Introduction to Physical and Chemical Change (Grade 10 NSC Matric Physical Sciences): Revision Notes

Introduction to Physical and Chemical Change

What is matter and why does it change?

Matter surrounds us everywhere - from the desk you sit at to the air you breathe and the water you drink. All of these are examples of matter, but matter doesn't always stay the same. It can transform in many different ways. Understanding how matter changes is fundamental to chemistry, and there are two main types of changes we need to understand: physical changes and chemical changes.

Physical changes in matter

Definition and key concept

A physical change occurs when matter changes its form or appearance, but the particles that make up the substance remain exactly the same. Think of it this way - the molecules don't break apart or join together in new ways, they simply rearrange themselves.

How physical changes work

When water is heated, for example, the temperature and energy of the water molecules increases. The liquid water then evaporates to form water vapour. Even though a change has occurred, the molecular structure of the water hasn't changed at all. This is a classic example of a physical change.

The equation for this change is:

$\text{H}_2\text{O (ℓ)} \rightarrow \text{H}_2\text{O (g)}$

Key characteristics of physical changes

1. Arrangement of particles

When a physical change occurs, the compounds may rearrange themselves, but the bonds between atoms don't break. For example, when liquid water boils, the molecules move apart but each molecule stays intact. Water doesn't break up into hydrogen and oxygen atoms.

The diagram above shows how water molecules arrange differently in liquid and gas phases, but each individual molecule remains unchanged.

2. Conservation of mass

In a physical change, the total mass, number of atoms, and number of molecules always stay exactly the same. You'll 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 do occur during physical changes, but these energy changes are normally much smaller than those that occur during chemical changes.

4. Reversibility

Physical changes are usually much easier to reverse than chemical changes. Methods such as filtration and distillation can reverse many physical changes. Changing temperature is another way to reverse a physical change. For example, if you dissolve salt in water, you can separate the mixture by filtration. Ice can be changed to liquid water and back again simply by changing the temperature.

Examples of physical changes

• State changes (melting, boiling, freezing)
• Dissolving one substance in another
• Energy transfer through conduction (heat moving through materials)
• Mixing substances together (where no new substances form)

Chemical changes in matter

Definition and key concept

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

Examples of chemical changes

Let's examine two important examples of chemical changes: decomposition and synthesis reactions.

Decomposition of hydrogen peroxide

The breakdown of hydrogen peroxide (H₂O₂) to form water (H₂O) and oxygen gas (O₂) demonstrates a chemical change perfectly. In this reaction, the chemical bonds between oxygen and hydrogen atoms in H₂O₂ are broken, and new bonds form between hydrogen and oxygen (to form H₂O) and between oxygen atoms (to form O₂).

The equation for this reaction is:

$2\text{H}_2\text{O}_2\text{(aq)} \rightarrow 2\text{H}_2\text{O (ℓ)} + \text{O}_2\text{(g)}$

Synthesis of water

The synthesis (formation) of water (H₂O) from hydrogen gas (H₂) and oxygen gas (O₂) is another excellent example of chemical change. The chemical bonds in the hydrogen and oxygen molecules are broken, and new bonds form to create water molecules.

Synthesis of iron sulphide

When iron and sulphur are heated together, they combine to form iron sulphide - a completely new substance with different properties from either iron or sulphur.

The equation is:

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

Key characteristics of chemical changes

1. Arrangement of particles

During chemical changes, the particles themselves change in some way. Using the hydrogen peroxide example, H₂O₂ molecules split into their component atoms. The number of particles changes because each H₂O₂ molecule breaks down into two water molecules (H₂O) and one oxygen molecule (O₂).

2. Energy changes

The energy changes during chemical reactions are much greater than those during physical changes. Energy is first used to break existing bonds, then energy is released when new bonds form.

3. Reversibility

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

4. Mass conservation

Mass is conserved during chemical changes, but the number of molecules may change. In the hydrogen peroxide decomposition, every two molecules of hydrogen peroxide create three new molecules (two water molecules and one oxygen molecule).

Conservation laws in chemical changes

Understanding conservation is crucial for grasping chemical changes. Let's examine this using the hydrogen peroxide decomposition:

Property	Reactants	Products
Mass is conserved	4(1.01) + 4(16.0) = 68.04	2(18.02) + 2(16.0) = 68.04
Atoms are conserved	4 oxygen atoms, 4 hydrogen atoms	4 oxygen atoms, 4 hydrogen atoms
Molecules	2 molecules	3 molecules
Energy changes	Energy taken in when bonds break	Energy given off when bonds form

Comparing physical and chemical changes

Aspect	Physical Change	Chemical Change
Particles	Rearrange but stay intact	Break apart and form new combinations
Energy changes	Smaller energy changes	Larger energy changes
Reversibility	Usually easy to reverse	Usually difficult to reverse
Mass	Always conserved	Always conserved
Number of molecules	Always stays the same	May change
Identity of substance	Stays the same	Changes to new substances