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ChemistryClass 9CBSE

What is the significance of the number of valence electrons in determining an element's chemical properties?

The number of valence electrons (electrons in the outermost shell) is crucial because these are the electrons involved in chemical bonding. Atoms tend to gain, lose, or share valence electrons to achieve a stable electron configuration, typically an octet. This behavior directly dictates an element's reactivity, the types of bonds it forms, and its chemical properties.

ChemistryClass 9CBSE

How did J.J. Thomson's 'plum pudding' model describe the atom? What was its major flaw?

Thomson's 'plum pudding' model proposed that the atom was a sphere of uniformly distributed positive charge, with negatively charged electrons (like 'plums') embedded within it. Its major flaw was its inability to explain the results of Rutherford's gold foil experiment, particularly the large deflections of alpha particles, which indicated a concentrated positive nucleus.

ChemistryClass 9CBSE

Differentiate between atomic mass and mass number. Why are they usually different?

Mass number (A) is the total count of protons and neutrons in a specific atom, always a whole number. Atomic mass (or atomic weight) is the weighted average of the masses of all naturally occurring isotopes of an element, often a decimal number. They differ because atomic mass accounts for the varying abundances of isotopes and the slight mass defect from nuclear binding energy, whereas mass number is just a count of nucleons for a single isotope.

ChemistryClass 9CBSE

Explain why atoms are generally electrically neutral. What happens if an atom gains or loses electrons?

Atoms are generally electrically neutral because they contain an equal number of positively charged protons in the nucleus and negatively charged electrons orbiting the nucleus, balancing out the overall charge. If an atom gains electrons, it becomes a negatively charged ion (anion). If an atom loses electrons, it becomes a positively charged ion (cation).

ChemistryClass 9CBSE

Consider an atom with 17 protons, 18 neutrons, and 17 electrons. Identify the element, determine its atomic number and mass number, and state whether it is a neutral atom or an ion.

The element is Chlorine (Cl) because it has 17 protons (atomic number = 17). Its atomic number is 17. Its mass number is 17 (protons) + 18 (neutrons) = 35. Since the number of protons (17) equals the number of electrons (17), it is a neutral atom.

ChemistryClass 9CBSE

Draw a simple diagram representing the atomic structure of an oxygen atom (atomic number 8, mass number 16). Label the nucleus, protons, neutrons, and electrons.

The diagram should show a central nucleus with 8 protons and 8 neutrons. The first electron shell should contain 2 electrons, and the second electron shell should contain 6 electrons. All components should be clearly labeled.

ChemistryClass 9CBSE

Define valency and explain how it is related to the electron configuration of an atom.

Valency is the combining capacity of an element, indicating the number of electrons an atom will gain, lose, or share to achieve a stable electron configuration (usually a full outermost shell). It is determined by the number of electrons in the outermost shell (valence electrons); atoms tend to react to achieve an octet (or duet for hydrogen/helium).

ChemistryClass 9CBSE

Discuss the limitations of Rutherford's nuclear model of the atom that led to the development of Bohr's model.

Rutherford's model could not explain why electrons orbiting the nucleus didn't continuously lose energy and spiral into the nucleus, causing the atom to collapse. It also failed to explain the discrete line spectra observed for elements, suggesting electrons could only exist in specific energy levels.

ChemistryClass 9CBSE

How does the atomic number differ from the mass number? What information does each provide about an atom?

The atomic number (Z) represents the number of protons in an atom's nucleus and uniquely identifies an element. The mass number (A) represents the total number of protons and neutrons in an atom's nucleus. The atomic number tells us the element and the number of electrons in a neutral atom, while the mass number indicates the atom's total nuclear mass.

ChemistryClass 9CBSE

Explain the concept of electron shells (or energy levels) and how electrons are distributed within them according to Bohr's model.

Bohr's model proposes that electrons orbit the nucleus in specific, fixed energy levels called electron shells. Each shell can hold a maximum number of electrons: 2 in the first shell, 8 in the second, and 18 in the third (for the first few elements). Electrons fill the lowest energy shells first.

ChemistryClass 9CBSE

What is an isotope? Provide an example to illustrate your explanation.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers. For example, Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons) are isotopes of carbon.

ChemistryClass 9CBSE

Describe Rutherford's Gold Foil Experiment and explain how its results led to the proposal of the nuclear model of the atom.

Rutherford's Gold Foil Experiment involved firing alpha particles at a thin sheet of gold foil. Most alpha particles passed straight through, suggesting atoms are mostly empty space. However, a small fraction were deflected at large angles, and some even bounced back, indicating the presence of a small, dense, positively charged nucleus at the center of the atom.

ChemistryClass 9CBSE

Explain the fundamental differences between protons, neutrons, and electrons in terms of their charge, mass, and location within an atom.

Protons have a positive charge (+1), a mass of approximately 1 atomic mass unit (amu), and are located in the nucleus. Neutrons have no charge (neutral), a mass of approximately 1 amu, and are also located in the nucleus. Electrons have a negative charge (-1), a very small mass (approximately 1/1836 amu), and orbit the nucleus in electron shells.

BiologyClass 9CBSE

A farmer wants to separate grains from their husks after harvesting. Which physical separation method would be most suitable for this purpose, and why?

The most suitable physical separation method for separating grains from their husks is winnowing. This method is based on the principle that grains are heavier than husks. When the mixture is dropped from a height in the presence of wind or a fan, the lighter husks are carried away by the wind, while the heavier grains fall straight down, forming a separate pile. This technique is effective due to the significant difference in density and weight between the grains and the husks.

BiologyClass 9CBSE

Why is 'water' considered a compound, while 'salt solution' is considered a mixture?

Water (H₂O) is considered a compound because: 1. It is formed by the chemical combination of two elements, hydrogen and oxygen, in a fixed ratio of 2:1 by atoms. 2. Its properties are entirely different from its constituent elements (hydrogen is combustible, oxygen supports combustion, but water extinguishes fire). 3. It cannot be separated into its constituent elements by physical means; chemical methods like electrolysis are required. Salt solution (e.g., common salt in water) is considered a mixture because: 1. It is formed by physically combining salt and water; there is no chemical reaction between them. 2. The components (salt and water) retain their individual properties. Salt still tastes salty, and water is still a liquid. 3. It can be separated into its components by physical means, such as evaporation (to get salt) and condensation (to get water).

BiologyClass 9CBSE

What is the difference between an aerosol and an emulsion? Give one example for each.

An aerosol is a colloidal system where a solid or liquid is dispersed in a gas. Example: Fog (liquid dispersed in gas), Smoke (solid dispersed in gas). An emulsion is a colloidal system where a liquid is dispersed in another liquid, and both liquids are generally immiscible. Example: Milk (fat globules dispersed in water), Face cream (oil in water or water in oil).

BiologyClass 9CBSE

You are given an impure sample of copper sulphate. How would you obtain pure copper sulphate crystals from it? Name the technique and describe the process.

To obtain pure copper sulphate crystals from an impure sample, the technique of crystallization would be used. Process:Dissolution: Dissolve the impure copper sulphate in the minimum amount of water at a higher temperature to form a concentrated solution. Filtration: Filter the hot solution to remove any insoluble impurities. Cooling: Allow the hot, saturated solution to cool slowly and undisturbed at room temperature. As the solution cools, the solubility of copper sulphate decreases, and the pure copper sulphate starts to crystallize out. Separation and Drying: Separate the crystals from the mother liquor by filtration and dry them between filter papers.

BiologyClass 9CBSE

What is meant by a saturated solution? How can you prepare an unsaturated solution from a saturated one without adding more solvent?

A saturated solution is a solution that contains the maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature. No more solute can be dissolved in it at that temperature. To prepare an unsaturated solution from a saturated one without adding more solvent, you can increase the temperature of the saturated solution. As temperature generally increases, the solubility of most solid solutes in liquid solvents also increases, allowing more solute to dissolve and thus making the previously saturated solution unsaturated.

BiologyClass 9CBSE

Suggest a suitable method to separate the components of a mixture of iodine and common salt. Explain the principle behind your chosen method.

The most suitable method to separate a mixture of iodine and common salt is sublimation. Iodine is a substance that sublimes, meaning it changes directly from a solid to a gaseous state upon heating without passing through the liquid phase. Common salt (sodium chloride) does not sublime. When the mixture is heated, iodine will vaporize and can be collected by cooling the vapors, while the common salt will remain behind in the solid state.

BiologyClass 9CBSE

What is the main difference between a true solution and a colloidal solution in terms of stability and appearance?

A true solution is transparent and stable; its particles do not settle down even on prolonged standing. The solute particles are extremely small (less than 1 nm) and invisible. For example, sugar solution. A colloidal solution (or colloid) is generally translucent and appears homogeneous but is actually heterogeneous. Its particles are larger than those in a true solution (1-100 nm) but too small to be seen with the naked eye and do not settle down. Colloids are also stable, meaning their particles do not settle. However, they scatter light (Tyndall effect), unlike true solutions.

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