Lesson 7

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text: from the history of mechanics

The history of modern mechanics being based on the discoveries of ancient scientists goes back to the 4th century when the first known attempts of the kind were conducted.

It took thousands of years for man to find scientific explanations for mechanical phenomena. There was little advance in mechanics until the 15th century when it began developing intensively. Following the discoveries in the field of mechanics, during this period, we should mention the name of Leonardo da Vinci (1452-1519).

In the l7th century mechanics was further enriched by Galileo Galilei (1564-1642). Being the follower of Galilei Isaac Newton developed mechanics to the level of a science. The mechanics of Galilei and Newton, now known as classical mechanics, made the foundation for the intensive development of that science.

Mechanics, thanks to its outstanding discoveries took the leading position among other sciences, having enriched them at the turn of the 18th century. Russian scientists made a great contribution to world science.

Answer the questions:

1. When were the first mechanical discoveries conducted?

2. How many years did it take for man to find scientific explanations for mechanical phenomena?

3. Who made the discoveries in the field of mechanics?

Lesson 8

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text: Metals and non-metals

The 105 elements do not, fortunately, exhibit 105 completely dif­ferent sets of properties. When the major properties are considered it is found that the elements fall into one of two groups, the metals or the non-metals. The contrast between the properties of these two groups is given below. It is not to be expected that all elements in one class will agree in every detail; some differ in one or two properties from the others of their class; these exceptions are indicated in brackets.

Metals Non-Metals

Physical properties

1. Solid at room. temperature (mercury is the only liquid metal) 2. Have a high density (except potassium and sodium) 3. Can be moulded by pressure, i. e. they are malleable 4. Have high melting points and boiling points 5. Are good conductors of heat and electricity 6. Can be drawn into wire, i.e. they are ductile

1. Many are liquids and gases at room temperature 2. Density is usually low 3. Solid non-metals are brittle 4. Have low melting points and boiling points 5. Are poor conductors of heat and electricity (graphite is a good conductor of electrici­ty) 6. Cannot be drawn into wire

Chemical properties

7. Have basic oxides 8. React with dilute acids form­ing salts 9. Form positive ions 10. Are liberated at the cathode during electrolysis (hydrogen acts as a metal)

7. Have acidic oxides 8. Salts of non-metals do not exist 9. Form negative ions 10. Are liberated at the anode during electrolysis

The chemical properties are much more conclusive than the phys­ical properties for deciding whether a particular element is to be re­garded as a metal or a non-metal, e.g.: if an element forms a basic oxi­de, it must be classified as a metal. A basic oxide is never formed by a non-metal. To determine if a particular element is a metal or a non-metal, pre­pare its oxide and examine this as follows:

a) if the oxide is soluble in water, use litmus to find out if the so­lution is acidic or alkaline;

b) if the oxide does not dissolve in water, an attempt must be made to find out if it neutralizes an acid and forms a crystalline salt. Only metallic oxides do this.