Focal length formula for mirror is 1/F=1/V+1/u, where V is the distance from pole to image and u is the distance from pole to object. Here we use sign conventions. Focal length is positive for a concave mirror and negative for convex mirror.

Which one of the following materials cannot be used to make a lens? Option A: Water Option B: Glass Option C: Plastic Option D: Clay

Answer: D

For lens preparation, transparent material may be used. Glass and water are transparent materials. There is a certain amount of plastic that is transparent so that the plastic can be used to make a lens, but the clay is an opaque substance so that the light can not be transmitted into it and the clay can not be used to create the lens.

The ability of metals to be drawn into thin wire is called ductility. It is one such property of metals that is used to make wires from metals. Metals such as copper have high ductility and thus can be drawn into long and thin wires without breaking.

UV light (ultraviolet light) is having a wavelength between 10 and 400 nm that is shorter than the visible light but longer than the X-rays and is a type of electromagnetic radiation. These are present in sunlight and contributes 10% of the total light from the sun.

Ozone layer depletion is the thinning of the ozone layer present in the upper atmosphere

Causes of ozone depletion:

• Methyl bromide
• Nitrogen oxides released from freezers, air conditioners, aerosol products and industrial solvents
• Compounds such as CFCs break down into chlorine atoms in the atmosphere. These chlorine atoms break down O₃ into oxygen (O₂) and nascent oxygen (O).

Considering the loss of classroom teaching time due to the Covid-19 pandemic and lockdown, CBSE reduced the syllabus of classes 9 to 12 with the help of suggestions from NCERT.

The CBSE syllabus has been rationalized keeping intact the learning outcomes so that the core concepts of students can be retained.

Deleted syllabus of CBSE Class 10 Science

2. Ionic bonds are formed because of transfer of electrons from metal to non-metal. In this course, metals get positive charge because of transfer of electrons and non-metal gets negative charge because of acceptance of electrons. In other words bond formed between positive and negative ion is called ionic bond

1. 

The chemical bond formed by the sharing of an electron pair between two atoms so that both the atoms get their octet complete is called covalent bond.

The molecules formed as a result of sharing of electron between two atoms are called as covalent molecules.

 (ii) and (iii)​

propene, propyne

Unsaturated compounds have double or triple bonds between the carbon atoms.

Among the given options, propene and propyne have double and triple bonds respectively. Hence, these are unsaturated compounds.

The mucus is a substance secreted along the tube by cells lining its walls .Mucus helps food to slide through the canal without doing too much damage to the lining. It also forms a protective covering which keeps the digestive juices, which are inside the lumen of the canal, from coming into contact with the living cells of the walls. So the inner wall of alimentary canal is not digested.

When we increase the value of current, the magnetic field associated with it also increases.
When we increase the distance from the coil, the magnetic field produced by a current carrying circular coil decreases.
We know that the magnetic field produced by a current carrying wire at a given point depends directly on the current passing through it. So if we increase the number of turns of the coil, more current will pass through the wire which indicates that the magnetic field associated with it also increases.

Planets are much closer to the earth and are seen as extended sources. So, a planet may be considered as a collection of a large number of point-sized light sources. Although light coming from individual point-sized sources flickers but the total amount of light entering our eye from all the individual point-sized sources average out to be constant. Thereby, planets appear equally bright and there is no twinkling of planets.

Aromatic compounds are those which contain at least one benzene ring.

BENZENE – Hexagon of Carbon atoms with 3 alternate double Bonds.

The cause of refraction of light is that light travels with different speeds in different media. This Change in the speed of light when it moves from one medium to another causes it to bend.

Mechanism of respiration involves the breathing mechanism and exchange of gases.

The gaseous exchange occurs by diffusion in the alveoli. It depends upon the pressure differences between blood and tissues, or atmospheric air and blood. The exchange of gases takes place at the surface of the alveolus.

The mechanism of breathing has already been explained above. Let us have a look at the steps involved in the exchange of gases.

Exchange of Gases

The exchange of gases takes place in the following manner:

Transport Of Oxygen

• Oxygen in the blood is carried to the tissue in two forms- Oxyhaemoglobin- chemical composition of oxygen with haemoglobin, and solution of oxygen in the blood plasma.
• The oxygen in the blood combines with haemoglobin when the concentration of oxygen is high in the blood.
• Oxyhemoglobin, being unstable, dissociates to release oxygen. Low oxygen, low pH and high temperatures stimulate the dissociation process.

Hydrogen gas is usually liberated when an acid reacts with an active metal. For testing the presence of hydrogen gas, bring a burning candle near the test tube in which reaction has taken place. If the gas burns with a popping sound, the gas is hydrogen.

An amphoteric oxide is one that can act as a base or as an acid. The non-metallic oxides are generally acidic and the metallic oxides are basic. Metallic oxides are known as amphoteric oxides which display both the basic and acidic nature. Elements that form amphoteric oxides have some metals and some of the non-metals characteristics. Amphoterism is based upon an oxide’s oxidation state. This oxide reacts with base and acid to produce salt and water as the main products.

Amphoteric oxides react with both acids and alkalis to form salt and water. Examples of amphoteric oxides are zinc oxide and aluminum oxide.

ZnO(s) + 2HCl(aq) →  ZnCl2(aq) + H2O(l)

ZnO(s) + 2NaOH(a) → Na2ZnO2(aq) + H2O(l)

Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l)

Al2O3(s) + 2NaOH(aq) → 2NaAlO2(aq) + H2O(l)

Parts of a Electric Motor

• Insulated Copper wire: A rectangular coil of wire ABCD
• Magnet Poles: A magnet as placed above ie North Pole and South Pole. This creates a magnetic field as shown above.
• Split Rings: Two disjoint C-shaped rings P and Q. It acts as a commutator (which can reverse the direction of current)
• Axle: The split rings are placed on the axle which can rotate freely.
• Brushes: The outside of the split rings are connected to conducting brushes X and Y.
• Source Battery: To source current.

Working

• When the current begins to flow, current flows through brush X, then A to B, B to C, C to D and then to brush Y and into the battery.
• Now applying Fleming's Left Hand Rule to wire AB, Current is along AB, Magnetic Field is as shown (North-> South), the motion of the wire is downwards.
• Now applying Fleming's Left Hand Rule to wire CD, Current is along CD, Magnetic Field is as shown (North-> South), the motion of the wire is upwards.
• The rectangular coil begins to move in the anti-clockwise direction
• Note that during anti-clockwise motion, the split rings and axle also move, whereas the brushes don't move.
• After half a rotation, Wire CD and Split ring Q moves to the left.  Wire AB and Split ring P moves to right. Brushes X and Y donot move.
• Now applying Fleming's Left Hand Rule to wire CD, Current is along DC. (Battery -> Split ring Q -> DC , Magnetic Field is as shown (North-> South), the motion of the wire is downwards.
• Now applying Fleming's Left Hand Rule to wire AB, Current is along BA. (Battery -> Split ring Q -> DC --> CB -> BA --> Split ring P) , Magnetic Field is as shown (North-> South), the motion of the wire is upwards.
• So, again the coil rotates in the anti-clockwise direction.
• The reversal of current in the coil results in the continous rotation of the coil. The reversal of current is achieved by the commutator rings

Applications

• Electric Fans
• Refrigerators
• Mixers
• Washing machines

An electric motor is used to convert electrical energy into mechanical energy. Let’s go through an instance: What does the mixer in your house do for you? The rotating blades mash and mix things for you. And if someone were to ask you how that works, what would you say? You would probably say that it works on electricity. Well, that’s not incorrect. Motors convert electric energy to mechanical work. The opposite is done by generators that convert mechanical work to electrical energy.

Different Parts of an Electric Motor and Their Function

A simple motor has the following parts:

• A power supply – mostly DC for a simple motor
• Field Magnet – could be a permanent magnet or an electromagnet
• An Armature or rotor
• Commutator
• Brushes
• Axle

Answer:

The name plaster of Paris came from the fact that it was first of all made by heating gypsum which was mainly found in Paris. The compound plaster of Paris is prepared by heating gypsum at 120^oC.

CaSO₄.2H₂O → CaSO₄.1/2H₂O + 3/2H₂O

The chemical formula for the plaster of Paris is (CaSO₄) H₂O and is better known as calcium sulfate hemihydrate.

Plaster of Paris is a popular substance that is utilized most commonly for sculpting materials and in gauze bandages. While we have seen many applications of this material in our everyday lives if we try to understand its chemistry, plaster of Paris is a white powdery chemical compound which is hydrated calcium sulfate that is usually obtained from calcining gypsum.

Plaster of Paris is also referred to as Gypsum plaster. The chemical formula of plaster of Paris is written as CaSO4·H2O or 2CaSO4·H2O.

(a) Most of the metals do not react with hydrogen. Only a few reactive metals reacts with hydrogen to form metal hybrides. 

     For example: when hydrogen gas is passed over heated sodium, then sodium hybride is formed.

(b) Non metals react with hydrogen to form covalent hybrides. 

     For example sulphur combines with hydrogen to form hydrogen sulphide. 

When a metal reacts with a non-metal few changes takes place with the compound or molecules undergoing in the reaction. Let us see what happens.

Transfer of electrons

When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.

Formation of ions

The metal atoms give electrons to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions.

Reactions

Few examples of the reaction between metals and non metals

• Sodium + chlorine → sodium chloride
• Magnesium + oxygen → magnesium oxide
• Calcium + chlorine → calcium chloride

Uses of Sodium Chloride

• It is widely used in food industries as a food preservative and as a flavour enhancer.
• It is a major raw material in the industrial manufacturing of various chemicals such as sodium carbonate, sodium hydrogen carbonate etc.
• This salt is used in glass production.
• In cold countries, it is used to prevent the build-up of ice on roads, bridges etc which is important for safe driving conditions.

Organised sector: All enterprises which are either registered or came under the purview of any of the relevant government rule, like Factory Act.
People enjoy working in the organised sector as there is a proper and systematic way to get the work done from the employees.

Atomic size is the distance between the center of nucleus and the outermost electron in the outer shell. Across a period from left to right there is decrease in atomic size with increase in nuclear charge of the element. Atomic size increases down the group because of addition of extra shell. 
The unit of measurement is Angstroms (A˚)

When sodium sulphate solution reacts with barium chloride solution results in double displacement reaction in which ions are exchanged.

Na2​SO4​+BaCl2​⟶2NaCl+BaSO4​

In this reaction barium sulphate and sodium chloride are formed,the solution in water consists only sodium chloride because sodium chloride dissolves in water while barium sulphate it settles down and a white precipitate is formed (insoluble).