The first menstrual flow begins at puberty and is termed menarche. The ova begin to mature with the onset of puberty.

The fusion of Male and Female gametes (sperm, ovum) is called fertilization. In human, fertilization occurs in fallopian tubes of women's ovary.

Secondary Sex characteristics are the physical features which distinguish male from female
Example:- Voice - In Male - husky voice. 
In Female - shrill voice.

(i) A band of strong fibrous tissue is called ligament.
(ii) It connects bone to bone. 

Our backbone which comprises of vertebrae is a cartilaginous gliding joint whịch allows limited movements. So our backbone is slightly movable.

Valency of an element can be defined as the number of hydrogen atoms which combine with one atom of it.

i) Anode rays travel in straight lines.
ii) Anode rays are made up of material particles.
iii) Anode rays are deflected by electric and magnetic fields.
iv) The properties of anode rays depend upon the nature of the gas taken inside in the discharge tube.
v) The mass of the particle is the same as the atomic mass of the gas taken inside the discharge tube.

Law of constant proportion: In a pure chemical compound the elements are always present in definite proportions by mass.

The increased green house effect is caused due to increase in the air pollutants and it results in the average increase of temperature of the atmosphere. This is Global warming.

Nitrogen reacts with non-metals like hydrogen, oxygen etc at high temperature to form their corresponding nitrogen compounds
\( 3 \mathrm{H}_{2}+\mathrm{N}_{2} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{NH}_{3} \)

Uses of Nitrogen
(i) Liquid nitrogen is used as a refrigerant.
(ii) It provides an inert atmosphere for conducting certain chemical reactions.
(iii) It is used to prepare ammonia (by Haber's process) which is then converted into fertilizers and nitric acid.
(iv) It is used for inflating tyres of vehicle.
(v) It is used for filling the space above mercury in high temperature thermometer to reduce the evaporation of mercury
(vi) Many explosives such as TNT, nitroglycerin and gun powder contain nitrogen.
(vii) It is used for the preservation of fresh foods, manufacture of stainless steel, reducing fire hazards and as part of the gas in incandescent light bulbs.

The process of depositing a layer of one metal over the surface of another metal by passing electric current is called electroplating.

The path through which electrons flow from one terminal to another terminal of the source is called electric circuit.

Earthing is the process of connecting the exposed metal parts of an electric circuit to the ground with the help of low resistance wire. It is a safety measure devised to prevent people from getting shocked if the insulation inside electrical devices fails.

Conduction is the process of heat transfer in solids from the region of higher temperature to the region of lower temperature without the actual movement of atoms or molecules

Three types ofheat transfer:
(i) Conduction
(ii) Convection
(iii) Radiation.

Effects of heat:
When heat energy is supplied to any substance, it brings about many changes.
The three important changes are:
(i) Expansion
(ii) Increase in temperature
(ii) Change in state.