5.3 Compounds and Elements

Matter

Mixtures physical Pure Substances

change

Mechanical Mixtures Solutions

CompoundsElements

Ordinary Suspensions Colloids chemical

Mechanical change

Mixtures

Shifting Views of the Elements

Ancient Greek Science: One of the earliest attempts to explain the properties of matter was proposed by ancient Greek Philosophers. The Greek Philosopher Empedocles thought that all substances consisted of varying amounts of four elements: Air, Fire, Earth, and Water.

Theory……. E.g, A warm breeze, would consist of air and fire and a

cool breeze, air and less fire.E.g., Lava, would contain earth and much fire.

The philosophers’view of the elements was not the only view, however. Hands-on investigations of matter were carried out for many centuries by people known as alchemists. Part pharmacist, part mystic, alchemists practised their craft all over Europe and the Middle East.

Alchemists:

These were the first experimenters. They had three main goals:

1. To turn base metals into gold.

2. To find the “elixir of life” that would give them eternal life.

3. To produce the “universal solvent” that would dissolve all substances.

Shifting Views of the Elements

• Sir Francis Bacon (1561-1626) – used scientific method to investigate the physical world. Published a book arguing that science should be built on the basis of experimental evidence rather than philosophical experimentation.

• Robert Boyle(1627-1691) – Expressed skepticism about the 4-element theory of the ancient philosophers. Also, recognized that elements could be combined to form compounds, which laid the foundation for our modern definition.

Bacon’s and Boyle’s ideas about elements led to the widespread search for elements using this new approach to investigating matter.

The Development of a Modern View of the Elements

• Antoine Lavoisier (1743- 1794)-Defined elements as pure substances that cannot be decomposed (broken down) into simpler substances by means of a chemical change. Earliest chemists to use “balanced” view of chemical changes. He identified 23 elements.

• Allesandro Volta(1745-1827)- invented the”voltaic pile” – a device now called the battery. Used this new tool to pass electricity through water and realized that it could be decomposed into hydrogen and oxygen.

• Humphry Davy (1778-1829)- Experimented with substances to see if they could be decomposed by using electric currents.

• Michael Faraday(1791-1867)- named the process of decomposing substances electrolysis.

• Joseph Proust(1754-1826)- Performed experiments to decompose compounds into elements and to measure the mass of elements. Noticed a pattern, now called the law of definite proportions.

DEMOnstration • Electrolysis:The process of decomposing a chemical compound

by passing electric current through it.• Law of Definite Proportions:A law stating that compounds are pure substances

that contain two or more elements combined in fixed (or definite) proportions.

• Law of Conservation of Mass:In a chemical change the total mass of the new

substances is always the same as the total mass of the original substances.

ElementsA type of pure substance that can not be broken down into simpler parts by ordinary chemical means.

E.g., Oxygen O2 : a gas needed for objects to burn

Hydrogen H2: a very volatile (explosive) gas

E.g., Sodium (Na): a very reactive metal

Chlorine (Cl): a toxic yellow-green gas

CompoundsA compound is a pure substance in which two or more elements are chemically combined in definite proportions.

E.g., Water is a combination of one oxygen atom and two hydrogen atoms (H2O). A liquid that puts flames out.

E.g., Table salt, or sodium chloride (NaCl) is a combination of one atom of sodium and one atom of chlorine.

The properties of a compound are different from the properties of the elements which are combined to make the compound.

Testing For Elements and Compounds

Imagine that you are a technician in a chemical lab. While cleaning up after a flood in the laboratory, you discover that the labels of three gas cylinders have been damaged. You suspect that the cylinders contain carbon dioxide, oxygen and hydrogen.

• How do you find out which cylinder contains which gas?

• Whether the cylinders have been contaminated by the flood water?

• Do they now contain water vapour?

There are four common tests used to identify four common gases:

Oxygen- The ‘Glowing Splint’ Test

1. Light a wooden splint.

2. Blow out the flame but leave the splint glowing.

3. Hold the glowing splint in a small amount of the unknown gas.

4. If the splint bursts in to a flame, the gas is oxygen.

• A common chemical reaction is combustion (burning).

• Oxygen must be present for combustion to take place.

• Substances such as wood and oil burn readily in air, which is about 20% oxygen. In pure oxygen they burn much more intensely.

• This chemical property- supporting combustion- allows you to identify it.

Hydrogen-The ‘Burning Splint’ Test

Hydrogen is known to be explosive in air

1. Light wooden splint

2. Hold the burning splint in a small amount of the unknown gas.

3. If you hear a loud “pop”, the gas is hydrogen.

Carbon Dioxide- the Limewater Test

• Carbon dioxide does not burn and does not allow other materials to burn. (If you put a burning splint into carbon dioxide , the flame will go out.

• The chemical test for carbon dioxide uses a liquid called limewater, a clear colourless solution of calcium hydroxide in water.

• Carbon dioxide reacts with the dissolved calcium hydroxide producing a precipitate.

1. Bubble the gas or add a few drops of limewater to the gas and swirl it around.

2. If the limewater turns cloudy or looks milky, the gas is carbon dioxide.

Water Vapour- The Chloride Test

• Water is a liquid at room temperature, but many chemical reactions produce water as a product.

• When water vapour touches a cold surface, it condenses to liquid water.

1. Hold cold surface water near the suspected water vapour.

2. Touch a piece of blue cobalt chloride paper to any liquid that condenses.

3. If the paper changes from blue to pink, water is present.