What is matter?

1. Matter is anything that has mass and takes up space.

2. All matter is made up of particles. Particles can be atoms, molecules or ions.

3. Matter can exist in three physical states - solid, liquid and gas.

What is the difference between ’gas’ and ’vapour’?
’Gas’ refers to a substance which exists naturally in gaseous state at room temperature. 
’Vapour’ is used to describe the gaseous state of a substance which is normally a solid or liquid at room temperature.
 e.g. oxygen gas and water vapour

4. Matter can undergo changes. The two main types of changes are physical change and chemical change. 

The table below shows the differences between physical change and chemical change.

What is a physical change?

1. A physical change is a change in a substance without changing its composition and identity. Hence, in any physical change, there will be no new substances formed.

2. Some physical changes involve heat change, others do not. For example, cutting or crushing a substance does not involve heat change. Expansion and contraction of a substance and changes in physical states require heat gain or heat loss. 

3. A physical change may be a temporary change and can be reversed easily.
e.g. An object expands when it is heated. It returns to its original size when it is cooled. 

Changes in Physical States

1. When a substance experiences a change from one physical state to another, phase change or state change takes place.

Changes in Physical States (cont.)

2. The physical state of a substance is determined by two factors:
- Average kinetic energy of particles (the motion)
- Strength of attractive forces between the particles

3. Heat energy is involved in the change of physical states of substances.

Melting

Melting is the process in which a substance changes from a solid to a liquid. 
 
Each pure substance melts at a fixed temperature called the melting point.

During melting, the temperature remains constant as there is no change in the average kinetic energy of particles. e.g. when ice melts, the temperature stays constant at 0°C.

The figure below shows the process of melting a solid subtance.

Boiling

Boiling is the process in which a substance changes from a liquid to a gas.

Each pure substance boils at a fixed temperature called the boiling point.

During boiling, the temperature remains constant because there is no change in the average kinetic energy of particles. e.g. when water boils, the temperature stays constant at 100°C.

The process of boiling a liquid substance is shown below.

Condensation

Condensation is the process in which a substance changes from a gas to a liquid.

Each pure substance condenses at a fixed temperature called the condensation point. The condensation point is the same as the boiling point in a pure substance.

During condensation, the temperature remains constant because there is no change in the average kinetic energy of particles. e.g. when water vapour condenses, the temperature stays constant at 100°C.

The process of condensation of a gas is shown below.

Freezing

Freezing is the process in which a substance changes from a liquid to a solid.

Each pure substance freezes at a fixed temperature called the freezing point. The freezing point is the same as the melting point in a pure substance.

During freezing, the temperature remains constant as there is no change in the average kinetic energy of particles. e.g. when water freezes, the temperature stays constant at 0°C.

The process of freezing a liquid is shown below.

What is a chemical change?

1. A chemical change takes place when atoms of chemical substances break apart and join or rearrange to form new substances.

2. Chemical changes are caused by chemical reactions. A chemical reaction is the process by which one or more substances are changed into one or more different substances with different properties. 

3. A chemical reaction follows the law of conservation of mass and energy. This means that the total mass and energy before and after a chemical change must be conserved. Neither mass nor energy is created nor destroyed in a chemical reaction. They only undergo changes.

4. A chemical change usually gives rise to a few common observations as follows:

Word Equation

A word equation is an equation in which the reactants and products in a chemical reaction are represented by words.

Causes of Chemical Change (Part 1)

The figure below shows the types of chemical reactions caused by heat.

Causes of Chemical Change (Part 3)

The figure below shows the types of chemical reactions caused by light, mixing substances and electricity.

What is manufacturing?

Manufacturing is a process in which raw materials are converted into useful products through chemical reactions. 
Raw material refers to the basic material from which a product is made. 

Sources of Raw Materials

The table below shows the sources of some raw materials and examples of their uses.

What is air pollution and how is it caused?

Air pollution is the presence of unwanted substances in the atmosphere that cause harm to living things and damage to the natural environment. These unwanted substances are called air pollutants.

Air pollution can be caused by natural processes (e.g. lightning and volcanic eruptions) or a result of human activities (e.g. burning of fuels and industrialisation). 

The table below shows the sources and effects of some common air pollutants.

What is global warming?

Global warming is an average increase in the temperature of the atmosphere near the Earth’s surface. It can cause global climate change. Climate change refers to long term changes in climate, including average temperature and the amount of rainfall.

The main greenhouse gases are water vapour, carbon dioxide and methane. These gases provide natural greenhouse effect on Earth, i.e. keep the Earth warm enough for the living things to survive on this planet.

The table below shows the human activities that have contributed to the increase in emissions of carbon dioxide and other greenhouse gases to the atmosphere.

Cause of Global Warming

Carbon dioxide and water vapour allow visible radiation from the Sun to reach the Earth’s surface, but trap some of the infrared radiation emitted by the Earth. Heat energy is thus retained in the atmosphere, producing a warming effect called the greenhouse effect.

When excessive greenhouse gases are emitted to the atmosphere, they reflect more radiation back to the Earth’s surface, causing the Earth’s average surface temeprature to rise. This phenomenon is called enhanced greenhouse effect and this leads to global warming.

Watch the video below to learn more about the greenhouse effect.

Effects and Consequences of Global Warming

The table below shows the effects and consequences of global warming.