Acids, bases and salts are three main categories of chemical compounds. They have certain definite properties which distinguish one class from the other. 

Bases: They are bitter in taste and soapy to touch. Sea water and detergents are some examples of substances that are basic. Many bases are oxide or hydroxide compounds of metals. Bases are compounds which yield hydroxide ion (OH-), when dissolved in water. Bases are corrosive in nature. ,show a pH value of more than 7.Example-NaOH

Acid :It is a compound which yields hydrogen ion (H+), when dissolved in water. Acid is sour to the taste and corrosive in nature. The pH value for acids is less than 7. Generally, all acids readily react with metal to release hydrogen gas. For example, metal zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.HCl is an acid.

Salts:These are formed by the combination of acid and base through neutralization reaction. The acidic and basic nature of salts depends on the acid and base combined in neutralization reaction. The pH of a salt solution depends on the strength of acids and bases combined in the neutralization reaction.

Acid Base Salt Example is NaCl.

I think yes, At high temperature endothermic reactions are spontenous but at low temperature they are not spontaneous.

its written in NCERT

A racemic mixture, or racemate  has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was racemic acid, which is a mixture of the two enantiomeric isomers of tartaric acid.

A racemic mixture is a 50:50 mix of two enantiomers. No matter how many molecules are in a mixture, it is racemic if there are equal numbers of the two enantiomers

The conjugate acid of ammonia is the ammonium ion, NH₄⁺ 

The conjugate base of any species is that species less a proton. Ammonia less a proton is the amide ion, NH₂⁻

Charles's law : When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume will be directly related.This directly proportional relationship can be written as:

V ∝ T
V/ T = k , where: V is the volume of the gas, T is the temperature of the gas (measured in kelvins), k is a constant.This law describes how a gas expands as the temperature increases; conversely, a decrease in temperature will lead to a decrease in volume.

Boyle's law :The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.

Boyle's law can be stated as
P ∝ 1/ V 
PV=k, where P is the pressure of the gas, V is the volume of the gas, and k is a constant.

The equation states that product of pressure and volume is a constant for a given mass of confined gas as long as the temperature is constant.The equation shows that, as volume increases, the pressure of the gas decreases in proportion. Similarly, as volume decreases, the pressure of the gas increases.

A conjugate acid, is a species formed by the reception of a proton by a base,it is a base with a hydrogen ion added to it.

NH₄⁺ is the conjugate acid to the base NH₃

Equilibrium :which is established between the unionized molecules and the ions in the solution of weak electrolytes is called Ionic equilibrium .The condition in which the rate of dissociation of unionized molecules is equal to the rate of combination of the ion is known as equilibrium.

Ionic equilibrium is the condition in which the rate of dissociation of non-ionised molecules is equal to the rate of combination of the ions.

         Rules for Writing SI Unit

1) When writing the name of the unit, the first letter of the unit should be in small case and it should be not written in capital case. Thus the unit of length should be written as ‘meter’ and not ‘Meter.’ For example it is correct to write 5 meter and not 5 Meter. Similarly, the unit of mass is written as kilogram and not Kilogram. Some of the units are written after the name of the scientist or other person, even these units should start with small letters. Thus the unit of power should be written as watt and not Watt. Similarly other units are newton, ampere, hertz etc.

2) When writing the symbols for the SI units, if the unit is named after the person, the symbol should start with the capital letter. Thus the symbol for newton is written as N, for ampere it is A, for hertz, it is Hz, for kelvin it is K and so on. If the unit is not the name after the person, its symbol should start with the small letter. Thus symbol for kilogram is kg, for meter it is m, and so on.

This nucleus has 56 protons and 137 − 56 = 81 neutrons.

Explanation:

Two numbers are used to identify a nucleus: Atomic number -- It is number of just protons in the nucleus. The atomic the number of protons determines the number of electrons as number of protons is always equal to number of electrons. Mass number- It is the number of protons plus neutrons. This is roughly proportional to the total mass of the atom. Mass number is also a key property of atoms because we need to know the masses of atoms to understand how a given amount of material we weigh out will react. When the element is barium, that means the aomic number is 56. "barium 137" means 56 protons. And the 137 for the mass number designates 137 total protons plus neutrons. To get just the number of neutrons you take the difference between protons and neutrons, thus 137 − 56 = 81 .