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

# Chemical Changes in Matter

## 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.

### Definition: Chemical change

The formation of new substances in a chemical reaction. One type of matter is changed into something different.

We will consider two examples of chemical change: the decomposition (breaking down) of hydrogen peroxide and the synthesis (forming) of water.

### Decomposition of hydrogen peroxide

The decomposition (breakdown) of hydrogen peroxide ($$\text{H}_{2}\text{O}_{2}$$) to form water ($$\text{H}_{2}\text{O}$$) and oxygen gas ($$\text{O}_{2}$$) is an example of chemical change. A simplified diagram of this reaction is shown in the figure below. The chemical bonds between $$\text{O}$$ and $$\text{H}$$ in $$\text{H}_{2}\text{O}_{2}$$ are broken and new bonds between $$\text{H}$$ and $$\text{O}$$ (to form $$\text{H}_{2}\text{O}$$) and between $$\text{O}$$ and $$\text{O}$$ (to form $$\text{O}_{2}$$) are formed. A chemical change has taken place.

The decomposition of $$\text{H}_{2}\text{O}_{2}$$ to form $$\text{H}_{2}\text{O}$$ and $$\text{O}_{2}$$

## Optional Experiment: The decomposition of hydrogen peroxide

### Aim

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

### Apparatus

Dilute hydrogen peroxide (about $$\text{3}\%$$); manganese dioxide; test tubes; a water bowl; stopper and delivery tube, Bunsen burner

#### Warning:

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

### Method

1. Put a small amount (about $$\text{5}$$ $$\text{mL}$$) of hydrogen peroxide in a test tube.

2. Set up the apparatus as shown above.

3. Very carefully add a small amount (about $$\text{0.5}$$ $$\text{g}$$) 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:

$2\text{H}_{2}\text{O}_{2}\text{(aq)} \rightarrow 2\text{H}_{2}\text{O (l)} + \text{O}_{2}\text{(g)}$

Note that the manganese dioxide is a catalyst and does not take part in the reaction, so we do not show it in the balanced equation. (A catalyst helps speed up a chemical reaction.)

### Fact:

This 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 less dense than water and does not dissolve in water, so the water is displaced downwards.

### Did You Know?

This reaction is often performed without collecting the oxygen gas and is commonly known as the elephant’s toothpaste reaction.

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

### The synthesis of water

The synthesis (forming) of water ($$\text{H}_{2}\text{O}$$) from hydrogen gas ($$\text{H}_{2}$$) and oxygen gas ($$\text{O}_{2}$$) is another example of chemical change. A simplified diagram of this reaction is shown in the figure below. The chemical bonds between $$\text{O}$$ in $$\text{O}_{2}$$ and between $$\text{H}$$ in $$\text{H}_{2}$$ are broken and new bonds between $$\text{H}$$ and $$\text{O}$$ (to form $$\text{H}_{2}\text{O}$$) are formed. A chemical change has taken place.

The synthesis of $$\text{H}_{2}\text{O}$$ from $$\text{H}_{2}$$ and $$\text{O}_{2}$$

## Optional Experiment: The synthesis of water

### Aim

To observe the synthesis of water.

### Apparatus

Hydrogen gas; balloon; string; candle; long stick: ear plugs; safety glasses

### Method

1. Half fill a balloon with hydrogen gas.

2. Fill the remainder of the balloon with oxygen gas. (You can also just use your breath to fill the balloon.)

3. Tie the balloon to one end of the string. Tie down the other end of the string so that the balloon is positioned in mid air, away from any people, objects, walls, ceilings etc.

4. Attach the candle tightly to the stick and light the candle.

5. Standing away from the balloon, carefully hold the candle to the balloon.

#### Warning:

This reaction can be highly explosive, for this reason it is best done outdoors. Always ensure that you wear ear protection or block your ears. Always have more oxygen than hydrogen in the balloon.

### Results

When you bring the candle close to the balloon you should see a flame and hear a loud bang.

### Conclusions

When a mixture of hydrogen and oxygen gas is set alight with a candle a chemical change occurs. Water is made according to the following equation:

$2\text{H}_{2}\text{(g)} + \text{O}_{2}\text{(g)} \rightarrow 2\text{H}_{2}\text{O (l)}$

### Did You Know?

A mixture of hydrogen and oxygen gas is used as a fuel to get rockets into space.

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 hydrogen peroxide that was used earlier, the $$\text{H}_{2}\text{O}_{2}$$ molecules were split up into their component atoms. The number of particles will change because each $$\text{H}_{2}\text{O}_{2}$$ molecule breaks down into two water molecules ($$\text{H}_{2}\text{O}$$) and one oxygen molecule ($$\text{O}_{2}$$).

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.

3. Reversibility

Chemical changes are far more difficult to reverse than physical changes. When hydrogen peroxide decomposes into water and oxygen, it is almost impossible to get back to hydrogen peroxide.

4. Mass conservation

Mass is conserved during a chemical change, but the number of molecules may change. In the example of the decomposition of hydrogen peroxide, for every two molecules of hydrogen peroxide that decomposes, three molecules are formed (two water and one oxygen).

The table below highlights these concepts for the decomposition of hydrogen peroxide.

 $$2\text{H}_{2}\text{O}_{2} \rightarrow 2\text{H}_{2}\text{O} + \text{O}_{2}$$ Molecules two molecules three molecules Energy changes energy taken in when bonds are broken energy given off when bonds are formed Mass is conserved $$4(\text{1.01}) + 4(\text{16.0}) = \text{68.04}$$ $$2(\text{18.02}) + 2(\text{16.0}) = \text{68.04}$$ Atoms are conserved $$\text{4}$$ oxygen atoms, $$\text{4}$$ hydrogen atoms $$\text{4}$$ oxygen atoms, $$\text{4}$$ hydrogen atoms

Table: Important concepts in chemical change