To distinguish sodium carbonate and sodium hydrogencarbonate, we cannot add water as both of them dissolve. We will heat the solid and the one remains stable is sodium carbonate while the one decompose on heating to release a colourless gas which can turn lime water milky is sodium hydrogencarbonate.
2NaHCO3(s) --> Na2CO3 +H2O + CO2
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To distinguish sulphate and sulphite solution, we can add acidified barium nitrate solution. The one which forms white precipitate is sodium sulphate while the solution which does not show observable change is sodium sulphite.
*barium sulphite is insoluble in water but redissolve in dilute nitric acid.
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To test for the presence of silver ions, add excess ammonia solution to see whether colourless solution form.
2008年4月23日 星期三
CE Chemistry: reagent store in brown bottle
Concentrated nitric acid, potassium permanganate solution, silver nitrate and chlorine water is usually kept in a brown bottle. It is because they are easily decompose by light.
ethanol, ethyl ethanote and 1,1,1-trichloroethene should be heat under water bath because they are flammable.
ethanol, ethyl ethanote and 1,1,1-trichloroethene should be heat under water bath because they are flammable.
2008年4月21日 星期一
CE Physics: conservation of energy cannot be applied to the question of inelastic collision
Two pendula of the same length are arranged as bob A of mass M at the equilibrium position and bob B of mass 2M hang aside of vertical height 0.45m . Bob B is relased and hit the other boba at the equilibrium position. Finally the two ball stick together. Find the common velocity of the bobs just after collision.
If you use the formula: potential energy of B at original position=Kinetic energy of B at the equilibrum= kinetic energy after collision, then you are wrong. Kinetic Energy is not conserve in the system of inelastic collision. So better find the velocity of B just before collision and use conservation of momentum to reach the answer.
If you use the formula: potential energy of B at original position=Kinetic energy of B at the equilibrum= kinetic energy after collision, then you are wrong. Kinetic Energy is not conserve in the system of inelastic collision. So better find the velocity of B just before collision and use conservation of momentum to reach the answer.
CE Physics: force of impact...
A ball of mass 3kg drops from rest to the ground from a height of 5m. After the impact it rebounds to the same height. Assume that the time of impact between the ball and the ground is 0.2s. What is the rorce of impact acting on the ground surface by the ball?
The answer is 330N, not 300N. 300 is the net force acting on the ball during impact. Talking about impact force acting on the ground surface by the ball, it is action force acting on the ground by the ball in other words, so we have to include weight in the calculation.
The answer is 330N, not 300N. 300 is the net force acting on the ball during impact. Talking about impact force acting on the ground surface by the ball, it is action force acting on the ground by the ball in other words, so we have to include weight in the calculation.
2008年4月19日 星期六
CE Physics:notes on atomic physics
In alpha particle scattering experiment, when gold foil is in 1mm thickness, The number of particles detected at the opposite side of the source becomes zero but not just decreases. (92Q7)
Usually a radioactive source with X to 1X hours half life emitting gamma radiation is used for injection in patient. The reasons include:
1.The half-life is long enough for doctor to diagnose the patient.
2.The half-life is short so that the radiation will not retain inside the body for a long time causing little harm to the body
3.Gamma ray get high penetrating power so that it can be detected outside the body.
(93Q7)
Usually a radioactive source with X to 1X hours half life emitting gamma radiation is used for injection in patient. The reasons include:
1.The half-life is long enough for doctor to diagnose the patient.
2.The half-life is short so that the radiation will not retain inside the body for a long time causing little harm to the body
3.Gamma ray get high penetrating power so that it can be detected outside the body.
(93Q7)
CE Physics: copper is not a magnectic material
93Q5(ai)
Describe a method to find the magnetic field pattern on the cardboard using iron filings.
Ans.Sprinkle iron filings on the board and tap the board gently. The magnetic field pattern is shown by the pattern of the filings.
How about using copper filings?
Copper, though being a metal, is not a magnetic material. So we cannot use copper filings.
Describe a method to find the magnetic field pattern on the cardboard using iron filings.
Ans.Sprinkle iron filings on the board and tap the board gently. The magnetic field pattern is shown by the pattern of the filings.
How about using copper filings?
Copper, though being a metal, is not a magnetic material. So we cannot use copper filings.
2008年4月18日 星期五
CE physics: conservation of energy 的真諦
設一架過山車在A點(設離地100米) 以30m/s 移動，向下滑動後再上B 點，那麼B點可否超過A點的高度?(超過離地100米?) 假設friction 可以neglected.
答案是可以。如果單純看potential energy，車在B點的potential energy高於A，那麼energy 便不conserved了?這是因為看漏了kinetic energy，留意車在A點是在移動，那麼只要車在B點的速度少於A點，彌補多出的potential energy，兩者整體的energy(KE+PE) 都是一樣，energy is conserved 。
答案是可以。如果單純看potential energy，車在B點的potential energy高於A，那麼energy 便不conserved了?這是因為看漏了kinetic energy，留意車在A點是在移動，那麼只要車在B點的速度少於A點，彌補多出的potential energy，兩者整體的energy(KE+PE) 都是一樣，energy is conserved 。
CE Biology: MC knowledge
1.Female sex hormone can maintain pregnancy.
2.Cotyledons in seeds do not carry out photosynthesis.
3.Small mammals have bigger surface area to volume ratio so that more heat is lost.
4.Sweat gland can only be found in mammal.
5.Concentration of urea and salt in glomerular filtrate is the same as plasma.
6.The highest concentration in glomerular filtrate is sodium chloride not urea.
7.Vertebral column consist of many small bone to enable body to bend.
8.Axon presents in the central nervous system are non-myelinated.
9.Both red blood cell and plasma involve in transportation of carbon dioxide towards the lung.
10.Calcium, which can be removed by sodium oxalate in solution, takes parts in blood clotting.
11.Double circulation maintain higher blood pressure and prevent the mixing of oxygenated and deoxygenated blood.
12.Canine absents in some mammals.
13.The widespread distribution of veins give mechanical support to the flattened blade of the leaf.
14.The vascular bundles of a dicotyledonous root are usually presence at the central core to provide better anchoring effect to act against the uprooting stress of wind or by-passing animals.
15.The vascular bundles of a dicotyledonous stem are usually presence along the periphery to provide better strengthening effect to act against the bending stress of wind.
16.At night, the accumulation of carbon dioxide stimulate sugar in the guard cell changing to starch thereby raise the water potential there and causing water to be drawn to be drawn to the surrounding which cause the stomata to close.
17.Each time before the level of the coloured water in the graduated U glass tube is measured, the leveling arm has to be moved up and down until the water levels in both the leveling arm are the same. This is to ensure that each time the volume of the gas trapped in the sample chamber is measured under the same pressure.
18.Contour ploughing prevents gully erosion.
19.Warm water decrease the solubility of gases and this make less oxygen available in water.this may lead to suffocation of certain aerobic life.
20.Carbon dioxide can be utilized by aquatic animals to combine with calcium for the formation of calcareous shells.
21.Bacteria carry out sexual reproduction conjugation.
Special thanks to Y.K.To, author of Complete Multiple choice Biology
2.Cotyledons in seeds do not carry out photosynthesis.
3.Small mammals have bigger surface area to volume ratio so that more heat is lost.
4.Sweat gland can only be found in mammal.
5.Concentration of urea and salt in glomerular filtrate is the same as plasma.
6.The highest concentration in glomerular filtrate is sodium chloride not urea.
7.Vertebral column consist of many small bone to enable body to bend.
8.Axon presents in the central nervous system are non-myelinated.
9.Both red blood cell and plasma involve in transportation of carbon dioxide towards the lung.
10.Calcium, which can be removed by sodium oxalate in solution, takes parts in blood clotting.
11.Double circulation maintain higher blood pressure and prevent the mixing of oxygenated and deoxygenated blood.
12.Canine absents in some mammals.
13.The widespread distribution of veins give mechanical support to the flattened blade of the leaf.
14.The vascular bundles of a dicotyledonous root are usually presence at the central core to provide better anchoring effect to act against the uprooting stress of wind or by-passing animals.
15.The vascular bundles of a dicotyledonous stem are usually presence along the periphery to provide better strengthening effect to act against the bending stress of wind.
16.At night, the accumulation of carbon dioxide stimulate sugar in the guard cell changing to starch thereby raise the water potential there and causing water to be drawn to be drawn to the surrounding which cause the stomata to close.
17.Each time before the level of the coloured water in the graduated U glass tube is measured, the leveling arm has to be moved up and down until the water levels in both the leveling arm are the same. This is to ensure that each time the volume of the gas trapped in the sample chamber is measured under the same pressure.
18.Contour ploughing prevents gully erosion.
19.Warm water decrease the solubility of gases and this make less oxygen available in water.this may lead to suffocation of certain aerobic life.
20.Carbon dioxide can be utilized by aquatic animals to combine with calcium for the formation of calcareous shells.
21.Bacteria carry out sexual reproduction conjugation.
Special thanks to Y.K.To, author of Complete Multiple choice Biology
2008年4月14日 星期一
Circle touching each other
When you come across this situation, DRAW TANGENT at the point touching each other
2008年4月8日 星期二
CE Mathematics: cancel out the given impossible option in probability
There are 3 balls in a bag, they are black(B), red(R) and white(W) respectively. Suppose the probability of drawing B is 0.1, R is 0.4 and W is 0.5. You draw a ball and it is known that it is black or red(that means it cannot be white). What is the probability of drawing a black ball?
Originally: 0.1+0.4+0.5=1
As drawing white ball is impossible, we cancel it out and it becomes
0.1+0.4=0.5......(*)
(*) x 2 to have one as the utimate possibility 1
0.2+0.8=1
Thus the probability of drawing a black ball is 0.2.
Originally: 0.1+0.4+0.5=1
As drawing white ball is impossible, we cancel it out and it becomes
0.1+0.4=0.5......(*)
(*) x 2 to have one as the utimate possibility 1
0.2+0.8=1
Thus the probability of drawing a black ball is 0.2.
CE Chemistry: describing cleaniness of fuels
low carbon content + complete combustion =clean fuels
high carbon content + incomplete combustion and particulates/CO/...= not clean fuels
high carbon content + incomplete combustion and particulates/CO/...= not clean fuels
2008年4月7日 星期一
CE Physics:machanics- newton's law and the definition of power
When situation is at rest or in constant acceleration, use newton's first law.
Accelerating, use second law.
Two objects, use third law.
They are some points in the (*) questions.
Power is the rate of change in energy, no matter it is potential energy, kinetic energy,etc
It can be express as P=Fv.
Sometimes differentiation can help solve the problem.
Accelerating, use second law.
Two objects, use third law.
They are some points in the (*) questions.
Power is the rate of change in energy, no matter it is potential energy, kinetic energy,etc
It can be express as P=Fv.
Sometimes differentiation can help solve the problem.
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