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Chemistry Review: Physical Change in Liquids

Look-alike liquids

Four plastic cups containg clear, colorless liquids
  • There are many different liquids that are clear and colorless such as water, alcohol, mineral oil, and vinegar. Even though they look very similar, they can act differently.
  • Why do liquids that look so similar act differently?

Physical Properties and Physical Change

A large glacier representing water in the solid phase
  • Liquids, just like solids and gases, have different physical properties.
  • Physical properties of a liquid are characteristics that can be observed or measured such as color, density, and surface tension.
  • Sometimes, the physical properties of a liquid can change like when the liquid dissolves a solid or mixes in with another liquid.

How liquids act on different surfaces

A sodium and chloride ion, shown with their corresponding positive and negative charges

Water beads up, while alcohol spreads out on a plastic bag.

  • Even though liquids may look similar, they may act differently when placed on a surface. Different liquids may spread out, sink in, or “bead” up.
  • The way water, alcohol, or any liquid acts on a surface depends on the characteristics of the liquid and the surface, and how they interact.
  • This interaction depends on the attractions the molecules of the liquid have for each other and also for the molecules of the surface. Let's start with water.

Inside Water

This animation shows a single water molecule. The water molecule is represented in two ways, both as a space-filling model and as a ball-and-stick model. The molecule is rotated 360 ° about the y axis to demonstrate how the hydrogen atoms are bonded to the central oxygen atom.

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  • Water molecules (H20), are made up of two hydrogen atoms (gray) bonded to one oxygen atom (red).
  • Because of the way oxygen and hydrogen atoms are bonded together, the area near the oxygen has a slight negative charge and the areas near the hydrogens have a slight positive charge.

More about water

This animation shows a single water molecule. The water molecule is represented in two ways, both as a space-filling model and as a ball-and-stick model. The molecule is rotated 360 ° about the y axis to demonstrate how the hydrogen atoms are bonded to the central oxygen atom.

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  • The positive and negative areas of the water molecule cause it to be attracted to other water molecules.
  • Notice that the oxygen from one water molecule is attracted to the hydrogen from another and vice versa.
  • This allows water molecules to form associations with other water molecules in several directions at the same time.

Surface Tension

A 'ball and stick' model of a water molecule, showing how the central oxygen atom is bonded to two hydrogen atoms
  • One reason why water and alcohol look different on a surface has to do with surface tension.
  • In a sample of water, the attractions for the water molecules at the surface are different than the attractions for the water molecules below the surface.
  • The water molecules at the surface have no water molecules anywhere above them. Therefore, they are attracted downward instead of in all different directions.
  • These surface molecules form attractions that are more stable and cause water to form a type of “skin”. This phenomenon is called surface tension.

Now, let's look at alcohol

A 'ball and stick' model of a water molecule, showing how the central oxygen atom is bonded to two hydrogen atoms

Isopropyl Alcohol. In this space filling model, red represents oxygen, gray represents hydrogen, and black represents carbon.

A 'ball and stick' model of a water molecule, showing how the central oxygen atom is bonded to two hydrogen atoms

Isopropyl Alcohol in water.

  • Isopropyl alcohol (or rubbing alcohol) that you buy in the store is a solution of isopropyl alcohol and water. Isopropyl alcohol, like all alcohols, has an oxygen atom bonded to a hydrogen atom.
  • This O—H bond in alcohol is similar to the O—H bonds in water. Like water, the oxygen has a slight negative charge and the hydrogen has a slight positive charge.
  • The carbon-hydrogen (C—H) part of the alcohol has only an extremely small amount of positive and negative charge.
  • Alcohol molecules interfere with the water molecules’ attraction for other water molecules. This disrupts the surface tension of the water and makes the surface tension of the alcohol not as strong as that of water.

Paper and wax paper

A cellulose molecule.

Cellulose

A glucose molecule.

Glucose

A 'ball and stick' model of a water molecule, showing how the central oxygen atom is bonded to two hydrogen atoms

C—H bonds in waxed paper

  • Now that we’ve looked at the liquids, let’s look at some different surfaces: Paper and waxed paper.
  • Paper is made from wood, which is made of cellulose. Cellulose is made of molecules of glucose held together by covalent bonds.
  • Glucose, like water, has oxygen atoms bonded to hydrogen atoms. These O—H bonds result in areas of positive and negative charge.
  • The wax on wax paper is made of carbon atoms bonded to hydrogen atoms. These C—H bonds have only a very small amount of positive and negative charge.

Alcohol and water on paper and wax paper

A 'ball and stick' model of a water molecule, showing how the central oxygen atom is bonded to two hydrogen atoms
  • On paper, the positive and negative areas on water and alcohol are attracted to the positive and negative areas of the cellulose. Both liquids absorb readily into paper.
  • On wax paper, the wax has very little positive and negative charge so the water is not very attracted to it. Water’s strong surface tension and its attraction to itself and not the wax causes the water to bead up on waxed paper.
  • The surface tension of the alcohol is not as strong as the water’s surface tension and causes the alcohol to spread out on the surface of the wax paper.

Liquids mixing in water

Results of mixing water with water Results of mixing water with saltwater Results of mixing water with alcohol. Results of mixing water with detergent.

Another way to distinguish between similar looking liquids is by seeing how they mix with a test liquid. Each liquid combines with water differently.

  • Water with water—Some area of green in middle but areas of yellow and blue remain.
  • Water with salt water—Quickly turns green throughout.
  • Water with alcohol—Slower mixing. Appears to “shake” where the water and alcohol meet.
  • Water with detergent—Turns green throughout but spreads out more than other mixtures.

Each liquid interacts with water in a characteristic way. The way the liquids mix is based on the interaction between water molecules and the molecules that make up each liquid.

Summary

  • Physical change is a change that alters the form or appearance of a material without changing the chemical composition. One example of a physical change is dissolving.
  • Salt is made of ions that are ionically bonded together in a crystal.
  • Sugar is made of molecules that are bonded together based on the positively and negatively charged areas.
  • The positive and negative areas of water molecules are attracted to the oppositely charged ions in salt and to the positive and negative areas on sugar molecules.
  • Because salt and sugar are made up of different atoms that bond together differently, water is attracted to them differently. Water interacts and dissolves salt and sugar in characteristic ways.
  • That’s why solids that may look similar behave differently.

Molecular Animation Downloads

The animations featured in the slideshow above are available for download by clicking on the "Download this animation" link below each file.

Each video is offered for download in both Quicktime Movie (.mov) and Windows Media Video (.wmv) format.

To play these videos, you will need either Quicktime or Windows Media Player.