A star sometimes appears brighter and some other times fainter. What is this effect called? State the reason for this effect.

List four advantages of water stored in the ground.

How does growing embryo get nutrition from the mother's blood?

List two advantages of vegetative reproduction practiced in case of an orange plant.

Burning fossil fuels is a cause of global warming. Justify this statement.

Define homologous series of organic compounds. List its two characteristics. Write the name and formula of the first member of the series of alkenes. 

An element 'M' has atomic number 11.

(a) Write its electronic configuration.

(b) State the group to which 'M' belongs.

(c) Is 'M' a metal or a non-metal.

(d) Write the formula of its chloride.

When we place a glass prism in the path of a narrow beam of white light a spectrum is obtained. What happens when a second identical prism is placed in an inverted position with respect to the first prism? Draw a labelled ray diagram to illustrate it.

F, Cl and Br are the elements each having seven valence electrons. Which of these (i) has the largest atomic radius, (ii) is most reactive? Justify your answer stating reason for each.

List four properties of the image formed by a convex mirror.