Dr. Uttama Mukherjee: Your Tutoring Companion

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Matter is broadly classified at the macroscopic level into two types of substances - pure substances and mixtures. Pure substances are further classified as elements and compounds.

A compound is made up of two or more elements combined together in a fixed ratio. Composition of a compound is fixed and its components cannot be separated by physical methods. Examples are - water (H2O), carbon dioxide (CO2), hydrochloric acid (HCl), urea (NH2.CO.NH2) etc.

Mixture, as the name suggests, is a mix of two or more pure substances in any ratio. The composition of a mixture is variable and its components can be separated by physical methods. Examples are - sugar or salt solution in water, milk, air, oil in water, biscuit etc.

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Magnetic separation is used to separate iron pins fron sand.

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The laws of chemical combination are as follows -

• Law of conservation of mass
• Law of definite/constant proportions
• Law of multiple proportions
• Law of reciprocal proportions
• Gay Lussac's law of combining volumes

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The difference between phenol, alcohol and ether lies in their different functional groups.

Phenol is indicated by a phenyl group and a hydroxyl group as C6H5OH. Other examples - catechol, resorcinol, hydroquinone (ortho, meta and para isomers respectively).

Alcohol too has a hydroxyl group but not as phenol. It is indicated as R-OH, where R = any alkyl group. Examples - C2H5OH (ethyl alcohol), PhCH2OH (benzyl alcohol) etc.

Ether is characterized as R-O-R where R = any alkyl group. -O- is the typical ether linkage. Examples - C2H5-O-C2H5 (diethyl ether), PhCH2-O-CH2Ph (dibenzyl ether).

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Steam produces more severe burns because the latent heat of vapourisation of steam is more than boiling water.

This means that steam has more heat energy than boiling water. Latent heat of steam (vapourisation of water) = 2264.7 kJ/kg.

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Popoff's rule states that during the oxidation of an unsymmetrical ketone, the cleavage of the C-CO bond is such that the keto group always stays with the smaller alkyl group.

Example: CH3COCH2CH2CH3 on oxidation gives two carboxylic acids --- CH3COOH and CH3CH2COOH. The keto group remains with the smaller alkyl group which is -CH3 while the -COOH group of the other carboxylic acid comes from the oxidation of -CH2 group of the bigger alkyl chain.  

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Sublimation is the method of separating naphthalene from common salt. Sublimation involves the conversion of a solid directly into its vapour state without being converted in the liquid state. Naphthalene vapours can be collected after sublimation and the salt remains as residue.

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The following factors affect evaporation -

1. Temperature - Higher is the temperature, greater is the rate of evaporation.

2. Wind velocity - Greater is the wind velocity, higher is the rate of evaporation.

3. Humidity - High humidity lowers the rate of evaporation. E.g. clothes dry very slowly during rainy season.

4. Surface area of the material - Greater surface area fecilitates evaporation.

5. Vapor pressure - More vapour pressure on the liquid surface hinders evaporation.

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The following factors affect evaporation -

1. Temperature - Higher is the temperature, greater is the rate of evaporation.

2. Wind velocity - Greater is the wind velocity, higher is the rate of evaporation.

3. Humidity - High humidity lowers the rate of evaporation. E.g. clothes dry very slowly during rainy season.

4. Surface area of the material - Greater surface area fecilitates evaporation.

5. Vapor pressure - More vapour pressure on the liquid surface hinders evaporation.

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The desert cooler cools better on a hot dry day because of evaporation which causes cooling effect. During a hot dry day, the temperature is high and humidity is low. This causes the rate of evaporation to be much higher as compared to any other cooler day. As the rate of evaporation increases, cooling effect also increases.