Science 2015 CBSE [All India] Set II

There are ten covalent bonds in a molecule of propane.

DNA is found in the nucleus of a cell.

The first trophic level in a food chain is always a green plant because they are the producers. They convert the energy of the sun into a usable form of energy for the next trophic level.

The four ways to make the residents aware that the improper disposal of waste is harmful to the environment and also for their own health are:

a) The waste serves as a breeding ground for mosquitoes that spread various diseases.

b) It releases harmful gases that make the environment unclean and unhygienic for all living organism.

c) It flows to water bodies with rain water and harms the aquatic organisms present in it.

d) It reduces the soil fertility and diminishes the beauty of the area.

The two advantages associated with water stored in the ground are:

1. Water does not get contaminated by pollutants.
2. Water does not get evaporated.

Organic compounds with similar general formula belong to a series called homologous series. Alkanes, alkenes, alkynes etc. forms such series and each member differ from its previous member by CH₂.

The given compounds are classified as:

Alkenes: C₃H₆ (propene), C₄H₈ (butene), C₅H₁₀ (pentene)

Alkynes: C₃H₄ (propyne), C₄H₆ (butyne), C₅H₈ (pentyne)

Two tests for experimentally distinguishing between an alcohol and a carboxylic acid are as follows:

a) Baeyer's reagent test: When alcohol is treated with few drops of a 5% alkaline potassium permanganate solution and kept in a hot water bath, the colour of potassium permanganate disappears. On the other hand, carboxylic acids do not show such reaction.

b) Sodium hydrogen carbonate test: Carboxylic acid on reaction with a solution of baking soda gives brisk effervescences due to evolution of carbon dioxide gas. When this gas is passed in lime-water, the later turns milky. Whereas, alcohol do not give effervescence with baking soda solution.

(a) All the three elements have a valency of 2. Therefore, they should belong to second group of the periodic table.

(b) Reactivity increases down the group due to increase in size. Therefore, ₄Be is least reactive among the three elements.

(c) Atomic size increases down the group due to addition of new shell. Therefore, ₂₀Ca has the largest size among the three elements.

In the modern periodic table, elements are arranged in 18 vertical columns called groups and 7 horizontal rows called periods. Elements with similar electronic configurations are placed in the same column. Number of valence electrons decides the group of the element and number of shells decides the period of the element.

The valency of an element is determined in the following manner:

a) If the number of electrons in the valence shell is less than 3, then the same number of electrons will represent the valency of that element.

b) If the number of valence electrons is either equal to or greater than 4, then that number is subtracted from 8 to obtain the valency of that element.

An element of atomic number 16 has the electronic configuration: 2, 8, 6
It has six valence electrons and three shells. So, its valency is two.
It is positioned in Period 3 and Group 16.