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SOLUTION

 

Read the text and check if your definitions were correct.

Solutions and Suspensions If a lump of sugar is stirred or shaken with a sufficient quantity of water, the sugar disappears in a rather short time: it is said to have dissolved in, or gone into solution in the water. This system composed of sugar and water is the solution. If the solution is placed in a bottle which is then stoppered to prevent evaporation, there will be no settling out of the sugar even though this substance is heavier than water. Passage of a solution through even the densest filter paper does not turn out to bring about any separation of the components.

It is common knowledge that if ordinary sand is shaken with water, the solid does not disappear but settles out quickly when the system is allowed to come to rest. However, if very finely divided sand or clay is shaken with water, a suspension is obtained from which the dispersed particles settle out very slowly.

We see mixtures of such materials as glue, soap or gelatin with water pass unchanged through a filter: the substances do not settle out, and no individual particles are visible to the naked eye. Thus the systems appear to be true solutions. But microscopic examination of them shows minute suspended particles to be present in the mixtures. In the true solutions the particles are too small to be seen in this way. The suspensions of glue, soap and the like are known as colloidal suspensions.

Solute and Solvent To distinguish between the substances which make up a solution it is customary to speak of the dissolved substance as the solute and the substance in which it is dissolved as the solvent. Sometimes the solvent is considered to be the component which is present in the greater proportion, while the other ingredient is the solvent, and the salt is the solute. These terms are used as a matter of convenience and their use is not intended to indicate any fundamental difference between the components of a solution.

Saturated, Unsaturated and Supersaturated Solutions The question now arises as to why a given solid at a constant temperature ordinarily dissolves in a particular solvent to a definite limiting concentration. If the solute tends to dissolve, why will not this tendency cause the solid to dissolve indefinitely?

Let us consider an aqueous solution of sugar. As the concentration of the solution increases, the number of sugar molecules returning to the solid in a unit of time is correspondingly greater. Finally, if a sufficient quantity of sugar is present, a condition of balance is supposed to be reached when the rates at which the substance goes into and out of solution are equal.

At this point no further increase in concentration at a given temperature is possible. The process of solution may seem to be at an end, when in reality it is in full operation, for there is every reason to believe that the two opposing forces are still in operation and that the particles of the solid are likely to be going into the dissolved state. But since an increase in concentration no longer takes place, the solution is said to be saturated.



So solutions can be classified by how they relate to the solubility value. In a saturated solution, some excess solute remains, and the amount that dissolves is equal to the solubility value for that temperature. Supersaturated solutions are known to contain more than the usual solubility, as long as there is no excess undissolved solute remaining.

 

Comprehension Aspect

 

Ex. 1. Choose the opposites to the underlined words from the ones given below:

1. The system is composed of water and sodium chloride. 2. Under certain conditions the solute goes into solution rather slowly. 3. The solute dissolves quickly. 4. The rate of the solution has increased. 5. The reaction is at an end.

a) in full operation, b) settled out, c) decreased, d) rapidly, e) out of, f) broken up into.

 

Ex. 2. Say whether the following statements are True or Erroneous. Correct the Erroneous ones.

1. Solvents are not always liquids. 2. Passage of a true solution through a filter brings about the separation of the solute and the solvent. 3. When sodium chloride is dissolved in water there will be settling out of the solute in a short time. 4. It is common knowledge that glue, gelatine and sand do not dissolve in water. 5. Sand forms a colloidal suspension with water. 6. All soluble substances dissolve indefinitely. 7. There is a fundamental difference between the solute and the solvent. 7. Equilibrium is such a state of solution when the quantity of the solute is equal to that of the solvent.

 

Ex. 3. Distinguish between solutions and suspensions.

 

Ex. 4. Identify the following as solutions or suspensions.

a) muddy river water,

b) chlorinated water in a swimming pool,

c) orange juice.

 

Ex. 5. Distinguish among unsaturated, saturated, and supersaturated solutions.

 

Ex. 6. Answer the questions

a) If you mixed silver nitrate with water, would it dissolve?

b) If you mixed potassium chloride with water, would it dissolve?

c) If you mixed the contents of these two combinations, would anything settle to the bottom?


GRAMMAR ASPECT

 

COMPLEX SUBJECT

(The Subjective Infinitive Construction)

NOUN (PRONOUN) + INFINITIVE

 

Complex Subject is used:

 

1. When the predicate is in the passive voice and is expressed by the verbs to know, to state, to report, to regard, to believe, to expect, to say and others. She is said to study at this University. √овор€т, что она учитс€ в этом университете.
2. When the predicate is in the active voice and is expressed by the verbs to prove, to seem, to appear, to happen, to chance, to turn out. He proved to be a good chemist. ќн оказалс€ хорошим химиком.
3. When the predicate is expressed by to be sure, likely, unlikely, certain. They are likely to take part in the conference. ќни, веро€тно, примут участие в конференции.

 

Ex. 1. Look through the text again and find the sentences with the Subjective Infinitive Construction.

 

Ex. 2. Choose those predicates of the possible sentences with the Subjective Infinitive Construction which can be translated as:

a) доказано, б) оказываетс€, в) найдено, г) ожидают, д) считают,

е) видно, ж) по-видимому, з) полагают, и) известно.

 

1. Е prove to protect Е 2. Е are supposed to beЕ 3. Е is assumed to increase Е 4. Е is believed to combine Е 5. Е are thought to be Е 6. Е are seen to prevent Е 7. Е are found to pass Е 8. Е are known to have obtained Е 9 Е turn out to cause Е 10. Е are expected to disappear Е 11. Е are considered to find Е 12. Е appear to have Е 13. Е proved to contain Е .

 

Ex. 3. Translate the following groups of words paying attention to the form of the Infinitive:

1. This salt is known to dissolve in water Е . 2. This salt is known to have dissolved Е . 3. This salt is known to have been dissolvedЕ . 4. The colour of the solution is seen to be changingЕ 5. The colour of the solution is expected to change Е . 6. The colour of the solution is found to have changed. 7. Many salts are supposed to be formed by direct union of their elements. 8. Many salts turn out to form aqueous solutions.

 

Ex. 4. Paraphrase the following sentences using the Subjective Infinitive Constructions as in the models:

Model 1: It is supposed that the substance is prepared by the reaction of some metals with water. The substance is supposed to be prepared by the reaction of some metals with water.

 

Model 2: It seems that tungsten exists in the same oxidation states as molybdenum. Tungsten seems to exist in the same oxidation states as molybdenum.

 

Model 3. It is believed that the substance was examined sufficiently. The substance is believed to have been examined sufficiently.

 

1. It is supposed that some properties of the solutions will be profoundly affected. 2. It appears that interactions between the ions and the solvent molecules were sometimes neglected. 3. It was found that solids dissolved in liquids only to a limited extent. 4. It is stated that apart from their mechanical properties, solid solutions do not differ greatly from the solutions of liquids. 5. It is known that solubility is measured in terms of the maximum amount of solute that can be dissolved in a solvent at chemical equilibrium. 6. It was believed that the speed of recrystallization would increase. 7. It is known that if the amount of solute exceeds the solubility, some solute remains undissolved.

Ex. 5. Translate the following sentences into Russian, mind the Subjective Infinitive Constructions.

1. Salt is said to be soluble in water because the solid salt seems to disappear if enough water is added; no solid is left on the bottom of the container. 2. Aqueous solutions of electrolytes are assumed to conduct an electric current by ionic migration. 3. Aqueous solutions of electrolytes are found to undergo electrolysis. 4. Electrolysis is known to take place when a direct current passes through the solution between two inert electrodes. A condition of equilibrium is supposed to be reached when rates at which the substance goes into and out of solution are equal. 6. The sugar is said to have dissolved in the water. 7. The process of solution may seem to be at an end. 8. A disperse system is known to denote any homogeneous medium containing dispersed entities of any size and state.

 


Date: 2015-01-29; view: 228


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