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.

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:

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?

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: CuCl2→Cu+Cl2CuCl2→Cu+Cl2

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:

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 2HgO→2Hg+ O 2

(2)

Figure 3: The decomposition of HgO HgO to form HgHg and O 2 O 2

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

Figure 4

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+B→AB

(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+O2→2MgO

(5)

Figure 5: The synthesis of magnesium oxide (MgOMgO) from magnesium and oxygen

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

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)?

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+S→FeS

(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

All reactions involve some change in energy. During a physical change in matter, such as the evaporation of liquid water to water vapour, the energy of the water molecules increases. However, the change in energy is much smaller than in chemical reactions.

When a chemical reaction occurs, some bonds will break, while new bonds may form. Energy changes in chemical reactions result from the breaking and forming of bonds. For bonds to break, energy must be absorbed. When new bonds form, energy will be released because the new product has a lower energy than the `in between' stage of the reaction when the bonds in the reactants have just been broken.

In some reactions, the energy that must be absorbed to break the bonds in the reactants is less than the total energy that is released when new bonds are formed. This means that in the overall reaction, energy is released. This type of reaction is known as an exothermic reaction. In other reactions, the energy that must be absorbed to break the bonds in the reactants is more than the total energy that is released when new bonds are formed. This means that in the overall reaction, energy must be absorbed from the surroundings. This type of reaction is known as an endothermic reaction. Most decomposition reactions are endothermic and heating is needed for the reaction to occur. Most synthesis reactions are exothermic, meaning that energy is given off in the form of heat or light.

More simply, we can describe the energy changes that take place during a chemical reaction as:

Total energy absorbed to break bonds - Total energy released when new bonds form

So, for example, in the reaction...

2Mg+O2 → 2MgO2Mg+O2→ 2MgO

Energy is needed to break the O-OO-O bonds in the oxygen molecule so that new Mg-OMg-O bonds can be formed, and energy is released when the product (MgOMgO) forms.

Despite all the energy changes that seem to take place during reactions, it is important to remember that energy cannot be created or destroyed. Energy that enters a system will have come from the surrounding environment and energy that leaves a system will again become part of that environment. This is known as the conservation of energy principle.

Definition 3: Conservation of energy principle

Energy cannot be created or destroyed. It can only be changed from one form to another.

Chemical reactions may produce some very visible and often violent changes. An explosion, for example, is a sudden increase in volume and release of energy when high temperatures are generated and gases are released. For example, NH4NO3NH4NO3 can be heated to generate nitrous oxide. Under these conditions, it is highly sensitive and can detonate easily in an explosive exothermic reaction.

The total mass of all the substances taking part in a chemical reaction is conserved during a chemical reaction. This is known as the law of conservation of mass. The total number of atoms of each element also remains the same during a reaction, although these may be arranged differently in the products.

We will use two of our earlier examples of chemical reactions to demonstrate this:

1. The decomposition of hydrogen peroxide into water and oxygen

2 H 2 O 2 → 2 H2O + O 2 2 H 2 O 2 → 2 H2O + O 2

Figure 7

Left hand side of the equation

Total atomic mass=(4×1)+(4×16)=68uTotal atomic mass=(4×1)+(4×16)=68u

Number of atoms of each element=(4×H)+(4×O)Number of atoms of each element=(4×H)+(4×O)

Right hand side of the equation

Total atomic mass=(4×1)+(4×16)=68uTotal atomic mass=(4×1)+(4×16)=68u

Number of atoms of each element=(4×H)+(4×O)Number of atoms of each element=(4×H)+(4×O)

Both the atomic mass and the number of atoms of each element are conserved in the reaction.

2. The synthesis of magnesium and oxygen to form magnesium oxide

Figure 8

Left hand side of the equation

Total atomic mass=(2×24,3)+(2×16)=80,6uTotal atomic mass=(2×24,3)+(2×16)=80,6u

Number of atoms of each element=(2×Mg)+(2×O)Number of atoms of each element=(2×Mg)+(2×O)

Right hand side of the equation

Total atomic mass=(2×24,3)+(2×16)=80,6uTotal atomic mass=(2×24,3)+(2×16)=80,6u

Number of atoms of each element=(2×Mg)+(2×O)Number of atoms of each element=(2×Mg)+(2×O)

Both the atomic mass and the number of atoms of each element are conserved in the reaction.

In the experiment above you should have found that the mass at the start of the reaction is the same as the mass at the end of the reaction. You may have found that these masses differed slightly, but this is due to errors in measurements and in performing experiments (all scientists make some errors in performing experiments).

In any given chemical compound, the elements always combine in the same proportion with each other. This is the law of constant proportion.

The law of constant composition says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a 2:1 ratio. If we look at the relative masses of oxygen and hydrogen in a water molecule, we see that 94% of the mass of a water molecule is accounted for by oxygen and the remaining 6% is the mass of hydrogen. This mass proportion will be the same for any water molecule.

This does not mean that hydrogen and oxygen always combine in a 2:1 ratio to form H2OH2O. Multiple proportions are possible. For example, hydrogen and oxygen may combine in different proportions to form H2O2H2O2 rather than H2OH2O. In H2O2H2O2, the H:O ratio is 1:1 and the mass ratio of hydrogen to oxygen is 1:16. This will be the same for any molecule of hydrogen peroxide.

In a chemical reaction between gases, the relative volumes of the gases in the reaction are present in a ratio of small whole numbers if all the gases are at the same temperature and pressure. This relationship is also known as Gay-Lussac's Law.

For example, in the reaction between hydrogen and oxygen to produce water, two volumes of H2H2 react with 1 volume of O2O2 to produce 2 volumes of H2OH2O.

2H2+O2→2H2O2H2+O2→2H2O

In the reaction to produce ammonia, one volume of nitrogen gas reacts with three volumes of hydrogen gas to produce two volumes of ammonia gas.

N2+3H2→2NH3N2+3H2→2NH3

This relationship will also be true for all other chemical reactions.

The following video provides a summary of the concepts covered in this chapter.