Class 9 Science Chapter 2 MCQ

In this reaction, X and Y cannot be broken down into simpler substances by chemical reactions, therefore, x and y are elements. A compound is a substance made up of two or more elements chemically combined in a fixed proportion by mass, therefore, P is a compound, having a fixed composition. Note: An element is a substance which cannot be broken down into two or more simpler substances by simple chemical methods of applying heat, light or electric energy.

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.

Actually, rusting of an article, made up of iron is called corrosion. Corrosion is a chemical change because rust is a chemical compound (hydrated iron oxide) which is totally different from element iron (Fe). Physical changes are those changes in which no new substances are formed. The substances do not change their identity and they can be easily returned to their original form by some physical processes. Example – melting of ice, glowing of an electric bulb, breaking of glass tumbler etc. On the other hand, chemical changes are those changes in which new substances are formed. The substances involved change their identity and they get converted into entirely new substances, which cannot be returned to their original form i.e. burning of paper, rusting of iron, burning of magnesium wire etc. Note: Dissolution is the process in which a solute is dissolved in a solvent to form solution.

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.

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

The chemical changes are permanent changes which are usually irreversible.

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.

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.