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Physical and Chemical Change

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 chapter, 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.

H2O(l) H2O(g)H2O(l)H2O(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 1: 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. A change in temperature is an example of a physical change.

You can think of a physical change as a person who is standing still. When they start to move (start walking) then a change has occurred and this is similar to a physical change.

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

  1. Arrangement of particles
    When a physical change occurs, the particles (e.g. atoms, molecules) may re-arrange themselves without actually breaking up in any way. In the example of evaporation that we used earlier, the water molecules move further apart as their temperature (and therefore energy) increases. The same would be true if ice were to melt. In the solid phase, water molecules are packed close together in a very ordered way, but when the ice is heated, the molecules overcome the forces holding them together and they move apart. Once again, the particles have re-arranged themselves, but have not broken up.
    H2O(s) H2O(l)H2O(s) H2O(l)
    (1)
    Figure 1 shows this more clearly. In each phase of water, the water molecule itself stays the same, but the way the molecules are arranged has changed. Note that in the solid phase, we simply show the water molecules as spheres. This makes it easier to see how tightly packed the molecules are. In reality the water molecules would all look the same.
    Figure 1: The arrangement of water molecules in the three phases of matter
    Figure 1 (CG10C4_001.png)
  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. Water vapour for example, can be changed back to liquid water if the temperature is lowered. Liquid water can be changed into ice by simply decreasing the temperature.

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?

Chemical Changes in Matter

When a chemical change takes place, new substances are formed in a chemical reaction. These new products may have very different properties from the substances that were there at the start of the reaction.

The breakdown of copper (II) chloride to form copper and chlorine is an example of chemical change. A simplified diagram of this reaction is shown in Figure 2. In this reaction, the initial substance is copper (II) chloride, but once the reaction is complete, the products are copper and chlorine.

Figure 2: The decomposition of copper(II) chloride to form copper and chlorine. We write this as: CuCl2Cu+Cl2CuCl2Cu+Cl2
Figure 2 (CG10C4_002.png)
Definition 2: Chemical change
The formation of new substances in a chemical reaction. One type of matter is changed into something different.

There are some important things to remember about chemical changes:

  1. Arrangement of particles
    During a chemical change, the particles themselves are changed in some way. In the example of copper (II) chloride that was used earlier, the CuCl2CuCl2 molecules were split up into their component atoms. The number of particles will change because each CuCl2CuCl2 molecule breaks down into one copper atom (CuCu) and one chlorine molecule (CuCl2CuCl2). However, what you should have noticed, is that the number of atoms of each element stays the same, as does the total mass of the atoms. This will be discussed in more detail in a later section.
  2. Energy changes
    The energy changes that take place during a chemical reaction are much greater than those that take place during a physical change in matter. During a chemical reaction, energy is used up in order to break bonds, and then energy is released when the new product is formed. This will be discussed in more detail in "Energy changes in chemical reactions".
  3. Reversibility
    Chemical changes are far more difficult to reverse than physical changes.

We will consider two types of chemical reactions: decomposition reactions and synthesis reactions.

Decomposition reactions

A decomposition reaction occurs when a chemical compound is broken down into elements or smaller compounds. The generalised equation for a decomposition reaction is:

AB A + B AB A + B

One example of such a reaction is the decomposition of mercury (II) oxide (Figure 3) to form mercury and oxygen according to the following equation:

2HgO 2Hg + O 2 2HgO2Hg+ O 2
(2)
Figure 3: The decomposition of HgO HgO to form HgHg and O 2 O 2
Figure 3 (CG10C4_003.png)

The decomposition of hydrogen peroxide is another example.

Experiment : The decomposition of hydrogen peroxide

Aim:


To observe the decomposition of hydrogen peroxide when it is heated.

Apparatus:


Dilute hydrogen peroxide (about 3%); manganese dioxide; test tubes; a water bowl; stopper and delivery tube

Important:
Hydrogen peroxide can cause chemical burns. Work carefully with it.

Figure 4
Figure 4 (CG10C4_004.png)

Method:

  1. Put a small amount (about 5 ml) of hydrogen peroxide in a test tube.
  2. Set up the apparatus as shown in Figure 4
  3. Very carefully add a small amount (about 0,5 g) of manganese dioxide to the test tube containing hydrogen peroxide.

Results:


You should observe a gas bubbling up into the second test tube. This reaction happens quite rapidly.

Conclusions:


When hydrogen peroxide is added to manganese dioxide it decomposes to form oxygen and water. The chemical decomposition reaction that takes place can be written as follows:

2 H 2 O 2 2 H2O + O 2 2 H 2 O 2 2 H2O + O 2
(3)
Note that the manganese dioxide is a catalyst and is not shown in the reaction. (A catalyst helps speed up a chemical reaction.)

Note:

The previous experiment used the downward displacement of water to collect a gas. This is a very common way to collect a gas in chemistry. The oxygen that is evolved in this reaction moves along the delivery tube and then collects in the top of the test tube. It does this because it is lighter than water and so displaces the water downwards. If you use a test tube with an outlet attached, you could collect the oxygen into jars and store it for use in other experiments.

The above experiment can be very vigourous and produce a lot of oxygen very rapidly. For this reason you use dilute hydrogen peroxide and only a small amount of manganese dioxide.

Synthesis reactions

During a synthesis reaction, a new product is formed from elements or smaller compounds. The generalised equation for a synthesis reaction is as follows:

A+B ABA+BAB
(4)

One example of a synthesis reaction is the burning of magnesium in oxygen to form magnesium oxide(Figure 5). The equation for the reaction is:

2Mg+O2 2MgO2Mg+O22MgO
(5)
Figure 5: The synthesis of magnesium oxide (MgOMgO) from magnesium and oxygen
Figure 5 (CG10C4_005.png)

Experiment: Chemical reactions involving iron and sulphur

Aim:


To demonstrate the synthesis of iron sulphide from iron and sulphur.

Apparatus:


5,6 g iron filings and 3,2 g powdered sulphur; porcelain dish; test tube; Bunsen burner

Figure 6
Figure 6 (CG10C4_006.png)

Method:


  1. Measure the quantity of iron and sulphur 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 1/3 full.
  3. This reaction should ideally take place in a fume cupboard. 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:


  1. After you removed the test tube from the flame, the mixture glowed a bright red colour. The reaction is exothermic and produces energy.
  2. The product, iron sulphide, 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:

Fe+S FeSFe+SFeS
(6)

Investigation : Physical or chemical change?

Apparatus:

Bunsen burner, 4 test tubes, a test tube rack and a test tube holder, small spatula, pipette, magnet, a birthday candle, NaClNaCl (table salt), 0,1MAgNO30,1MAgNO3, 6MHCl6MHCl, magnesium ribbon, iron filings, sulphur.

Warning:
AgNO3AgNO3 stains the skin. Be careful when working with it or use gloves.
Method:

  1. Place a small amount of wax from a birthday candle into a test tube and heat it over the bunsen burner until it melts. Leave it to cool.
  2. Add a small spatula of NaClNaCl to 5 ml water in a test tube and shake. Then use the pipette to add 10 drops of AgNO3AgNO3 to the sodium chloride solution. NOTE: Please be careful AgNO3AgNO3 causes bad stains!!
  3. Take a 5 cm piece of magnesium ribbon and tear it into 1 cm pieces. Place two of these pieces into a test tube and add a few drops of 6MHCl6MHCl. NOTE: Be very careful when you handle this acid because it can cause major burns.
  4. Take about 0,5 g iron filings and 0,5 g sulphur. Test each substance with a magnet. Mix the two samples in a test tube and run a magnet alongside the outside of the test tube.
  5. Now heat the test tube that contains the iron and sulphur. What changes do you see? What happens now, if you run a magnet along the outside of the test tube?
  6. In each of the above cases, record your observations.

Questions:

Decide whether each of the following changes are physical or chemical and give a reason for your answer in each case. Record your answers in the table below:

Table 1
Description Physical or chemical change Reason
melting candle wax    
dissolving NaClNaCl    
mixing NaClNaCl with AgNO3AgNO3    
tearing magnesium ribbon    
adding HClHCl to magnesium ribbon    
mixing iron and sulphur    
heating iron and sulphur    

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