PHYSICAL AND CHEMICAL CHANGES

Physics and Chemistry are scientific disciplines that have the purpose of explaining the world in which we live. Since in it, everything is constant change, its main objective is to explain these changes.

Both focus their study on matter. If matter has well-defined boundaries, then we refer to it as an object or a body. The atmosphere is a material system, since it does not have well-defined limits. A piece of iron, or your pencil, are objects, since they do.

Matter can be so small as to need a microscope to see it, or as large as the entire universe. In either case, it can undergo two types of change.

Understand, think, investigate...

1 The text says that scientific knowledge is the work of many people. Name two of those people and search the Internet for other scientists whose work was used to advance science.

2 Round table. Given the properties of scientific knowledge, explain why horoscopes are not scientific knowledge.

3 What is the origin of scientific knowledge? What do we do if a hypothesis fails?

4 Discuss whether the statement ‘everything in the world is in constant change’ is true or false.

5 Indicate whether the following changes are physical or chemical and why: an object falls, we crumple paper and then burn it, we dissolve salt in water, a cloud forms, fruit ripens.

■ Physical changes

If we raise the temperature of a piece of iron until it melts, we still have iron. At the beginning it was in a solid state and afterward it was in a liquid state. It is the same when an ice cube melts.

Physical changes are those in which after the change, we still have the same substance.

■ Chemical changes

However, if we leave a piece of iron in the open air, it will eventually rust and the final substance is different from the original.

Chemical changes are those in which the substances at the end differ from those at the beginning.

Normally, chemical changes are accompanied by physical changes that help us recognise them. For example, a gas emission, colour change or bubbling.

Physical and chemical changes: examples

Iron melts when it reaches a temperature of 1 540 °C. When this happens, a new substance is not formed, but the substance does change state. This is a case of a physical change. When there is a change of state, these cases are physical changes.

However, when iron rusts, it forms a new substance, iron oxide, with very different properties. Unlike iron, iron oxide is not a good conductor of electrical current for example. These cases, in which the final substances differ from the originals, are chemical changes.