Class 9 Science Chapter 2 MCQ

A2B is a compound made up of two elements A and B in a fixed ratio. The properties of a compound (e.g. A2B) are entirely different from those of its constituent elements (e.g. A and B). The composition of a compound is fixed.

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.

Metals are good conductors of heat and electricity. This refers that metals allow heat and electricity to pass through them easily. Metals are generally good conductor of heat. Silver metal is the best conductor of heat. Gold and silver have the highest thermal conductivity. Copper and Aluminium metals are also very good conductors of heat.

Generally, metals have high densities this refers that metals are heavy substances. As an example, the density of iron metal is 7.8 gram per centimetre cube which is quite high. There is some exception such as sodium and potassium metals have low densities. Thus, they are very light metals.

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

It is the properties of colloids that the size of particles in a colloid is bigger than those in a true solution but smaller than those in a suspension. It is between 1 nm and 100 nm in diameter. The particles of most of the colloids or colloidal solution cannot be seen even with a microscope.

A soap solution scatters a beam of light passing through it and renders its path visible, therefore, soap solution is a colloid (or colloidal solution).

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.

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.

i) If the substance does not have a fixed melting point, boiling point, etc., it is a mixture. ii) If the substance has a fixed melting point, boiling point, etc., it is considered as a compound.