Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 1

Some Definitions First

CHEMISTRY studies MATTER.

MATTER is anything that has a mass and a volume.

Each of the particular kinds of matter (solid or liquid or gas) is called a MATERIAL.

CHEMISTRY is thus defined as the science of the composition and the structure of materials and of the changes that the materials undergo.

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 2

Chemistry is an Experimental Science and is CENTRAL

All experimental sciences are based on observation, collecting data, and analysing these in an SYSTEMATIC WAY.

This is known as THE SCIENTIFIC METHOD.

The scientific method is a systematic approach to research.

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 3

Scientific Method

When data are collected and represented, by equations, symbols, etc., next step is form a hypothesis, a tentative explanation of observations. Observations may be repeated to refine the hypothesis further.

When a large amount of data have been collected, they may be summarised in a short-hand way by coming up with a mathematical equation. This is known as a law.

Tested hypotheses result in a theory, a principle that explains facts and laws that are based on them.

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 4

Classification of MatterMatter can be classified into two broad groups:• Substances – this the type of matter that has definite composition

and distinct properties. Substances can be either elements or compounds

• Mixtures – a combination of two or more substances, where the substances retain their unique identities and can be separated by physical means

• Mixtures can be homogeneous or heterogeneous

• Elements cannot be separated into simpler substances by chemical means. There are 92 natural and 26 created elements. Each element is identified by a symbol.

• Elements interact with one another to yield compounds, defined as substances formed from two or more elements combined by chemical means, with properties not similar to properties of the constituent elements.

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 5

Schematic Classification of Matter

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 6

States of Matter

There are 3 states of matter:

• Solids – units packed in definite order• Liquids – semi packed with some order• Gases – no packing with units moving freely

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 7

Physical and Chemical Properties of MatterSubstances are identified according to their properties and composition.

Physical properties of a substance can be measured and/or observed without changing its composition or identity.

For example, melting of ice is a physical property of water, observed while it is melting. Thus melting is a physical process.

Chemical properties of a substance can be observed during a chemical change.

For example, burning of hydrogen (an element) in air is a chemical property, observed while it is burning. Thus burning is a chemical process.

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 8

Extensive-Intensive Properties

All measurable properties of matter fall into either of the following additional groups

Extensive Property – A property that depends on how much matter is under study. For example, volume and mass of a sample of matter depends on the amount of the sample. Extensive properties of the same kind can be added together.

Intensive Property – A property that does not depend on how much matter is under study. For example, temperature and pressure of a sample does not depend on the amount of sample. Such properties are not additive.

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 9

Measurements

Scientists use instruments (also called tools) to measure macroscopic properties of substances directly. For example, a ruler is used to measure the length of a sheet of paper.

Microscopic properties cannot be measured as such; indirect methods have to be devised for these.

Each measurement results in a value. Depending on what tool is used, this value may change. In order to have a unified system across the scientific world, an international set of units was agreed upon in 1960 and named as SI Units. This set has 7 base units for seven basic properties. All units for all other properties can be derived from these.

These units are modified in decimal steps by a series of prefixes.

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 10

SI Units

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 11

Some Other Common UnitsSome quantities represented by SI Units are too large for experimentation. It has been customary to use submultiples of these in scientific research.

For example, we usually measure mass in grams (g) instead of kilograms. Centimetres (cm) and millimetres (mm) may be more convenient than metres.

Converting from one unit to another is a true mathematical operation and is done via a conversion factor, which in fact, is a proper mathematical equation.

For example, to change centimetres into metres we use this conversion factor.

1 m 100 cm

Chemistry: The Study of Change

General Chemistry I CHM 111 Dr Erdal Onurhan Slide 12

Precision and Accuracy

Accurate and Not Accurate Neither AccuratePrecise But Precise Nor Precise