chemical reaction and equation: Why do fire flies glow at night?

1. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?

Answer:  KCl (aq) + AgNO3 (aq) → AgCl (s) + KNO3 (aq)

It is a double displacement and precipitation reaction.

2. Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

Answer:  2FeSO4(s) --H---e---at→ Fe2O3(s) + SO2(g) + SO3(g)

It is a thermal decomposition reaction

3. Why do fire flies glow at night?

Answer: Fire flies have a protein which in the presence of an enzyme undergoes aerial oxidation. This is a chemical reaction which involves emission of visible light. Therefore, fire flies glow at night.

4. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Answer: Grapes when attached to the plants are living and therefore their own immune system prevents fermentation. The microbes can grow in the plucked grapes and under anaerobic conditions these can be fermented. This is a chemical change.

5. Which among the following are physical or chemical changes?

(a) Evaporation of petrol 

(b) Burning of Liquefied Petroleum Gas (LPG)

(c) Heating of an iron rod to red hot.

(d) Curdling of milk

(e) Sublimation of solid ammonium chloride

Answer:  (a), (c) and (e) — are physical changes. 

(b) and (d) are chemical changes

6. During the reaction of some metals with dilute hydrochloric acid, following observations were made.

(a) Silver metal does not show any change

(b) The temperature of the reaction mixture rises when aluminium (Al) is added.

(c) The reaction of sodium metal is found to be highly explosive

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.  

Explain these observations giving suitable reasons.

Answer:(a) Silver metal does not react with dilute HCl

(b) The temperature of the reaction mixture rises when aluminium is added because it is an exothermic reaction.

(c) Reaction of sodium metal is found to be highly explosive because it is an exothermic reaction

(d) When lead is treated with hydrochloric acid, bubbles of hydrogen gas are evolved

                  Pb + 2HCl → PbCl2 + H2

6. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

Answer: Calcium oxide CaO(s) + H2O(l) → Ca(OH)2(aq)

7. Write a balanced chemical equation for each of the following reactions and also classify them.

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.

(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

Answer:(a) Pb(CH3COO)2 + 2HCl → PbCl2 + CH3COOH

 Double displacement reaction

(b) 2Na + 2C2H5OH → 2C2H5ONa + H2

 Displacement reaction

(c) Fe2O3 + 3CO → 2Fe + 3CO2

 Redox reaction

(d) 2H2S + O2 → 2S + 2H2O

 Redox reaction

8. Why do we store silver chloride in dark coloured bottles?

Answer:  Silver chloride on exposure to sunlight may decompose as per the following reaction.

2AgCl → 2Ag + Cl2 Therefore, it is stored in dark coloured bottles.

9.Take about 2 g barium hydroxide in a test tube. Add 1 g of ammonium chloride and mix with the help of a glass rod. Touch the bottom of the test tube with your palm. What do you feel? Is this an exothermic or endothermic reaction?

Answer:  Ba(OH)₂ +2NH₄ Cl ----------------à BaCl2 + 2NH₄(OH)      Endothermic reaction

10. What is the difference between the displacement and double displacement reactions? Write equations for these reactions.  

Answer: In displacement reaction, one element from its salt is displaced by a more reactive element e.g. in following example, Cu is displaced by Zn from CuSO₄ because Zn is more reactive.

CuSO₄ + Zn ---> ZnSO₄ + Cu  

Also, Zn(s) + CuSO4 (aq) → ZnSO4 (aq) + Cu(s) ;   Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

Zinc and lead are more reactive elements than copper. They displace copper from its compounds

In double displacement reaction, exchange of ions takes place between two reactants to form new products.

Na₂SO₄ + BaCl₂ ---> BaSO₄ + 2NaCl

Here, the white precipitate of BaSO4 is formed by the reaction of SO4^-2 and Ba²⁺               

Science Questions - Ponder over it?

1. Why does a rider feel a tendency to fall when the horse starts running of stops suddenly?
Ans: This is caused by inertia. When the horse starts running, the rider, being at rest, falls back. When a running horse stops suddenly, the rider being in motion has a tendency to fall ahead.

2. Why does a person carrying a bucket full of water in his right hand bend towards the left?
Ans: He bends towards his left so that the centre of gravity falls within the base. This enables him to keep up balance, otherwise he may fall.

3. Why does tea cool more rapidly in a saucer than in a cup?
Ans: In a saucer evaporation takes place more rapidly than in a cup. Cooling is caused by evaporation.

4. Ice packed in sawdust does not melt quickly. Why?
Ans: Ice packed in sawdust does not melt quickly, because sawdust being bad conductor, it cuts the heat rays.

5. Why do you heat a metal rim before fitting it to a wheel?
Ans: By heating, the rim expands and fits easily over a slightly bigger radius than that of the rim. On being cooled it contracts and has a firm grip on the wheel.

6. Why do wet clothes dry slowly on a rainy day?
Ans: On a rainy day the atmosphere air contains more water vapor than on a dry day and hence evaporation is slower.

7. Why does a substance thrown up come to the ground?
Ans: Because of the gravitational force of the earth.

8. Why does moisture gather outside of a glass of cold water on a warm day?
Ans: The temperature of the outer surface of glass of cold water is lower than that of its surroundings. The vapor in the surrounding atmosphere gets condensed on the outer surface of the water appears in the forms of tiny particles of water.

9. What happens to iron when it rusts?
Ans: It reacts with the oxygen of the air and forms iron-oxide which is the rust.

10. How would you distinguish between welding and soldering?
Ans: Welding is the joining of metals by raising the temperature of the metals to fuse them together by melting their ends. Soldering is the joining of metals with the help of an alloy called solder.

11. How does a ball which falls down, bounce up?
It is due to the operation of Newton’s Third Law of motion. To every action there is equal and opposite reaction. A ball falling to the ground is slightly deformed. Due to the elastic force of the material of which the ball is made, the ball tries to recover its original shape. In doing so it forces the ground which pushes the ball upward due to reaction and the ball bounces up.

12. An iron nail floats on mercury but sinks in water. Give reasons.
Ans: The specific density of iron is lower than of mercury, hence it floats on mercury, while it is higher than that of water, and hence it sinks in water.

13. How do you convert Centigrade in to Fahrenheit?
Ans: With the help of the following formula – C/100 = F-32/180

14. It is advisable to work electric appliances when they are earthed suitably. Why?
Ans: In case of short-circuiting, the current passes to the earth without harming the user, if an electrical appliance is properly earthed.

15. How does a refrigerator keep food fresh?
Ans: Food is kept fresh so long as fermentation does not set in; refrigeration prevents fermentation by providing low temperature.

16. Ice wrapped in a blanket does not melt away quickly. Why?
Ans: Blanket being bad conductor of heat prevents the outside heat from creeping inside.

17. Why can a petrol fire not be extinguished by throwing water on it?
Ans: The heat of the petrol fire is so intense that the water thrown on it gets evaporated and decomposed. Hence it is not effective in extinguishing the petrol fire.

18. A train stops when the chain is pulled. Why?
Ans: When the chain is pulled, one small valve gets opened and air/atmospheric pressure is admitted to the under side of the piston head of the brake cylinder through pipe connection. Initially there is vacuum on both top and bottom side of the piston head. When the air/atmospheric pressure enters the under side of the piston head, the piston raises up due to difference of pressure and pulls the brake rigging to which it is connected with links and levers. Thus the brakes are applied and the train stops.

Courtesy: Articlediary.com

Salt - The Spice of Life

Salt - The Spice of Life: For every meal there is one thing people generally reach for before they even take a bite – the salt shaker. Salt is one of the oldest spices used and is a key component to humans, animals, and plants.

Its flavor is unique and versatile, salt has been a staple throughout time. Enhancing almost every dish, salt is added to breads, meats, fruits and vegetables to sauces and desserts.

Additionally, salt aids foods in a variety of ways like:

Preservation – helps protect against microorganisms, bacteria through dehydration and preventing growth of bacteria, which slows or prevents spoilage.

Texture Aid – in bread making, allows the dough to rise by giving helping the gluten hold more water and carbon dioxide. In meats it improves tenderness and in cheeses it aids in consistency of the cheese and the hardness of the rind.

Binder – in processed meats it helps retain water which reduces the loss of meat when cooking.

Color Developer – in ham, bacon, and other processed meats it helps obtain the desired color. It also helps create a golden crust for breads.

Fermentation Control – slows and controls the fermentation process in:

Pickling

Cheese production Sauerkraut production Summer sausage production

Why aqueous solution of sodium carbonate is basic in nature?

Question: Why aqueous solution of sodium carbonate is basic in nature?


Answer: Sodium bicarbonate is an amphoteric compound. Aqueous solutions are mildly alkaline due to the formation of carbonic acid and hydroxide ion:

NaHCO− 3 + H₂O → H₂CO₃ + OH⁻  

Question: Why is an aqueous solution of ammonium chloride acidic in nature?

Answer: A basic component is the one that produces hydroxide (OH^-) ions when dissolved in water. In aqueous solution of ammonium chloride, ammonium ions (NH4⁺) first associate with H₂O and form ammonia and hydroxide ions.

NH₄Cl + H₂O = NH₄⁺ + HCl ( equation 1)

NH₄⁺ + H₂O = NH₃ + OH^- (equation 3)

Since ammonium ions produce hydroxide ions, NH4⁺ are considered to be the basic components.
For more Questions: Visit: www.jsuniltutorial.weebly.com/ 
and:
http://chemistryadda.blogspot.in/2011/06/extrascore-science-acid-base-and-salt.html

Why are commercial samples of bleaching powder not completely soluble in water?

Why are commercial samples of bleaching powder not completely soluble in water?


Because it is essentially chlorinated CaO, which is insoluble. It releases chlorine on adding to water. Stable bleaching powder has about 37% available chlorine when fresh.

Why aqueous solution of sodium carbonate is basic in nature?

cbse Chemisrty Amazing Facts

Hydrogen is the first element on the periodic table. It has an atomic number of 1. It is highly flammable and

is the most common element found in our universe. Morehydrogen facts.

Liquid nitrogen boils at 77 kelvin (−196 °C, −321 °F). More nitrogen facts.
Around 1% of the sun’s mass is oxygen. More oxygen facts.
Helium is lighter than the air around us so it floats, that's why it is perfect for
the balloons you get at parties. More helium facts.
Carbon comes in a number of different forms (allotropes), these include diamond, graphite and impure forms such as coal. More carbon facts, diamond facts, and coal facts.
Although it is still debated, it is largely recognized that the word 'chemistry' comes from an Egyptian word meaning 'earth'.
The use of various forms of chemistry is believed to go back as long ago as the Ancient Egyptians. By 1000 BC civilizations were using more complex forms of chemistry such as using plants for medicine, extracting metal from ores, fermenting wine and making cosmetics.
Things invisible to the human eye can often be seen under UV light, which comes in handy for both scientists and detectives.
Humans breathe out carbon dioxide (CO2). Using energy from sunlight, plantsconvert carbon dioxide into food during a process called photosynthesis.
Chemical reactions occur all the time, including through everyday activities such as cooking. Try adding an acid such as vinegar to a base such as baking soda and see what happens!
Above 4 °C, water expands when heated and contracts when cooled. But between 4 °C and 0 °C it does the opposite, contracting when heated and expanding when cooled. Stronger hydrogen and oxygen bonds are formed as the water crystallizes into ice. By the time it's frozen it takes up around 9% more space.
Often formed under intense pressure over time, a crystal is made up of molecules or atoms that are repeated in a three dimensional repeating pattern. Quartz is a well known example of a crystal.
Athletes at the Olympic Games have to be careful how much coffee they drink. The caffeine in coffee is a banned substance because it can enhance performance. One or two cups are fine but they can go over the limit with more than five. (update - as of 2004 caffeine has been taken back off the WADA banned list but its use will be closely monitored to prevent future abuse by athletes.)

Hydrogen Facts

The chemical symbol of hydrogen is H. It is an element with atomic number 1, this means that 1 proton is found in the nucleus of hydrogen.
Hydrogen is the lightest, simplest and most commonly found chemical element in the Universe, making up around 75% of its elemental mass.
Hydrogen is found in large amounts in giant gas planets and stars, it plays a key role in powering stars through fusion reactions.
Hydrogen is one of two important elements found in water (H2O). Each molecule of water is made up of two hydrogen atoms bonded to one oxygen atom.
In 1766, during an acid metal reaction, Henry Cavendish first formally recognized hydrogen. In 1781 he also found that hydrogen produces water when burned. While Cavendish is usually given credit for the discovery of hydrogen as an element, it had been produced by earlier scientists who were unaware of hydrogen as a unique chemical element.
It wasn’t until a few years later (1783) that hydrogen was given its name. The word hydrogen comes from the Greek word hydro (meaning water) and genes (meaning creator).
Hydrogen gas has the molecular formula H2. At room temperature and under standard pressure conditions, hydrogen is a gas that is tasteless, odorless and colorless.
Hydrogen can exist as a liquid under high pressure and an extremely low temperature of 20.28 kelvin (−252.87°C, −423.17 °F). Hydrogen is often stored in this way as liquid hydrogen takes up less space than hydrogen in its normal gas form. Liquid hydrogen is also used as a rocket fuel.
Under extreme compression hydrogen can also make a transition to a state known as metallic hydrogen. Laboratory research into this area is ongoing as scientists continue efforts to produce metallic hydrogen at low temperature and static compression

Nitrogen Facts
Nitrogen is a chemical element with the symbol N and atomic number of 7.
Under normal conditions nitrogen is a colorless, odorless and tasteless gas.
Nitrogen makes up around 78% of the air you breathe.
Nitrogen is present in all living things, including the human body and plants.
Nitrogen gas is used in food storage to keep packaged or bulk foods fresh. It is also used in the making of electronic parts, for industrial purposes and has many other useful applications.
Nitrogen gas is often used as an alternative to carbon dioxide for storing beer in pressurized kegs. The smaller bubbles it produces is preferred for some types of beer.
Titan, the largest moon of Saturn, has an atmosphere nearly entirely made of nitrogen (over 98%). It is the only moon in our solar system known to have a dense atmosphere.
Nitrogen is in a liquid state when at a very low temperature. Liquid nitrogen boils at 77 kelvin (−196 °C, −321 °F). It is easily transported and has many useful applications including storing items at cold temperatures, in the field of cryogenics (how materials behave at very low temperatures), as a computer coolant (a fluid used to prevent overheating), removing warts and much more.
Decompression sickness (also known as the bends) involves nitrogen bubbles forming in the bloodstream and other important areas of the body when people depressurize too quickly from scuba diving. Similar situations can occur for astronauts and those working in unpressurized aircraft.
Nitrous oxide (also known as laughing gas or by its chemical formula N2O) is used in hospitals and dental clinics as an anesthetic (removing or reducing pain and general awareness for various surgeries).
Nitrous oxide is also used in motor racing to increase the power of engine and speed of the vehicle. When used for this purpose it is often referred to as nitrous or NOS.
Nitrous oxide is a considerable greenhouse gas and air pollutant. By weight is has nearly 300 times more impact than carbon dioxide.
Nitroglycerin is a liquid used to create explosives such as dynamite. It is often used in the demolition and construction industries as well as by the military.
Nitric acid (HNO3) is a strong acid often used in the production of fertilizers.
Ammonia (NH3) is another nitrogen compound commonly used in fertilizers.




Oxygen Facts
Oxygen is an element with the chemical symbol O and atomic number 8.
Oxygen is a very reactive element that easily forms compounds such as oxides.
Under standard temperature and pressure conditions two oxygen atoms join to form dioxygen (O2), a colorless, tasteless and odorless gas.
Oxygen is essential to human life, it is found in the air we breathe and the water we drink (H20).
Oxygen makes up around 21% of the air you breathe. It is also the most common element in the Earth’s crust (around 47%) and the third most common element in the Universe (but far less than hydrogen and helium, the two most common).
The large amount of oxygen on Earth is supported by the oxygen cycle which involves the movement of oxygen between the air, living things and the Earth’s crust. Photosynthesis (a process that converts carbon dioxide into organic compounds using sunlight) plays a major role in this cycle.
Ozone (O3) is an allotrope (different form) of oxygen that combines three oxygen atoms together. While ground level ozone is an air pollutant, the ozone layer in the Earth’s upper atmosphere provides protection from the suns harmful rays by filtering UV light.
The sun’s mass is made up of around 1% oxygen.
Between 1770 and 1780, Swedish pharmacist Carl Wilhelm Scheele, British clergyman Joseph Priestley and French chemist Antoine Laurent Lavoisier researched, documented and helped discover oxygen. The name oxygen was first used by Lavoisier in 1777.
Oxygen therapy is used as a common medical treatment. You may have seen patients on TV or in real life using an oxygen mask or nasal cannula (a plastic tube that fits behind the ears and delivers oxygen through the nostrils).
Oxygen has a number of other practical uses such as smelting metal from ore, water treatment, as an oxidizer for rocket fuel and a number of other industrial, chemical and scientific applications.
Concentrated oxygen promotes fast combustion. While a spark or heat is still needed to start a fire, having concentrated oxygen near various fuels can be very dangerou