The concept of chemical element is in relationship with the chemical substance. The chemical element is a substance which is having a single type of atom. It is noticed by a particular number of protons in the nucleus of its atoms. This number is termed as the atomic number of the element.

It is the quantity of an element  whose mass in grams is numerically equal to its atomic mass. In simple terms, atomic mass of an element expressed in grams is the gram atomic mass or gram atom.For example, the atomic mass of oxygen = 16 amu therefore gram atomic mass of oxygen = 16 g. The gram atom should not be confused with the mass of one atom of the element in grams, which is the actual mass of the atom.

All around us are substances that are made of matter. Matter has mass and occupies some space. Different kinds of substances are made up of different matter. Matter can be thus be classified in many ways. Two basic classifications of matter are based on its physical and chemical properties. The physical classification of matter shows its physical appearance that describe the shape, hardness, softness, melting and boiling points. The chemical classification of matter shows the ability of the matter to change its composition to form new substances.

A compound is a pure substance made up of two or more elements combined in a definite proportion by mass, which could be split by suitable chemical methods.

A mole is the amount of a substance which contains many elementary entities like atoms, molecules or ions. It is a pure number, having no units and can describe any type of elementary object.The number of moles of a substance present in one litre of a solution is known as molarity.

When a solid is in the form of a Lamina, its length and breadth increases on heating. Hence, its area also increases. This is known as the superficial expansion.

If a metal wire is used in a pendulum, its length changes with temperature. During summers, its length increases and thus increasing its period and the pendulum clocks go slow in summer.

In perfect or ideal gas the change in density is directly related to the change of temperature and pressure as expressed by the Ideal Gas Law. PV=RT is the gas equation for one mole of gas.

Ice has a relative density of 0.92. When ninety-two volumes of water at 4^oC is frozen, it will expand to hundred volumes. Thus, ice is lighter than water and hence it floats on water.

Liquid thermometers depend on the thermal expansion of liquids such as mercury, alcohol etc. The commonly used liquid thermometer is mercury thermometer which uses mercury as the thermometric liquid. The thermometer consists of a cylindrical glass bulb containing mercury to which a graduated capillary tube of uniform bore is attached. The change in temperature is shown by the apparent expansion of mercury in glass and the resulting movement of the mercury meniscus in the system of the thermometer.As mercury freezes at -38.8 ⁰C and boils at 356⁰ C, the mercury thermometer can not be used for very low or very high temperature measurements. A mercury thermometer can be used to measure temperatures up to 300⁰C

When certain salts form crystals with a definite number of molecules of water, they chemically combine to do so in a definite proportion. A salt containing water of crystallisation is called a hydrous salt or a hydrate. This water can be expelled out also.e.g.CaCl₂, 6H₂O,Na₂CO₃, 10 H₂O etc.

Biogas is a clean and efficient fuel. It is a mixture of methane (CH₄), carbon dioxide (CO₂), hydrogen (H₂) and hydrogen sulphide (H₂S).The chief constituent of biogas is methane (65%). 

 On heating from 0^oC upto 4^oC its volume decreases, reaching a minimum at 4^oC, such that the density of water is maximum at 4^oC. On heating further, water behaves like any other liquid. Similarly, during cooling process, the volume of water decreases up to 4^oC, reaching a minimum at 4^oC. If further cooled upto 0^o C, its volume increases instead of decreasing and hence, density of water at 0^oC is lesser than that at 4^oC.

Solid NaCl is a norml salt. Salts which produce one simple cation and one simple anion in aqueous solution  are called normal salts.The ions present in simple salt can be tested easily.

Molality is another common measurement of concentration. Molality is defined as the moles of solute divided by kilograms of solvent (the substance in which it is dissolved, like water).Molality is sometimes used in place of molarity at extreme temperatures because the volume can contract or expand.

Soil covers a major portion of the earth's land surface. It is an important natural resource that either directly or indirectly supports most of the planet's life. Life here depends upon soil for food. Plants are rooted in soil and obtain needed nutrients from it. Animals get their nutrients from plants or from other animals that eat plants. Many animals make their homes or are sheltered in the soil. Microbes in the soil cause the breakdown and decay of dead organisms, a process that in turn adds more nutrients to the soil. It does not pollute water.

A colloid is a solution in which the particle size ranges between 10^-7 and 10^-5 cm.For example, milk, blood, honey, smoke, ink, gum, starch solution etc.

A true solution is a homogeneous solution in which the solute particles have diameters less than 10^-7cm. i.e., the solute particles are of molecular dimensions. The particles are invisible even under powerful microscopes.For example, sodium chloride in water is a true solution. Most ionic compounds form true solutions in water. Organic compounds like sugar and urea also form true solutions in water.

H₂O is a compound because, 1.The components hydrogen and oxygen cannot be separated by physical methods such as filtration, evaporation. 2.Hydrogen and oxygen are present in a fixed proportion of 1: 8. 3.Energy changes accompany the formation of a compound i.e., heat and light are given out. 4.Properties of water are entirely different from the constituents, hydrogen and oxygen. 5.The boiling point of water is 100^oC at 76 cm Hg i.e., one atmospheric pressure

The property by which certain hydrous crystals lose their water of crystallisation, completely or partially, when exposed to air and crumble into a powder is called efflorescence.for examples Na₂SO₄.10H₂O.Such substances are termed as efflorescent substances.

Atoms in a  molecule are not a type of a mixture.The number of atoms in a molecule is the atomocity of the molecule. 

This branch deals with the qualitative and quantitative analysis of various substances.

A mixture of  water and salt is an impure substance since the ratio of combining elements is not fixed.

Sublimation is a direct change from the solid state to gaseous state, without passing through the liquid state. If a mixture contains two solids, one sublimable, and the other non-sublimable, they can be separated by the process of sublimation. The solid that sublimes, escapes in the form of vapours. When these vapours are collected on a cold surface, they condense to form the sublimable solid, known as the sublimate.

Certain hygroscopic substances, not only absorb moisture from the air, but also dissolve themselves in that moisture, undergoing a change of state. Such substances are called deliquescent substances. The process is called Deliquescence.It is a deliquescent substance.