Class 9 Science Chapter 2 MCQ

Rusting of iron takes place in the presence of air (O2) and water vapour. Action of air and moisture on iron converts it to its oxide. This process is called corrosion and it leads to the formation of a new compound. Therefore, it is a chemical change.

Sulphur dissolves completely in carbon disulphide. So, it forms a homogeneous solution. As the particles of solute are too small to scatter light, they will not cause tyndall effect.

Melting of iron metal, bending of an iron rod and drawing a wire of iron metal are physical changes, because in three processes, iron changes its form, not the chemical composition. In rusting of iron, its chemical composition is changed.

Here, all these statements are correct. Hence your answer will be option [D].

Here, are all the statements are correct. Hence, your answer will be option [D].

In a true solution (like sugar solution), the solid particles are so small that they cannot scatter light rays falling on them. For example, if a beam of light (from a torch) is put on true solution (like sugar solution) kept in a beaker in a dark room, the path of light beam is invisible inside the solution when seen from the side. The beam of light can become visible only when the solid particles are big enough to reflect light falling on them. Since the particles of a true solution do not a scatter light.

The use of process of evaporation for separating a mixture is based on the fact that liquids vaporize easily whereas solid do not vaporize easily. The changing of a liquid into vapors or gas is called evaporation. Evaporation is used to separate a solid substance that has dissolved in water or any other liquid. The dissolved substances is left as a solid residue when all the water or liquid has evaporated.

The separation of a dissolved solids from a liquid can be done by a number of other methods such as evaporation, crystallization, distillation and chromatography etc.

Crystallization is a better technique than evaporation of dryness because some solids decompose or get charred on heating to dryness during evaporation. There is no such problem in crystallization. The soluble impurities do not get removed in the process of evaporation, but such impurities get removed in crystallization.

The process in option [B] is a chemical change because rusting causes formation of a new compound. Other statements are examples of physical changes where no new compounds are formed. In the process mentioned in other statements, original form of iron can be obtained back from moulded forms.