Chemistry » Physical and Chemical Change » Introduction to Physical and Chemical Change

Physical Changes in Matter

Physical and chemical change: Introduction

Matter is all around us. The desks we sit at, the air we breathe and the water we drink are all examples of matter. But matter doesn’t always stay the same. It can change in many different ways. In this section, we are going to take a closer look at physical and chemical changes that occur in matter.

Physical changes in matter

A physical change is one where the particles of the substances that are involved in the change are not broken up in any way. When water is heated for example, the temperature and energy of the water molecules increases and the liquid water evaporates to form water vapour. When this happens, some kind of change has taken place, but the molecular structure of the water has not changed. This is an example of a physical change. All changes in state are physical changes.

\[\text{H}_{2}\text{O} \text{(l)} \rightarrow \text{H}_{2}\text{O (g)}\]

Conduction (the transfer of energy through a material) is another example of a physical change. As energy is transferred from one material to another, the energy of each material is changed, but not its chemical makeup. Dissolving one substance in another is also a physical change.

Definition: Physical change

A change that can be seen or felt, but that doesn’t involve the break up of the particles in the reaction. During a physical change, the form of matter may change, but not its identity.

There are some important things to remember about physical changes in matter:

  1. Arrangement of particles When a physical change occurs, the compounds may re-arrange themselves, but the bonds in between the atoms will not break. For example when liquid water boils, the molecules will move apart but the molecule will stay intact. In other words water will not break up into hydrogen and oxygen atoms.

    The figure below shows this phase change. Note that the water molecules themselves stay the same, but their arrangement changed.

    6f57d79b610dcbf3a4823d7d6fb56668.png

    The arrangement of water molecules in the liquid and gas phase

    Did You Know?

    The bonding of hydrogen and oxygen to form water is explosive and if the water molecule broke apart every time water boiled, life on Earth would not exist for very long!

  2. Conservation of mass

    In a physical change, the total mass, the number of atoms and the number of molecules will always stay the same. In other words you will always have the same number of molecules or atoms at the end of the change as you had at the beginning.

  3. Energy changes

    Energy changes may take place when there is a physical change in matter, but these energy changes are normally smaller than the energy changes that take place during a chemical change.

  4. Reversibility

    Physical changes in matter are usually easier to reverse than chemical changes. Methods such as filtration and distillation can be used to reverse the change. Changing the temperature is another way to reverse a physical change. For example, a mixture of salt dissolved in water can be separated by filtration, ice can be changed to liquid water and back again by changing the temperature.

Optional Activity: Physical change

Use plastic pellets or marbles to represent water in the solid state. What do you need to do to the pellets to represent the change from solid to liquid?

Make a mixture of sand and water. Filter this mixture. What do you observe?

Make a mixture of iron filings and sulfur. Can you separate the mixture with a magnet?

Optional Experiment: The synthesis of iron sulfide

Aim

To demonstrate the synthesis of iron sulfide from iron and sulfur.

Apparatus

\(\text{5.6}\) \(\text{g}\) iron filings and \(\text{3.2}\) \(\text{g}\) powdered sulfur; porcelain dish; test tube; Bunsen burner

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Method

  1. Measure the quantity of iron and sulfur that you need and mix them in a porcelain dish.

  2. Take some of this mixture and place it in the test tube. The test tube should be about one third full.

  3. Heat the test tube containing the mixture over the Bunsen burner. Increase the heat if no reaction takes place. Once the reaction begins, you will need to remove the test tube from the flame. Record your observations.

  4. Wait for the product to cool before breaking the test tube with a hammer. Make sure that the test tube is rolled in paper before you do this, otherwise the glass will shatter everywhere and you may be hurt.

  5. What does the product look like? Does it look anything like the original reactants? Does it have any of the properties of the reactants (e.g. the magnetism of iron)?

Warning:

When working with a Bunsen burner, work in a well ventilated space and ensure that there are no flammable substances close by. Always tuck loose clothing in and ensure that long hair is tied back.

Results

After you removed the test tube from the flame, the mixture glowed a bright red colour. The reaction is exothermic and produces heats. The product, iron sulfide, is a dark colour and does not share any of the properties of the original reactants. It is an entirely new product.

Conclusions

A synthesis reaction has taken place. The equation for the reaction is:

\[\text{Fe (s)} + \text{S (s)} \rightarrow \text{FeS (s)}\]

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