Give reason for ...

when you rub your hands with and antiseptic hand gel, you feel your hands are cold.

Due to the absorption of heat from the hands when the alcohol present in the hand gel evaporates

The transformation of a substance from one state to another is accompanied  by a thermal change

Because it is accompanied by a loss or absorption of thermal energy to or from the surroundings

The sublimation process (from changes in state of matter) is an endothermic process

Because it is accompanied by the absorption of thermal energy from the surrounding

The refrigerator works to preserve the food inside it from spoilage

Because it works by withdrawing heat from inside the refrigerator and transferring it to the surrounding medium (air) using the refrigerant, so it keeps the food inside cool

dissolution process is accompanied by a thermal change

Because it is accompanied by a loss or absorption of thermal energy to or from the surroundings

the dissolution of anhydrous lithium chloride salt in water is exothermic

Because it is accompanied by an increase in the temperature of the solution 

the process of converting anhydrous salts to hydrated salts is a reversible process

Because when drops of water are added to anhydrous salts they turn into hydrated salts and upon heating they turn back into anhydrous salts again

it is preferable to use several polystyrene cups in laboratory experiments

To increase thermal insulation 

when diluting concentrated nitric acid, the acid is dripped onto the inner walls of water container

Because water absorbs the released heat while adding concentrated nitric acid to it without causing damage

instant cold compresses are used to reduce the severity of swelling

Because they work by narrowing the blood vessels which reduces blood flow to the injured area and thus reducing the swelling

instant hot compresses are used to relieve pain associated with muscle strain (fatigue)

Because they work by widening the blood vessels which increases blood flow to the stressed areas causing relaxing the contracting muscles and relieving pain

chemical reactions usually involve thermal changes

Because in most reactions, the amounts of thermal energy absorbed and released during the breaking and forming of bonds in the same reaction are not equal, so the reaction absorbs or release thermal energy which fulfills the law of conservation of energy

The reaction of acids with alkalis are exothermic

Because the amount of energy released during the formation of new bonds in the product molecules is greater than the amount of energy absorbed during the breakage of the bonds in the reactant molecules.

Coffee can be warmed in self-heating cans without the need for an external heat source

Because an exothermic reaction occurs between calcium oxide and water, with releasing heat and used in warming coffee

The thermite reaction is used in railway tracks welding

Because it is an exothermic reaction that results in the release of a significant amount of thermal energy, leading to melting iron which is produced from the reaction

When barium hydroxide powder is added to ammonium chloride powder, the temperature of the surroundings decreases

Because it is an endothermic reaction accompanied by the absorption of thermal energy from the surroundings leading to decreasing the temperature

The decomposition reaction of ozone gas in the atmosphere is endothermic

Because the amount of energy released from the forming new bonds in the product molecules is less than the amount of energy absorbed during breakage of the bonds in the reactant molecules

Ethyl alcohol is considered a renewable energy resource

Because it is prepared from plants rich in starch such as corn plant

The use of biofuel is preferred over fossil fuel

Because biofuel is a renewable resource, while fossil fuel is a non-renewable resource

A very small amount of ethanethiol gas is added to both natural gas and butagas

To take appropriate safety measures in case of gas leaks, as both are odourless and ethanethiol gas is characterized by a garlic smell.

The flame of a paraffin candle placed under a bell jar goes out in minutes

Due to the depletion of the amount of oxygen gas inside the glass bell jar

The combustion of a piece of steel wool (scouring pad) in a cylinder containing pure oxygen is faster than its combustion in atmospheric air

To increase the concentration of pure oxygen in the cylinder compared to its concentration in the air, and the rate of combustion increases with the increase in the percentage of oxygen gas

Bunsen flame has different colours

Due to the difference in the amount of oxygen mixed with the fuel, which may be limited so the flame turns yellow, or abundant when the air hole in the Bunsen burner is opened so the flame turns blue

Ignition of a matchstick when it is struck against a rough surface

Because friction generates the heat necessary to start an exothermic chemical reaction

The ignition point determines how dangerous the fuel is to use

Because as the ignition point of the fuel is low, its flammability increases, so its danger increases, and vice versa.

The combustion of a hydrocarbon in air leads to the formation of carbon dioxide gas and water vapour through an exothermic reaction

Because hydrocarbons contain carbon and hydrogen elements

The oxyacetylene flame is used in cutting and welding metals

Because the oxyacetylene flame reaches a temperature of 3000℃ which is sufficient for that

The car engine cannot operate without fuel

Due to the incomplete combustion triangle (absence of fuel)

Frying pan fires are extinguished by covering them with a metal lid.

To isolate oxygen and thus the fire triangle is incomplete and the fires is extinguished

The presence of a bucket filled with sand in school laboratories

To extinguish fires of some metals such as sodium and magnesium by isolating them from oxygen

It is strictly forbidden to extinguish sodium or potassium fires with water

Because water increases the intensity of the fire instead of extinguishing it as a result of its reaction with these metals in an exothermic reaction accompanied by the ignition of the released hydrogen

Carbon dioxide gas is used in extinguishing fires

Because it is a gas that doesn’t burn and doesn’t support burning

Water is used in extinguishing paper and wood fires.

Because of the high specific heat of water, which makes it absorbs a large amount of thermal energy and thus the temperature of the burning materials decreases

Water is not used in extinguishing of petroleum oil fires

Because petroleum oil floats on the surface of water due to its lower density compared to that of water, which leads to the spread of fires instead of extinguishing them

Oxidation and reduction processes are two simultaneous processes

Because when a substance gains oxygen (is oxidized), another substance must lose this oxygen (is reduced) and when a substance loses hydrogen (is oxidized), another substance must gain this hydrogen (is reduced)

Air evacuation preserves food for a longer period of time

Because this provides a low-oxygen environment, which reduces oxidation and reduction reactions that lead to spoiling of food

Mass is a scalar physical quantity, while acceleration is a vector physical quantity

Because to define mass, only its magnitude needs to be specified, while to define acceleration, both its magnitude and direction must be specified

The bag placed on a table remains stationary unless someone moves it from its place

Because a stationary object remains stationary unless acted upon by external unbalanced forces that change its state, according to Newton’s first law (Law of Inertia)

Rushing the bus passenger forward when the bus stops suddenly

Because the inertia of the passenger causes him to resist the sudden change in the state of motion when pressing the brake to keep the state of motion they were in

Rushing the passenger of the stationary bus backward when it moves suddenly

Because the inertia of the passenger causes him to resist the sudden movement of the bus to maintain his previous state of rest, so he rushed backward.

The electric fan blades continue to rotate for a few seconds after the electric current is switched off.

Because the inertia of the fan blades causes them to resist the sudden stop after the electric current is switched off, to maintain the state of motion they were in.

The greater the mass of the moving object, the longer the stopping time and distance when the brakes are applied

Because as the mass of the object increases, its inertia increases, making it more difficult to stop, which is indicated by the increased time it maintains its state of motion and thus increases its stopping time and distance.

When a person jumps forward form the front of a small boat, the boat moves backward

Because the movement of the boat represents the reaction to the force of person jumping (action), and they act in opposite directions without any time difference

The tower crane is an example of complex machines

Because it consists of many simple machines

Scissors is a first class lever

Because the fulcrum is located between the effort and the load

Nutcracker is a second class lever

Because the load is located between the fulcrum and the effort

• The mechanical advantage of second class levers is always greater than 1

• All second class levers save effort

Because the effort arm is always greater than the load arm

Pliers saves effort

Because the effort arm is greater than the load arm (its mechanical advantage is greater than 1)

Seesaw doesn’t save effort

Because the effort arm equals the load arm (its mechanical advantage equals 1)

• Third class levers don’t save effort

• Coal tongs doesn’t save effort

Because the effort arm is less than the load arm (its mechanical advantage is less than 1)

All first class levers can change the direction of the applied force

Because the direction of load is in opposite direction of effort

Using third class levers although they don’t save effort

Because it is used for precise and dangerous tasks

Standing on tiptoes represents a second class lever

Because the load (body weight) is located between the effort (the calf muscles) and the fulcrum (the tips of the toes)

Human arm from the elbow joint to the palm is considered a third class lever

Because the effort (The biceps muscle) is located between the fulcrum (elbow joint) and the load (the object held in the palm)