Science:2015:CBSE:[Delhi]:Set-II

C_nH_2n+2 is the general formula for the alkane series. The second member of the alkane family is ethane having formula C₂H₆.

Power of lens is defined as the reciprocal of its focal length(in metres).

a. We need to manage our resources carefully because the human population is increasing at tremendous rate due to improvement in health care. As a result the demand for all the resources is increasing at exponential rate which are very limited in nature.
b. The management of natural resources requires a long term perspective because the demand of these natural resources is increasing exponentially and in order to meet the need and demand of next generations, the natural resources must be used carefully.

Four measures that can be taken to conserve forests are:
1. Indiscriminate cutting of trees or deforestation should be prohibited.
2. Forest fire should be controlled.
3. Alternative sources of energy, like biogas should be used instead of fuel wood.
4. Reforestation of the deforested area should be undertaken.

(i) When we move from left to right across a period in the periodic table, atomic radius decreases due to increase in nuclear charge which pulls the outermost electron closer to the nucleus. Therefore, sodium has the largest atomic radius among these elements.

(ii) Among the given elements, aluminium is the least reactive element because when we move from left to right across a period in the periodic table, the reactivity decreases due to increased nuclear charge which makes the loss of outermost electron by the element difficult.

(i) Among the given elements, ₁₉K has one electron in the outermost shell as its electronic configuration is 2, 8, 8, 1.

(ii) The elements ₄Be and ₂₀Ca belong to the same group, i.e., Group 2 as both elements have two electrons each in their outermost shells.

Electronic configuration of ₄Be = 2, 2.
Electronic configuration of ₂₀Ca = 2, 8, 8, 2.

(iii) The atomic number of element X = 17
Electronic configuration of element X = 2, 8, 7.
Number of valence electrons in X = 7
Valency of X = 8 − 7 = 1

Similarly, Atomic number of element K = 19

Electronic configuration of K is 2, 8, 8, 1.
Number of valence electron in K = 1
Valency of K = 1

Therefore, on combining the elements K and X, we get the formula of compound as KX.

The compound KX is of ionic nature.

The compounds having the same molecular formula but different structural formulae are known as isomers.
The two structural isomers of butane are:


Lower alkanes i.e. methane, ethane and propane do not exhibit isomerism as they are too short to have branched structures required to show structural isomerism.

Soap molecules are sodium or potassium salts of long-chain carboxylic acids or higher fatty acids. They form scum upon reaction with calcium and magnesium ions present in hard water.

Detergent molecules are ammonium or sulphonate salts of long-chain carboxylic acids. They do not form insoluble precipitates with calcium and magnesium ions.

Cleansing action of soaps: The dirt or oil droplet present on clothes is organic in nature and is insoluble in water. Hence, it cannot be removed by only washing with water. When soap is dissolved in water, its hydrophobic ends attach themselves to the dirt. Then, the molecules of soap arrange themselves in micelle formation and trap the dirt at the centre of the cluster. These micelles remain suspended in the water. When cloth is rinsed in water, the dirt particles are easily washed away.