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Chemistry Review: Physical Change in Solids

Curious Crystals

Two small piles of similar looking white crystals
  • Some substances like salt and sugar look very similar but are actually very different.
  • Why do solids that look similar behave differently?

What are physical properties?

  • Some substances look the same, but behave differently. To begin to understand these differences, let's look at the physical properties of substances.
  • Physical properties of a solid are characteristics which can be observed or measured, such as shape, size, color, and texture.
  • Some physical properties are harder to measure, like density and hardness.

Physical Change

A large glacier representing water in the solid phase
  • Sometimes, physical properties can change. A physical change is something that alters the form or appearance of a material without changing the chemical composition.
  • For example: When water (H2O) changes from solid ice to liquid water, that is a physical change. The chemical makeup (hydrogen and oxygen) is not altered.
  • When salt and sugar dissolve in water it is a physical change. The chemical makeup of the salt and sugar are not changed.

Why do salt and sugar dissolve differently?

A sodium and chloride ion, shown with their corresponding positive and negative charges A three-dimensional diagram of a salt crystal
  • To understand why salt dissolves, we need to look at salt and water on the molecular level.
  • Salt (NaCl) is made from a positive sodium ion (Na+) and a negative chloride ion (Cl-).
  • Since positive and negative attract, the sodium ion and the chloride ion form an ionic bond, which results in NaCl.
  • Many sodium and chloride ions attract each other and form ionic bonds to make a salt crystal.

The ionic bonds in salt

This animation depicts the structure of a salt crystal. The ions bond together in a repeating three-dimensional pattern.

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  • An ionic bond forms from the attraction between two oppositely charged ions.
  • When sodium and chloride ions bond to form salt, they make a repeating, three-dimensional pattern.
  • With salt, each sodium ion (gray sphere) is surrounded by six chloride ions (green spheres) and each chloride ion is surrounded by six sodium ions. This is why salt looks like a cube.

Water on the molecular level

A 'ball and stick' model of a water molecule, showing how the central oxygen atom is bonded to two hydrogen atoms
  • A water molecule (H2O), is made up of two hydrogen atoms (gray) bonded to one oxygen atom (red).
  • The oxygen in the water molecule has a greater attraction for electrons than the hydrogen has. Therefore, electrons spend more time near the oxygen than the hydrogens.
  • Because electrons have a negative charge, this makes the area around the oxygen slightly negatively charged and the area around the hydrogen slightly positively charged.

Dissolving salt

This animation depicts the process of dissolving salt at the molecular level. Because salt is composed of ions which have positive and negative charges, the small areas of positive and negative charge on water are attracted to the salt ions. Eventually, when enough water molecules surround a salt ion, the ion becomes more attracted to the water than it is to other salt ions molecules. The water molecules pry the salt ions away from their corresponding salt ions one by one. This is dissolving.

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  • When salt crystals are placed in water, the positive ends of the water molecules attract the negative chloride ions.
  • The negative ends of the water molecules attract the positive sodium ions.
  • When attraction between the water and the ions overcomes the attraction the ions have for each other, the salt dissolves. This video appears courtesy of Roy Tasker.
    © Roy Tasker, VisChem Project.

Salt dissolved in water

This animation shows how salt ions associate with water after a salt crystal has been dissolved. The ions, having negative and positive charges, associate with corresponding areas of positive and negative charge in water molecules.

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  • Once the salt crystal is dissolved in water, the positively charged sodium ions and the negatively charged chloride ions are associated with the oppositely charged polar ends of many water molecules. This video appears courtesy of Roy Tasker.
    © Roy Tasker, VisChem Project.

Sugar on the molecular level

A sodium and chloride ion, shown with their corresponding positive and negative charges
  • Once the salt crystal is dissolved in water, the positively charged sodium ions and the negatively charged chloride ions are associated with the oppositely charged polar ends of many water molecules.

Sugar dissolves in water

This animation depicts the process of dissolving sugar at the molecular level. Because sugar is polar (having areas of positive and negative charge) and water is also polar, water is attracted to sugar molecules. Eventually, when enough water molecules surround a sugar molecule, a sugar molecule becomes more attracted to the water than it is to other sugar molecules. The water molecules pry sugar molecules away from one another one at a time. This is dissolving.

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  • Because sucrose has positive and negative areas and water has positive and negative areas, water molecules are attracted to the oppositely charged area on the sucrose molecules.
  • When the attraction that water molecules have for sucrose molecules overcomes the attraction that sucrose molecules have for each other, the water separates the sucrose molecules from each other, and they dissolve.

Solubility is a characteristic property

  • The amount of attraction water molecules have for particles of a solid compared to the attraction the solid particles have for each other determines the solubility of the solid. Therefore, solubility is a characteristic property of a solid.
  • Water molecules interact differently with the ions in salt than they do with the molecules in sugar
  • That's why salt and sugar and other solids will dissolve in water differently.

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.