Hydrochloric acid is used in combination with nitric acid to form a mixture called aqua regia. Aqua regia is a highly corrosive and effective mixture used to dissolve gold and platinum, which are resistant to single acids. The nitric acid acts as an oxidizing agent, while the hydrochloric acid provides chloride ions that form soluble complexes with gold, enabling its extraction.

Litmus is a water-soluble mixture of different dyes extracted from lichens. The main use of litmus is to test whether a solution is acidic or basic. Blue litmus paper turns red under acidic conditions, and red litmus paper turns blue under basic or caustic conditions.

A reduction is a process in which electronegative atoms are removed. In chemistry, Reduction is a reaction that adds electrons to a substance.

Oxidation reaction is a chemical process. It is defined as a process that occurs when a chemical species gains oxygen or loses hydrogen. In electronic terms loss of electrons is called oxidation.

A chemical change occurs when the substance's composition is changed. Cooking rice is a chemical change because during this process changes in the chemical properties of rice occur and we get a new substance that is cooked rice.

A physical change involves a change in physical properties, such as melting, transition to a gas and change of strength. For burning candle, wax melts which is known as physical change. Also the wax near the flame burns and generates CO2, light and heat which is a chemical change. Hence, burning of candle is a physical as well as chemical change.

Atoms are electrically neutral because they have equal numbers of protons (positively charged) and electrons (negatively charged). If an atom gains or loses one or more electrons, it becomes an ion. If it gains one or more electrons, it now carries a net negative charge, and is thus "anionic."

Hydrogen is the lightest and simplest element on earth ever found. The atomic number and mass number of hydrogen is the same.

A dry cell is an electric battery comprises of zinc anode (cylindrical drum-acts as negative electrode), a piece of carbon cathode (central rod-acts as positive electrode), electrolyte (ammonium chloride in the form of paste) and the vacuum is filled up with manganese dioxide. 

The combination of two nuclides is called isotope if they have same proton number-Z, but different neutron (mass) number-N. Example: Carbon-12, Carbon-14.