Alloy is a homogeneous mixture of two or more metals. One of them can be a non- metal also. Solder cons its of lead and tin. It has low melting point which makes it suitable for welding electrical wires.

(i) Testes in males produces hormone testosterone. Ovaries in females produces hormone oestrogen
(ii) Pituitary gland present in the brain is responsible for body growth, development of bones and muscles. If the release is in excess it causes gigantism and if it is produced less then it causes dwarfism.

(i) Positive phototropism: Shoots growing towards light.
(ii) Negative phototropism: Roots growing away from light towards ground.
(iii) Positive geotropism: Growth of roots towards earth due to the pull of the earth, i.e. gravity
(iv) Negative geotropism: Shoots growing away from gravity.
(v) Hydrotropism: Roots growing towards the source of water.
(vi) Chemotropism: Growth of pollen tubes towards the ovary

'X' is more reactive than 'Y' because it displaces 'Y' from its salt solution.

X - Carbon; Y - Diamond and Z - Graphite

Comparison between hormonal mechanisms are as follows :

Auxins promote cell elongation and are present at the shoot tip. When tendril comes in contact with a support, auxin stimulates faster growth of the cells on the opposite sides. Thus, the tendril coils around the support.

Cerebellum is the part of hindbrain that maintains posture and equilibrium of the body.

Hydrogen gas is produced when dilute hydrochloric acid is added to a reactive metal.
Fe(s)    +    H₂SO₄ (aq)      \(\rightarrow\) FeSO₄ (aq)    + H₂(g)
Iron        Dil. Sulphuric acid  Ferrous sulphate   Hydrogen 

Sodium metal being reactive highly reacts so vigorously with oxygen that it catches fire if kept in open air. Therefore, to protect it from accidental fires, sodium is kept immersed in kerosene oil.

The structural and functional unit of the nervous system is called neuron

Two functions of neuron: .
(a) Neurons receive signals from a sense organ.
(b) Neurons send signals to a muscle or a gland.

(i) The nature of the solution is acidic. It is a mixture of conc. HNO₃, and conc. HCI in the ratio of 1 : 3. It can dissolve gold as it is an oxidising agent.
(ii) Pure gold is 24 carat gold which is very soft.

The directional movement of the plant in response to light is called phototropism. The shoots respond by bending towards light and roots respond by bending away from the light.
This happens as follows:
(i) When sunlight falls straight on the plant, the auxin hormone synthesised at the tip of the stem spreads uniformly down the stem and due to equal concentration of auxin, stem grows straight.
(ii) When sunlight falls on only one side of the plant, the auxin diffuses towards the shady side of shoot. The concentration of auxin stimulates the cells to grow longer. Therefore, the stem appears to bend towards the source of light in the directional movement
 

(i) B is the most active metal as it displaces iron from its salt solution.
(ii) B will displace Cu from CuSO₄ solution because B is more reactive than copper.
(iii) B > A > C > D.