by the shape of the solid.

In the liquid phase the molecular forces are weaker than in a solid. A liquid will take the shape of its container with a free surface in a gravitational field. In microgravity, a liquid forms a ball inside a free surface. Regardless of gravity, a liquid has a fixed volume. In the gas phase the molecular forces are very weak.

A gas fills its container, taking both the shape and the volume of the container. Give students time after the activity to record their observations by answering the following questions on their activity sheet. Once they have answered the questions, discuss their observations as a whole group.

The yellow and blue food coloring will spread faster in hot water than in cold. The colors will combine and turn green in the hot water while the colors will remain separate longer in the cold water. Students should agree that the food coloring mixes faster in the hot water because the molecules in hot water move faster than they do in cold water.

Show the molecular model animation Heating Water. Move the slider at the bottom of the window all the way to the right to show that the water molecules are moving faster and are a little farther apart in hot water. Explain that the little balls represent the particles of a liquid, in this case water molecules.

Let students know that for now, they will use circles or spheres to represent atoms and molecules, but eventually they will use a more detailed model. For now, students should focus on the motion of the molecules, how they interact, and their distance from one another. Have students fill in the blank with the word increases or decreases on their activity sheet as you read each sentence. Project the image Water Molecules at Different Temperatures.

Have students refer to the drawing of room temperature water on their activity sheet and discuss how they should represent the molecules in cold and hot water.

Students should realize that since the molecules in the cold water are moving slower, they should have fewer motion lines than the molecules in room temperature water.

The slower motion also allows the attractions to bring the molecules a little closer together than in room temperature water, so the circles should be drawn a little closer together. Have students read and discuss the Take It Further question on the activity sheet. After the class discussion, have students write their own response to the following question in the space provided on the activity sheet. The American Chemical Society is dedicated to improving lives through Chemistry.

Skip Navigation. Lesson 1. Engage Ask students to help you design an experiment to see if the speed of water molecules is different in hot water compared to cold water. Ask students questions such as the following: Is the speed of water molecules different in hot and cold water?

What can we do to find out? Ask students: Should we use the same amount of hot and cold water in our experiment?

Should we use the same type of cup for the hot and cold water? The presence of such a charge on each of these atoms gives a water molecule a net dipole moment. The electrical attraction between water molecules caused by this dipole pulls individual molecules closer together, making it more difficult to separate the molecules, and therefore raising the boiling point.

This type of attraction is known as hydrogen bonding. The molecules of water are constantly moving in relation to each other, and the hydrogen bonds are continually breaking and reforming at intervals briefer than femtoseconds x 10 seconds. Many of the physical and chemical properties of water including its capacity as a solvent are partly to the acid-base reactions it can be part of. The gaseous phase of water is known as water vapor or steam and is characterized by a transparent cloud.

Water also exists in a rare fourth state called supercritical fluid, which occurs only in extremely uninhabitable conditions. Water freezes to form ice, ice thaws to form liquid water, and both water and ice can transform into the vapor state. Phase diagrams help describe how water changes states depending on the pressure and temperature.

Phase diagram of water : The three phases of water — liquid, solid, and vapor — are shown in temperature-pressure space. The polar nature of water is a particularly important feature that contributes to the uniqueness of this substance.

The water molecule forms an angle with an oxygen atom at the vertex and hydrogen atoms at the tips. Because oxygen has a higher electronegativity than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge.

The oxygen end is partially negative, and the hydrogen end is partially positive; because of this, the direction of the dipole moment points from the oxygen toward the center position between the two hydrogens. This charge difference causes water molecules to be attracted to each other the relatively positive areas are attracted to the relatively negative areas , as well as to other polar molecules.

Polarity of the water molecule : Owing to the electronegativity difference between hydrogen H and oxygen O atoms, and the bent shape of the H 2 O molecule, a net dipole moment exists.

The figure indicates the partial charges that the atoms possess. A water molecule can form a maximum of four hydrogen bonds by accepting two hydrogen atoms and donating two hydrogen atoms.

One such property is its relatively high melting and boiling points; more energy is required to break the hydrogen bonds between molecules in order to change to a higher energy phase. Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force. Liquids and solids share a common attribute: a clear and discernible phase boundary that gives the sample a simple but definite shape.

Liquids and solids share also something else: most of their molecular units are to some extent in relatively close contact. However, liquids, like gases, are fluids, meaning that their molecular units can move more or less independently of each other. Whereas the volume of a gas depends entirely on the pressure, the volume of a liquid is largely independent of the atmospheric pressure. Therefore, gases are compressible while liquids are very nearly not. The molecules in a sample of a liquid that find themselves fully in the interior volume are surrounded by other molecules and interact with them based on the attractive intermolecular forces that are present for molecules of this type.

However, the molecules at the interface with another medium usually air do not have other like molecules on all of their sides namely, on top of them , so they cohere more strongly to the molecules on the surface and immediately below them.

The result is a surface film which makes it more difficult for an object to pierce through the surface than for it to move once submerged in the sample of liquid. Therefore, the cohesive forces result in the phenomenon of surface tension. Surface tension is responsible for the shape of a liquid droplet. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer.

In the absence of other forces, including gravity, drops of virtually all liquids would be perfectly spherical.

If no force acts normal perpendicular to a tensioned surface, the surface must remain flat. But if the pressure on one side of the surface differs from pressure on the other side, the pressure difference times the surface area results in a normal force. In order for the surface tension forces to cancel out this force due to pressure, the surface must be curved.

When all the forces are balanced, the curvature of the surface is a good measure of the surface tension, which is described by the Young-Laplace equation:. Even though the needle is denser than water, it floats because surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force.

This property is caused by cohesion of similar molecules and is responsible for many of the behaviors of liquids. Note that the forces from the surface tension are symmetrical. In imagining the shape of a liquid droplet or the curvature of the surface of a liquid, one must keep in mind that the molecules at the surface are at a different level of potential energy than are those of the interior.

A gas will change volume to fit the volume of the container. In general, solids are denser than liquids, which are denser than gases. The particles in the solid are touching with very little space between them. The particles in a liquid usually are still touching but there are some spaces between them.

The gas particles have big distances between them.