important questions from all chapters of chemistery of class 9th (CBSE) that mainly comes in annual exam
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Answer:
What is isotopes and isobar. Atomic mass and atomic number
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MOST COMMON QUESTION ASKED IN EXAM..
Given below are some very important long answer type questions for CBSE class 9 Science exam:
Question 1. (a) Which method will you use to separate : (i) components of air (ii) acetone and water? Explain each.
(b) What is the difference in the apparatus used for distillation and fractional distillation?
Answer.
(a) (i) Separation of constituents of air by fractional distillation:
Air consists mainly of nitrogen and oxygen. Carbon dioxide and other gases are also present in trace amounts. To separate these different gases first we need to liquify the air. When the air is compressed and cooled down, at 0°C, water present in air freezes to ice. On further cooling, carbon dioxide separated as dry ice at about -78°C. on further cooling under pressure, air gets liquefied at -200°C. After this, through fractional distillation each gas can be separated on the basis of their boiling points. The gas having lower boiling point will be obtained first and the gas having highest boiling points will be obtained at last. Nitrogen has a boiling point of -196°C while oxygen has -183°C. The nitrogen gas will start to escape through the outlet and it is collected. The liquid oxygen will be collected in the fractionating column.
(ii) Separation of acetone and water:
It can be done by simple distillation method. In this method, the components of mixture are separated on the basis of difference in boiling points of constituent liquids. First, the lower boiling point liquid will start boiling and its vapours can be collected and condensed to obtain the liquid. In this case acetone will come out first and left over will be water only.
(b) Fractional distillation uses a complex apparatus with a fractionating column which is long tube packed with metal wire, metal ribbon or glass beads that give extra condensing surfaces, allowing the liquid to vaporize and condense at every minute change reduce in temperature.
Simple Distillation includes a simple apparatus with a flask to contain the mixture, a condenser and a flask to collect purified components.
Question 2. (a) List any four properties of a colloid and mention any two properties in which colloids differ from suspension.
(b) State what is Tyndall effect ? Which of the following solutions will show Tyndall effect ? Starch solution, sodium chloride solution, Tincture iodine, air
Answer.
(a) Properties of a colloid (any four)
(i) A colloid is a heterogeneous mixture.
(ii) The size of particles of a colloid is too small to be individually seen by naked eyes.
(iii) Colloids are big enough to scatter a beam of light passing through it and make its path visible.
(iv) They do not settle down when left undisturbed, that is, a colloid is quite stable.
(v) They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation can be used to separate the colloidal particles.
Two properties in which colloids differ from suspension are (ii) and (iii) as the particles of a suspension are large enough to be observed with a naked eye and these particles settle down well when the mixture is left undisturbed.
(b) Tyndall effect is the phenomenon of scattering of a beam of light by the particles of a colloid.
Starch solution and tincture iodine are colloid and thus will show Tyndall effect.