Dry HCl gas does not contain any H⁺ or H₃O⁺ ions, so it does not show any acidic property. Hence, it does not change the colour of dry litmus paper. To show its acidic behaviour, it needs wet litmus paper.

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.

(i) The twinkling of a star is due to atmospheric refraction of starlight. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the actual position. This apparent position of the star is not stationary but keeps on changing slightly as the physical conditions of the earth's atmosphere are not stationary.
(ii) Since the stars are very distant, they act as point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount light entering the eyes flickers-the star sometimes appears brighter and at other times appears fainter, which is the twinkling effect.

It reaches only to target organs and it cannot recreate the signal again

Since, the child cannot see distant objects, it means that he is suffering from myopia. The image is formed before the retina in case of myopic eye. It can be corrected by using concave lens of suitable focal length.

Dilution of a concentrated acid is a highly exothermic reaction and a lot of heat is generated. Care must be taken while mixing concentrated acid with water. The acid must always be added slowly to water with constant stirring. If water is added to the concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.

Distilled water does not contain any ions and have no electrolytes. Hence, it does not conduct electricity. But in rain water, a small amount of electrolyte is present in the form of acids. These acids are produced due to dissolution of acidic oxides (e.g. SO₂, NO₂) in rain water and make it a better conductor of electricity.

Differences between movement of leaves and movement of shoot towards light are as follows:

(1) Since the light rays from a nearby object are focused at a point behind the retina, the defect shown in the ray diagram is hypermetropia.
(2) This defect arises either because the focal length of the eye lens is too long or the eyeball has become too small.
(3) To correct this defect of vision, a convex lens is used, which has the ability to converge incoming rays for forming the image on the retina.
(4) Since a convex lens has the ability to converge incoming rays, it can be used to correct this defect of vision. The ray diagram for the corrective measure for a hypermetropic eye is shown in the given figure:

(i) For the swimmer the aeroplane will appear to be higher than it actually is. As water is a denser medium, so when light travels from water to air, i.e. denser to rarer, it will bend away from the normal as a result of which aeroplane will appear to be higher than its actual position.
(ii) Atmospheric refraction
(iii) Stars seem to be at higher distance than they actually are.

Alcohols and glucose contain hydrogen but are not categorised as acids because they do not ionise in the solution to produce H⁺ (aq) ions and hence can't conduct electricity.
Activity Set the apparatus as shown below and test for the solution of HCI, alcohol and glucose.
Observation
(i) The bulb glows when dil. HCI was added to beaker.
(ii) The bulb did not glow when alcohol and glucose solution was added to the beaker.
Conclusion
(i) Alcohol and glucose cannot form ions in solution and hence, can't conduct electricity.
(ii) Dil. HCl solution forms (aq) H⁺ ions and hence, can conduct electricity suggesting that electric current is carried through the solution by ions formed by them.

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.

Functions of a neuron
The neuron receives information from receptors as electrical impulse present at its dendritic end. The impulse then travels from dendrite to the cell body and further along axon to its end. At the end of axon, electrical impulse leads to the release of some chemicals.
These chemicals cross the synapse and reach the next neuron. This is how nerve impulses travel through the body. Thus, neurons are important in receiving information from the surroundings and in sending it to the effector.

Working of Human eye
Human eye consists of various parts which helps us in seeing the objects, the function of various parts are:
(a) Cornea: It is the transparent membrane which refracts the light entering our eye.
(b) Iris: Iris controls the size of pupil.
(c) Pupil: It allows the light entering our eye to pass through it.
(d) Lens: Refracts the light of the objects at different places on retina.
(e) Ciliary muscles: Helps in changing the focal length of the lens.
(f) Retina: It is the screen of the eye on which image is formed. It consists of rods and cones .
(g) Optic nerve: It carries the electrical signals from retina to brain.