CBSE class 9 th chemistry extra questions for SA2 exams
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Q1. Define polyatomic ion . Give one example.
Answer. As it is evident from its name polyatomic ion, poly means many. A polyatomic ion is defined as a group of atoms which are bounded by covalent bond have a net charge, monoatomic ions are atomic version and polyatomic ions are molecular version, in monoatomic ions, the number of protons and electrons are unequal, on the same way the number of electrons and protons are unequal in a group of covalently bonded atoms, so they have a net charge in a single roof. Polyatomic ions are also of two kinds cation polyatomic ion and anion polyatomic ions, the example of cation polyatomic ion is and anion polyatomic ion is .
Q2. What is the law of mass conservation and law of constant proportions?
Answer. The law of conservation of mass states that neither the mass is created nor it is destroyed during a chemical or physical change, the mass of products participating in a chemical reaction is always equal to the mass of reactants during a chemical reaction. This principle is used in the chemical industry to predict the amount of products formed from a particular amount of reactants. As an example, in the following reaction, the mass of reactants hydrogen and oxygen is equal to the mass of product water.
4×1 + 2×16 → 2(2×1 + 16)
36 u = 36 u
Law of constant proportion- The elements combined in a definite proportion during the formation of a compound, whether it is formed by any method or found anywhere within the universe. The law of constant proportion is applied to decide the proportion of elements required to produce a compound. As an example, the ratio of hydrogen and oxygen in water is 1 : 8 explained bellow, whether it is formed by any method or found anywhere within the universe.
In the mass of hydrogen is 2×1 = 2u and the mass of oxygen is =16u, the ratio of hydrogen and oxygen is 2 : 16= 1: 8
Q3. Define the term valency. What is the valency of magnesium and copper?
Answer. The tendency of an atom to make octet in its outermost orbit by the way of exchange of electrons is known as valency, valency is actually holding capacity of an atom of the element while combining with other elements. The way of evaluating the valency of an element is determined by the valence electrons of an atom. If the number of valence electrons is 4 or less than 4 the valency is as it is as the number of valence electrons but if there are more than 4 valence electron then that number of valence electrons must be subtracted from 8 to get the valency.
Magnesium, Mg, atomic number(12), its electronic distribution = 2,8,2 s0 in its outermost orbit the electrons are 2, therefore the valency of magnesium is 2.
Copper, Cu, atomic number(29), its electronic distribution = 2,8,18,1, hence its valency is 1 but it is one of the transition elements so it has variable valency, it has two valencies 1 and 2.
Q4.What is atomicity? What is the atomicity of phosphorus and nitrogen?
Answer. The number of atoms, a molecule of an element is made up of, known as atomicity of the element. The phosphorus molecule is made of 4 atoms and nitrogen molecule is composed of 2 atoms, therefore the atomicity of phosphorus is 4 and the atomicity of nitrogen is 2.
Q5.Write three isotopes of the hydrogen.
Answer. Three isotopes of the hydrogen are which are called protium, deuterium, and tritium respectively.
Q6.According to you, among different atomic models which atomic model is correct and why?
Answer. According to Rutherford’s atomic model electrons revolve around the nucleus made of positive charge, but electrons are charged particles so it is assumed that during it orbits around the nucleus it will radiate energy and ultimately collapse at the nucleus and the atom finish while atom is a stable particle, this drawback of Rutherford’s atomic theory gave birth to Neils Bohr’s atomic model, he modified Rutherford’s theory.
According to Neils Bohr’s theory of atomic model.
- Electrons revolve around the nucleus in discrete orbitals called energy cell or energy level, in these discrete orbitals electrons, don’t lose energy and do not fall into the nucleus.
- The energy of an electron in these orbital is stable but during the exciting stage, the electron can jump to the higher energy level where its energy increases and when atom returned to the original state the electron jump from higher energy level to lower energy level where the energy of electron decreases.
- The number of electrons around the nucleus in different orbitals accommodated by the formula 2n² where n is a positive integer,n=1,2,3,4……..
- Neils Bohr’s named the orbitals around the nucleus K,L,M,N…..etc.
Neils Bohr’s model is most accurate because it explains the stability of the atom, the valency of elements can easily be explained from Neil’s Bohr’s atomic model.
Q6. Give the difference between isotopes and isobars.
Answer. (i) Isotopes have the same number of protons but different atomic mass as an example have 1 proton and their atomic masses are 1,2 and 3 respectively whereas Isobars have same atomic mass and different atomic number or number of protons as an example have their same atomic mass and different atomic number.
(ii) Isotopes have the same chemical and physical properties whereas Isobars have different chemical and physical properties as example calcium is a solid and argon is a gas.
(iii) Isotopes have the same electronic configuration because the number of protons are equal to the number of electrons whereas Isobars have different electronic configuration because of the different number of electrons in them.
(iv) Isotopes have the same atomic numbers so they are placed on the same position in the periodic table whereas Isobars are placed on different positions in the periodic table.
(v) Isotopes have many applications as an example U-235 is used as fuel in a nuclear reactor, I-131 is used in the treatment of goiter, co-60 is used in the treatment of cancer but Isobar has no any significant applications.
Q7.You are given two samples of water labeled as A and B.Sample A boils at 100 °C and sample B boils at 102°C . Which sample of the water will not freeze at 0°C? Comment?
Answer. The boiling point of pure water is 100°C, therefore sample A is of pure water and since the impurities in a liquid increase its boiling point so the water in sample B will be impure and further it is known to us that freezing point of pure water is 0°C when impurities are added to it, its freezing point decreases, therefore water of sample B will freeze before the temperature reduced to zero, in other words, the water of sample B will not freeze at 0°C.
Hint: The boiling temperature of the water is increased when impurities is added on it because the concentration of mixture increases which affects intermolecular forces to be increased between the molecules of water hence boiling point is increased and freezing point decreased.
Q8. A group of students took an old shoe box and covered it with a black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample contained in a beaker/tumbler in the box as shown in the fig bellow. They were amazed to see that milk taken in the tumbler was illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it?
(a) Explain why the milk sample was illuminated. Name the phenomenon involved.
(b) Same results were not observed with a salt solution. Explain.
(c) Can you suggest two more solutions which would show the same effect as shown by the milk solution?
Answer. (a) The milk sample was illuminated because milk is a colloidal mixture, the size of colloidal particles is large, these particles scatter the light resulting in particles of the milk illuminated. Such an effect is known as a Tyndal effect.
Study midpoint theorem
(b) The particles of salt solutions are very less in size as compared with colloid, and hence are unable to scatter sunlight, thus light passes through the solutions without showing the Tyndal effect.
(c) The Tyndal effect can be seen through the suspension of water and flour, a beam of light could be seen through fog, the particles of water droplets scatter the light.
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