Finding Out about Matter

Matter Chemists specialize in matter and the properties of matter. Properties such as colour and taste are characteristics of matter; they describe matter specifically and can be used to identify matter. Boiling point is another important property used to identify matter.

Purifying Matter Muddy water is not pure, and will separate into layers of dirt and clear water  a mixture A mixture is two or more kinds of matter that have separate identities. Matter that is easily separated into component parts is called a mixture or said to be impure. Matter can be classified as pure forms and mixtures. Solutions are mixtures that look uniform throughout and do not scatter light. It is harder to identify a solution as a mixture

Separating the parts of a Solution If you boil water containing dissolved salt, the salt will be left behind in the pot. If you recover the boiled water, solutions like salt water and sugar water can be separated into their component parts by distillation.

Most mixtures containing water can be separated by distillation, but some cannot. Ammonia and water cannot be fully separated by distillation Many mixtures can be extremely difficult to separate. Only when there is no way of separating the matter into components can you conclude that it is pure.

Characteristics of Pure Substances Boiling Point As a solution of salt is boiled, the temperature gradually rises as the water boils away. Pure substances have a constant boiling point; mixtures ordinarily do not.

The freezing point and melting point of pure substances is the same The temperature will remain at the freezing point until all of the matter has changed from a liquid to a solid.

Chemical and Physical Changes Certain substances such as sugar or baking soda do no melt when heat is applied, they undergo chemical changes that cannot be easily reversed. These substances undergo decomposition Changes that produce a new kind of matter with different properties are called chemical changes. Certain substances such as moth flakes may be heated, and left to cool, and will return to their original properties. Melting and other changes that are easily reversed to get the original material back again are described as physical changes.

Compounds and Elements Although many pure substances like sugar and baking soda decompose when they are heated, the majority do not. Another way to decompose compounds is by electrolysis. Electrolysis involves passing an electric current through a substance, causing it to decompose into new kinds of matter.

Electrolysis of Water Water is a poor conductor of electricity, so sulfuric acid is usually used to conduct. The products of electrolysis have none of the properties of the original material, and they cannot be changed back by mixing them together.

Electrolysis represents a chemical change in which matter decomposes to form new kinds of matter. Pure substances that can be decomposes into new kinds of matter are called compounds. Pure substances that cannot be decomposed, and are the building blocks of all kinds of matter are called elements. There are 118 known elements (109 at the time Heath Chemistry was written) 94 elements are believed to occur naturally on Earth, the rest have been made in atomic reactors Virtually all matter on Earth is made of only 85 elements. Eight of the elements account for 99% of the Earth’s crust. All other elements make up only 1% of its mass.

Compounds Have a Definite Composition An important difference between mixtures of elements and compounds of elements is that the mixtures can have almost any composition that is desired, but the compounds will have a definite composition. Law of Definite Composition The volume of hydrogen gas obtained from water is always twice the volume of oxygen gas obtained. Every 9g of water contains 1g of hydrogen and 8g of oxygen. It is possible to make several compounds from the same elements The fact that two or more compounds can be made from the same elements is known as the law of multiple proportions.