Matter in Our Surroundings — Science STD 9 — Question

When water is cooled to a temperature x, it gets converted into ice at temperature x by a process called P. And when ice at temperature x is warmed, it gets reconverted into water at the same temperature x in a process called Q.

1. What is the value of temperature x in Kelvin?
2. What is the process P known as?
3. What is the name of energy released during process P?
4. What is the process Q known as?
5. What is the name of energy absorbed during process Q?

Explain the formation of (i) sodium ion, and (ii) chloride ion, form their respective atoms giving the number of protons and number of electrons in each one of them. What is the reason for positive charge on a sodium ion and a negative charge on a chloride ion?

The atomic number and the mass number of certain elements are given below in the table.

Element	A	B	C	D	E	F
Atomic number	1	3	8	18	20	1
Mass number	2	7	16	40	40	4

1. Select a pair of isobar and isotope from the above table.
2. What would be the valency of elements B and E?

What is the difference between the molecule of an element and the molecule of a compound? Give one example of each.

The nucleus of an atom has 5 protons and 6 neutrons. What would be the:

1. Atomic number.
2. Mass number.
3. The number of electrons.
4. The number of valence electrons, per atom of this element?

What are isobars? Give one example.

Why does a gas exert pressure?

Which produces more severe burns: boiling water or steam? Why?

What is an octet? How do elements reach an octet?