CBSE Class 10 Science — NCERT Solutions

(i) The substance is calcium oxide or quick lime; formula: CaO.

(ii) Its reaction with water is:

$$\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{heat}$$

Given: Magnesium ribbon burns in air (oxygen) to form magnesium oxide.

Reaction:
$$2\mathrm{Mg}(\mathrm{s}) + \mathrm{O}_2(\mathrm{g}) \rightarrow 2\mathrm{MgO}(\mathrm{s})$$

Concept: A substance is said to be oxidised if it gains oxygen during a reaction.

Conclusion: Magnesium gains oxygen to form magnesium oxide. Therefore, magnesium is being oxidised in this reaction.

Take one test tube at a time and test it with red litmus.

• The test tube in which red litmus turns blue contains the basic solution.
• The test tube in which red litmus shows no change may contain either distilled water or an acidic solution, because red litmus does not change in acid or neutral solution.
• Now use the test tube identified as basic. Dip the same red litmus into the other two test tubes.
• If it turns blue there, that test tube is the base.
• If it does not change, that test tube is either acidic solution or distilled water.
• To separate these two, first dip a fresh strip of red litmus into one of them. The acidic solution will keep red litmus red, and the distilled water will also keep it red.

So, with only red litmus, you can identify the base directly, but you cannot distinguish acid from distilled water using red litmus alone. To identify all three uniquely, you need an additional indicator or prior identification of the base.

Curd and sour substances should not be kept in brass and copper vessels because they contain acids. The acids react with these metals to form harmful salts, which can contaminate the food and may be poisonous. Hence, such substances are stored in non-reactive containers.

From the chapter:

• Mercury is the only metal mentioned as liquid at room temperature.
• Sodium is so soft that it can be cut with a knife.
• The best conductors of heat are silver and copper; choosing one example, silver.
• Lead is mentioned as a poor conductor of heat.

Malleable means a substance, especially a metal, can be beaten into thin sheets.

Ductile means a substance can be drawn into thin wires.

In carbon dioxide, carbon has valency 4 and each oxygen has valency 2. So the carbon atom shares two electron pairs with each oxygen atom. Therefore the structure is O=C=O, with two double bonds.

Diffusion is too slow to meet the oxygen needs of large multicellular organisms like humans because many cells are not in direct contact with the environment. In such organisms, oxygen has to reach all cells quickly, but simple diffusion over long distances cannot do this efficiently. Also, the body surface is not enough for all cells, so a transport system is needed.

The leaves of the sensitive plant fold quickly in response to touch and this movement is independent of growth. In contrast, movement of a shoot towards light is a growth-related movement, so it is slow and depends on growth.

DNA copying is important because it passes the hereditary information from parents to the next generation. It also helps make new cells with the same basic body design. Small variations may occur during copying, and these variations are the basis of evolution and help species survive changing conditions.

In an asexually reproducing species, a trait that is found in a larger part of the population is more likely to have arisen earlier, because it had more time to be passed on to many descendants. Since trait B is present in 60% of the population, while trait A is present in only 10%, B is likely to have arisen earlier.

When light travels from air to water, it goes from a rarer medium to a denser medium. Therefore, it bends towards the normal because its speed decreases in water.

Correct option: (b) accommodation.

The ability of the eye to adjust its focal length (by changing the curvature of the eye lens through ciliary muscles) to focus on objects at different distances is called accommodation. Presbyopia is the loss of this ability in old age; near-sightedness and far-sightedness are refractive defects, not the focusing mechanism itself.

An electric circuit means a continuous and closed path for the flow of electric charges (current). If the path is broken anywhere, current stops flowing.

A compass needle is itself a small magnet. When it is brought near a bar magnet, the magnetic field of the bar magnet exerts a force on the needle. So the needle turns and aligns itself with the magnetic field, causing deflection.

Trophic levels are the different steps or levels in a food chain, showing who eats whom.

Example of a food chain: grass → goat → human

• 1st trophic level: grass (producers)
• 2nd trophic level: goat (primary consumer)
• 3rd trophic level: human (secondary consumer)

So, each step in the food chain is a trophic level.