The Respiratory system (or ventilatory system) is the biological system that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveolar region of the lungs. Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes anatomical features such as holes on the undersides of leaves known as stomata.

Enzymes are complex chemical compounds made up of proteins which act as catalysts in our body i.e. the enzymes increase the rate of chemical reactions taking place in our body. For example, the digestion of food in our takes place in 4-5 hours due to action of enzymes. In the absence of these enzymes it is estimated that the digestion of food will take place in approximately 50 years.

The chemical compounds that work on the surface of a liquid and reduce its surface tension are called surfactants. In simple words we can say that surfactants are those chemicals that increase surface area of a liquid and enhance its wettability by reducing its surface tension. E.g. Synthetic detergents and soaps etc. Almost all the synthetic detergents act as surfactants. They diminish the surface tension of water and help in deep penetration of water molecules in oil, grease and dirt particles present in dirty clothes. Thus surfactats help in dissolution and removal of oil, grease and dirt particles from dirty clothes.

We shall understand electrolytic refining of metals by taking the example of refining of copper. In case of copper, a thick block of impure copper is made anode and a thin block of pure metal is made cathode and copper sulphate solution is used as an electrolyte. On passing electric current, pure copper metal from the electrolyte solution deposits on the cathode. At the same time an equal amount of impure copper dissolves from anode into the electrolyte solution. The soluble impurities settle down in the solution below the anode and are called as anode mud.
Many metals like Cu, Zn, Pb, Cr, Ni, Ag, and Au are refined electrolytically for refining of an impure metal by electrolysis.

This method is used for concentration of sulphide ores of copper, lead and zinc. In this method, powdered ore is put in a tank full of water. And then, some Pine oil is added to it. In the tank the particles of sulphide ore are wetted by pine oil whereas the gangue particles are wetted bywater. Then air is passed through this mixture. This results in the agitation of water in tank, which cause the sulphide ore particles to stick with the oil and rise to the surface in the form of froth. The gangue particles being heavier remain behind at the bottom of water tank. The froth is separated and concentrated sulphide ore is obtained from it. 

Metals are the elements which can form positive ions by losing electrons. For example, Na is a metal because it can form positive ion (Na+) ion by losing an electron from its outermost shell. In the same way Iron, aluminium, copper, gold, silver, magnesium, potassium and calcium are also metals because all of these elements can form positive ions by losing electrons.

It should be noted that the oxides of metals are basic in nature. All metals are solid at room temperature except mercury which is liquid metal.

Almost all metals react with oxygen to form metal oxides. But different metals react with oxygen at different intensities. For example, sodium metal is always kept immersed in kerosene oil. Because, if we keep it open, it reacts so vigorously with oxygen present in air that it catches fire. We have already discussed that the oxides of metals are basic in nature. As all the metals have different reactivity so they combine with oxygen at different temp. 

On heating, magnesium metal burns in air giving magnesium oxide. 

Every living organism requires food to fulfill the requirement of energy for their various activities. Green plants are autotrophic in nature. So, they synthesize their food by a very important process called photosynthesis. The term ‘photosynthesis’ consists of two words ‘photo’ means ‘light’ and ‘synthesis’ means ‘to make’. Thus, photosynthesis is the process of making food with the help of sunlight.

It may be defined as the process of making food by green plants from carbon dioxide andwater by chlorophyll in the presence of sunlight. In the process of photosynthesis oxygen gas is liberated.  The process of photosynthesis takes place in leaves of a plant, because these contain the green colouring pigment chlorophyll which is necessary for synthesis of food in plants. Thus, leaves are the site of preparation of food in plants. That is why; leaves are also called food factories of a plant.

The carbon dioxide required for photosynthesis is obtained from the air which enters the leaves through very minute pores called stomata. But, aquatic plants use carbon dioxide dissolved inwater. The water is absorbed from the soil by the roots of plants and transported to leaves by a network of tiny veins called xylem vessels. The sunlight energy is absorbed by chlorophyll. A chemical reaction using the trapped sunlight energy takes place in the chloroplasts (chlorophyll containing organelles of cells) to prepare food. The food prepared by the leaves of a plant is usually in the form of carbohydrates.

An atom consists of three fundamental particles: electrons, protons and neutrons. The electrons have negative charge; protons have positive charge while the neutrons have no charge. Structurally, an atom is divided into two parts: nucleus and extra nuclear portion. The nucleus is a tiny, heavy sphere which is present at the centre of an atom. The nucleus of an atom consists of protons and neutrons. Nucleus is positively charged due to the presence of protons in it. The diameter of nucleus ranges from 10^-5 to 10^–4 m.

The extra nuclear portion consists of negatively charged electrons which revolves around the positively charged nucleus in circular paths called orbits. In an atom, the number of electrons is always equal to that of protons. So, atoms are always electrically neutral.

Usually all metals react with water to produce metal oxide (or metal hydroxide) and hydrogen gas. But, all metals do not react with water at equal intensity. The metals which are very reactive can react even with cold water while the other metals react with hot water or with steam. 

For example: Sodium, potassium and calcium metal can react with cold water to produce their hydroxides and hydrogen gas.