Explain the formation of scum when hard water is treated with soap.
Soap does not work properly in hard water. A soap is a sodium or potassium salt of long chain carboxylic acid. Hard water contains salts of calcium and magnesium. When soap is added to hard water, sodium and potassium ions present in soap displace calcium or magnesium ions from the water molecules forming an insoluble substance called scum. A lot of soap is wasted in the process.
For example:
Sodium Stearate + Calcium chloride → Sodium chloride + Calcium Stearate (scum)
Formation of scum hinders the formation of foam, so soap is thus unable to clean in hard water.
Draw the structure for the following compounds.
(i) Ethanoic acid (ii) Bromopentane*
(iii) Butanone (iv) Hexanal.
*Are structural isomers possible for bromopentane?
What is an homologous series? Explain with an example.
How would you name the following compounds?
(i) CH3 -- CH2 -- Br
(ii)
(iii)
What will be the formula and electron dot structure of cyclopentane?
Explain the nature of the covalent bond using the bond formation in CH3Cl.
Why are carbon and its compounds used as fuels for most applications?
How many structural isomers can you draw for pentane?
Explain the mechanism of the cleaning action of soaps.
What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
How would you distinguish experimentally between an alcohol and a carboxylic acid?
Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
What is a good source of energy?
Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like humans?
What are trophic levels? Give an example of a food chain and state the different trophic levels in it.
What changes can you make in your habits to become more environment-friendly?
What is the difference between a reflex action and walking?
What is the importance of DNA copying in reproduction?
If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Why should a magnesium ribbon be cleared before burning in air?
Define the principal focus of a concave mirror.
Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?
(i) Write the electron-dot structures for sodium, oxygen and magnesium.
(ii) Show the formation of Na2O and MgO by the transfer of electrons.
(iii) What are the ions present in these compounds?
What advantage over an aquatic organism does a terrestrial organism have with regard to obtaining oxygen for respiration?
Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).
Food cans are coated with tin and not with zinc because
(a) zinc is costlier than tin.
(b) zinc has a higher melting point than tin.
(c) zinc is more reactive than tin.
(d) zinc is less reactive than tin.
Which of the following constitute a food-chain?
(a) Grass, wheat and mango
(b) Grass, goat and human
(c) Goat, cow and elephant
(d) Grass, fish and goat
The magnification produced by a plane mirror is +1. What does this mean?
The position of three elements A, B and C in the Periodic Table are shown below –
Group 16 Group 17
- -
- A
- -
B C
(a) State whether A is a metal or non-metal.
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?
Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic