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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.

BiologyClass 9CBSE

How would you separate a mixture of sand and salt? Describe the steps involved.

To separate a mixture of sand and salt, the following steps can be used: Dissolution: Add water to the mixture of sand and salt. Salt is soluble in water, while sand is not. Stir the mixture to dissolve the salt. Filtration: Pour the mixture through a filter paper in a funnel. The sand, being insoluble, will remain on the filter paper as residue, while the salt solution (filtrate) will pass through. Evaporation: Heat the salt solution (filtrate) gently in an evaporating dish. The water will evaporate, leaving behind the solid common salt.

BiologyClass 9CBSE

Why is air considered a mixture and not a compound? List two reasons.

Air is considered a mixture and not a compound for the following reasons: 1. Variable Composition: The composition of air is not fixed; the proportion of its constituent gases (nitrogen, oxygen, argon, carbon dioxide, etc.) can vary slightly from place to place and time to time. In a compound, elements are combined in a fixed ratio. 2. Retention of Properties: The components of air (like oxygen and nitrogen) retain their individual properties. For example, oxygen in the air still supports combustion, and nitrogen still acts as a diluent. In a compound, the properties of the constituent elements are lost, and the compound has entirely new properties.

BiologyClass 9CBSE

A student prepared a solution by dissolving 20 g of common salt in 180 g of water. Calculate the mass by mass percentage of the solution.

Given: Mass of solute (common salt) = 20 g Mass of solvent (water) = 180 g Mass of solution = Mass of solute + Mass of solvent Mass of solution = 20 g + 180 g = 200 g Mass by mass percentage of solution = (Mass of solute / Mass of solution) × 100 Mass by mass percentage = (20 g / 200 g) × 100 Mass by mass percentage = (1/10) × 100 = 10% Therefore, the mass by mass percentage of the solution is 10%.

BiologyClass 9CBSE

Explain the process of chromatography. For what kind of mixtures is this technique most effectively used?

Chromatography is a separation technique used to separate components of a mixture that are dissolved in a fluid (mobile phase) by passing it through a stationary phase. The separation occurs because different components travel at different speeds depending on their differential affinities for the stationary and mobile phases. Components that are more soluble in the mobile phase and have less affinity for the stationary phase travel faster. This technique is most effectively used for separating components of a mixture where the components have different solubilities in the same solvent, such as separating colors in a dye, pigments from natural colors, or drugs from blood.

BiologyClass 9CBSE

What is an element? How is it different from a compound? Provide one example for each.

An element is a pure substance that cannot be broken down into simpler substances by chemical means. It consists of only one type of atom. Example: Oxygen (O), Iron (Fe). A compound is a pure substance formed when two or more elements chemically combine in a fixed ratio. It has properties entirely different from its constituent elements. Example: Water (H₂O), Carbon dioxide (CO₂). The key difference is that elements are the simplest forms of matter, while compounds are formed by the chemical combination of elements.

BiologyClass 9CBSE

You are given a mixture of oil and water. Which separation technique would you use to separate them? Explain the principle behind this technique.

To separate a mixture of oil and water, I would use a separating funnel. The principle behind this technique is that oil and water are immiscible liquids, meaning they do not mix and form separate layers due to differences in their densities. Oil, being less dense, forms the top layer, while water, being denser, forms the bottom layer. The separating funnel allows the heavier (bottom) layer to be drained off carefully, leaving the lighter (top) layer behind.

BiologyClass 9CBSE

Differentiate between physical and chemical changes. Give two examples of each, explaining why they are classified as such.

A physical change is a change in the form or appearance of a substance, but not in its chemical composition. No new substance is formed, and the change is often reversible. Examples: Melting of ice: Ice (solid water) changes to liquid water, but it's still H₂O. No new substance is formed. Dissolving sugar in water: Sugar molecules are dispersed in water, but their chemical identity remains the same. A chemical change (or chemical reaction) results in the formation of new substances with different chemical properties. It is usually irreversible. Examples: Burning of wood: Wood combines with oxygen to produce ash, carbon dioxide, and water vapor, all new substances. Rusting of iron: Iron reacts with oxygen and water to form iron oxide (rust), a new substance with different properties.

BiologyClass 9CBSE

What is meant by the 'concentration' of a solution? Describe two different ways to express the concentration of a solution, providing the formula for each.

The concentration of a solution refers to the amount of solute present in a given amount of solution or solvent. Two common ways to express concentration are: Mass by Mass Percentage of a Solution: Formula: (Mass of solute / Mass of solution) × 100 Mass by Volume Percentage of a Solution: Formula: (Mass of solute / Volume of solution) × 100

BiologyClass 9CBSE

Imagine you have a mixture of iron filings, common salt, and ammonium chloride. Suggest a sequence of separation techniques you would use to separate each component from this mixture. Justify your choices.

Magnetic Separation: First, use a magnet to separate the iron filings, as iron is magnetic and the other components are not. Sublimation: Next, heat the remaining mixture (salt and ammonium chloride). Ammonium chloride sublimes (changes directly from solid to gas) upon heating, leaving behind the common salt. The ammonium chloride can be collected by condensation. Dissolution and Evaporation: Finally, dissolve the remaining common salt in water. Filter if any insoluble impurities are present. Then, evaporate the water to obtain pure common salt.

BiologyClass 9CBSE

Describe the Tyndall effect. Which types of mixtures exhibit this effect, and why? Provide a common example where you might observe the Tyndall effect.

The Tyndall effect is the scattering of a beam of light by colloidal particles as it passes through a colloid. It is exhibited by colloids and some suspensions (when particles are not settled). Solutions do not show the Tyndall effect because their particles are too small to scatter light. Colloidal particles are large enough to scatter light but small enough not to settle.A common example is the path of light visible when a sunbeam enters a dusty room, where dust particles (a type of colloid/suspension) scatter the light.

BiologyClass 9CBSE

Explain why a solution is considered a homogeneous mixture, while a suspension is a heterogeneous mixture. What are the key differences in their particle sizes and visibility?

A solution is homogeneous because its components are uniformly distributed throughout, and individual particles are not visible even under a microscope. The particle size in a solution is less than 1 nm. For example, sugar dissolved in water. A suspension is heterogeneous because its components are not uniformly distributed, and the particles are large enough to be seen with the naked eye and settle down upon standing. The particle size in a suspension is greater than 100 nm. For example, sand in water.

BiologyClass 9CBSE

Distinguish between a pure substance and a mixture based on their composition and properties. Provide two examples for each category from your daily life.

A pure substance consists of only one type of particle (atoms or molecules) and has a fixed, uniform composition and definite properties. For example, water (H2O) and gold (Au) are pure substances. A mixture, on the other hand, contains two or more pure substances physically combined in any proportion. Its composition can vary, and its properties are a combination of the properties of its components. Examples include air (mixture of gases) and lemonade (mixture of water, lemon juice, and sugar).

BiologyClass 9CBSE

What is the effect of wind speed on the rate of evaporation? Explain with a practical example.

Increasing wind speed increases the rate of evaporation. This is because wind carries away the water vapor molecules that have evaporated from the surface of the liquid. By removing these vapor molecules, the concentration of water vapor above the liquid surface decreases, creating a steeper concentration gradient and allowing more liquid molecules to escape into the air. A practical example is that clothes dry faster on a windy day than on a still day, as the wind blows away the water vapor, preventing saturation of the air near the clothes.

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