GAT Test - 9

Concept:

For the resistance connected in series, the equivalent resistance is:

⇒ Req = R1 + R2  ------ (1)

For the resistance connected in parallel combination, the equivalent resistance,

The correct answer is Ohm’s law.

• In 1828, George Simon Ohm, a German physicist, derived a relationship between electric current and potential difference.

Key Points

• This relationship is known as Ohm’s law.
• Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points.
• It is given by the expression I = V/R. In this relation, I is the current through the conductor in amperes, V is the voltage across the conductor in volts, and R is the resistance of the conductor in units of ohms.

CONCEPT:

• Power: The rate of work done by electric energy is called power. It is denoted by P.
• The SI unit of power is the watt (W).

• Watt is a small unit that's why kilowatt-hour is used as the unit for electrical energy.
• 1 unit of electric energy: When a one-kilowatt load works for 1 hour then the energy consumed is called 1 unit of electricity.
• 1 Unit of electricity = 1 KWh = 1000 Watt-hour = 3.6 × 106 J
• 1 Kilo-watt = 1000 Watt
• Work done (W) = Energy consumed = Heat (H) = Power (P) × Time taken (t)

CALCULATION:

• The rating of the heater is given i.e. P = 1000 W,  V = 200 V
• From the above formula, the resistance of heater R_h can be calculated as

Explanation:

Drift velocity is given as:

where i is the current flowing through the conductor, n is the number of electrons and A is the area of the cross-section of the conductor.

∴ v_d ∝ i  as n, e, A are constants.

∵ Temperature is kept constant then according to Ohm's law,

Potential difference V ∝ i 

∴ (Drift velocity) v_d ∝ V

Thus, on doubling the potential difference the drift velocity also doubles.

Hence, the correct option is (3)

Concept:

• Metals react with bases and release hydrogen gas.
• The hydrogen gas released burns with a pop sound.
• This is the test for the presence of hydrogen gas. 
• Hydrogen gas is Colourless and odourless. 

Explanation:

• Sodium Hydroxide is a base and aluminium foil. 
• The reaction gives hydrogen gas which gives a pop sound on burning. 
• Hydrogen is the Colourless and odourless gas that produces a ‘pop’ sound when a burning match stick is brought near it.
• The chemical equation for the process is 

2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂

Important Point

• Bases are bitter in taste and have a soapy feel.
• Acids are sour in taste.  
• Metals more reactive than hydrogen reacts with acid and releases hydrogen gas. 

The correct answer is Dissociation of hydrogen molecule is highly exothermic

Concept:-

• Atomic Hydrogen: Atomic hydrogen consists of individual hydrogen atoms. It is a very reactive form of hydrogen and can easily react with other elements and compounds.
• Molecular Hydrogen: Molecular hydrogen consists of two hydrogen atoms bonded together by a covalent bond. It is a stable and unreactive form of hydrogen.
• Bond Dissociation Energy: Bond dissociation energy is the energy required to break a bond between two atoms in a molecule.
• Exothermic Reaction: An exothermic reaction is a reaction in which heat is released.

Explanation:-

Atomic hydrogen atoms can recombine to form molecular hydrogen (H₂) with the release of a large amount of heat. This reaction is very fast and exothermic.

Bond Energy:
The hydrogen-hydrogen bond in a hydrogen molecule (H₂) is a very strong bond. The bond dissociation energy of H₂ is 436 kJ/mol. This means that it takes 436 kJ of energy to break the bond between two hydrogen atoms in a hydrogen molecule.

Exothermic Reaction:
When atomic hydrogen atoms recombine to form molecular hydrogen, the bond dissociation energy is released as heat. This makes the reaction highly exothermic.

Welding:
The heat released by the recombination of atomic hydrogen can be used for welding metals. In atomic hydrogen welding, a stream of hydrogen gas is passed through an electric arc, which dissociates the hydrogen molecules into atomic hydrogen. The atomic hydrogen then recombines on the surface of the metal, releasing heat and melting the metal.

Conclusion:-

The recombination of atomic hydrogen to form molecular hydrogen is a highly exothermic reaction. This heat can be used for welding metals and other applications.

The correct answer is Y, Water, Z, X.

Concept:

Density:

Equal quantities of liquids are poured into a test tube containing 5 ml of water. Thus, the order of liquids settled from top to bottom is Y, Water, Z, X.

Concept:

• An alloy is considered a homogeneous mixture because, by definition, a homogeneous mixture has the same uniform appearance and composition throughout.
• Since an alloy is a mixture of two or more metals or a mixture of a metal with some other element, and these mixtures are uniform throughout, they fit the definition of being homogeneous. 

Some common mixtures are :

Amalgam:

• Amalgam is a mixture of mercury with another metal.
• The Amalgam of Silver and Mercury is used for tooth fillings.

Explanation:

• Amalgam = Mercury + Metal
• Steel = Iron (metal) + Carbon (non-metal)
• Bronze = Copper (metal) + Tin (metal)
• Brass = Copper (metal) + Zinc (metal)

Concept:

Acid and Base

Indicators

• The indicators are chemicals or natural substances which change their colour in presence of acid or base. 
• Litmus paper is an indicator that is obtained from lichens. 
• Some indicators and their colour change are given in the table below:

Explanation:

• Turmeric is a natural indicator. 
• It remains yellow in acidic solution and turns red in basic solution. 
• This property of turmeric is also used in painting. 
• Hydrochloric acid is an acid hence turmeric will remain yellow in that solution. 
• Sodium hydroxide is the base, so turmeric will turn red in the sodium hydroxide solution. 

So, the correct option is 'Yellow in hydrochloric acid and red in sodium hydroxide.'

Concept:

The changes occurring in the environment can be divided into two types - Physical changes & Chemical changes.

Physical change -

• Properties such as shape, size, colour and state of a substance are called its physical properties. A change in which a substance undergoes a change in its physical properties is called a physical change. A physical change is generally reversible.

Chemical change -

• A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction.
• The chemical change is irreversible.

Explanation:

• A - When food gets spoiled, it produces a foul smell because of the microbial growth, which results in discolouration & change in food's quality.
• B - A slice of apple acquires brown colour when kept out for some time because the apple contains iron and when it is cut with the knife the iron present in the apple reacts with the atmospheric oxygen to form iron oxide. This represents an oxidation reaction.
• C -  When an ant bites, calamine is used to ease the irritation on the skin because the ant's bite contains an acid that gets neutralised when rubbing with the basic calamine. This represents a neutralisation reaction.
• All three points represent a chemical change.