Science: 2017:CBSE: [All India]:Set–III

The molecular formulas of 2^nd and 3^rd member of this series are C₃H₄ (propyne) and C₄H₆ (butyne) respectively.

Variation is important for the survival of species as it helps species to adapt to the changing environmental conditions.

According to ten percent law of energy, only 10% energy is transferred from one trophic level to another. Thus, if 20, 000 J of energy is available to plants, then only 2000 J of energy will be available to sheep and 200 J of energy will be available to man in the given food chain.

Given that the object is placed at a distance 15 cm from lens.

i.e, object distance (u) = -15 cm

Focal length of concave lens, f = -30 cm

Image distance (v) =?

By lens formula we know that

Four characteristics of the image formed are,

(i) Position: The value of (v) shows that the image is formed at a distance of 10 cm from the optical center.

(ii) Nature: The negative sign of the (v) shows the image formed is virtual and erect.

(iii) Size of the image:

For lens magnification,

As the value of magnification is less than 1. Hence the image formed is diminished.

(iv) The positive sign of magnification (m) shows that the image is formed above the principal axis.

Four activities that we can do on our own are:

• Use of efficient engines for complete combustion of fuels.
• Car pool for same destination.
• Avail the facility of public transport like metro trains.
• Recycle paper bags, use both sides of paper, envelope etc.

A natural resource is anything that occurs naturally in the environment and can be used by living organisms. Coal and petroleum are fossil fuels. Fossil fuels are formed naturally from the remains of dead animals and plants which have been subjected to various biological and geological processes.

Coal and petroleum are exhaustible or non-renewable natural resources, i.e., finite quantity of these resources are available in nature. If we continue to use these exhaustible resources at the same rate, nature would not be able to meet our demands and these resources would disappear. Hence, in order to maintain the continuity of these resources, it is necessary to use them judiciously.

(i) Use of ester: Esters are used as flavouring agents.

(ii) Use of saponification process: Saponification processis used in making soaps.

When ethanol is heated at 443K with excess of conc. H₂SO_4,ethene(C₂H₄) is obtained.

In this reaction, sulphuric acid act as a dehydrating agent.

According to Modern Periodic Law, physical and chemical properties of elements are periodic functions of their atomic numbers.

Elements belonging to same group have same number of electrons in the valence shell. So, they show similar properties.

As we move from left to right in a period, tendency of elements to gain electrons increases.

This is because as we move from left to right in a period, electrons are added in same valence shell and effective nuclear charge (no. of protons in a shell) increases. So, attraction for the incoming electron increases.

Electronic configuration of element ‘X’ with atomic number 20 = 2,8,8,2

Electronic configuration of element ‘Y’ with atomic number 17 = 2,8,7

Number of valence electrons of X = 2
Valency of X = 2

Number of valence electrons of Y = 7
Valency of Y = 8 - 7 = 1

Therefore, the formula of compound the compound is XY₂.

Ionic bond will be formed between element X and Y. Element ‘X’ with atomic number 20 has electronic configuration 2,8,8,2. Thus it gives its two electrons to two atoms of Y and forms X²⁺ ions. While element ‘Y’ with atomic number 17 has electronic configuration 2,8,7. Thus two atoms of Y gain one e^- each and forms 2Y^– ions. So the electron dot structure for this divalent halide will be as follows—