Choose a course and topic, attempt each question and open its complete worked solution when you are ready.
If resistance stays constant and potential difference is halved, what happens to current?
Ohm's law gives \(I=\frac{V}{R}\). With constant R, current is directly proportional to voltage; halving V halves I.
The current becomes one-half.
Spectacle powers are -5.5 D for distant vision and +1.5 D for near vision. Calculate both focal lengths.
Use \(f=\frac{1}{P}\) in metres.
From the refractive-index table, identify the optically densest and least dense media.
Optical density rises with refractive index. Diamond has the greatest listed value (about 2.42), while air has the smallest value (about 1.0003; vacuum is exactly 1).
Highest: diamond. Lowest listed medium: air.
Outline a project for determining which coat colour is dominant in dogs.
Select dogs of two clearly distinguishable coat colours and record several generations of reliable pedigrees; do not conduct unsafe or indiscriminate breeding. Identify true-breeding lines where possible. If all offspring of parents with contrasting true-breeding colours show one colour, that colour is the candidate dominant. Examine offspring from candidate heterozygotes: a near 3:1 dominant-to-recessive ratio supports simple dominance. Use a large sample, record sex and ancestry, and recognise that dog coat colour may involve multiple genes.
State the functions of seminal vesicles and the prostate gland.
Seminal vesicles add a fructose-rich fluid that nourishes sperm and contributes most of semen volume. The prostate adds an alkaline secretion that improves sperm motility and helps neutralise acidity in the urethra and female tract. Together the fluids form a transport medium for sperm.
They add nutritive, protective and alkaline fluids to sperm to form semen.
Explain how auxin causes a tendril to coil around a support.
When a tendril touches a support, auxin moves away from the contacted side. Cells on the opposite side receive more auxin and elongate faster than cells on the touched side. This unequal growth bends the tendril towards and then around the support.
Unequal auxin distribution causes greater growth away from contact and coils the tendril.
Explain how the small intestine is adapted for absorption of digested food.
It is long and highly folded. Millions of finger-like villi, covered with microvilli, provide a very large surface. The epithelium is one cell thick, giving a short diffusion path. Each villus has a dense blood-capillary network for sugars and amino acids and a lacteal for fats. Continuous blood and lymph flow maintain concentration gradients.
Villi and microvilli give a large, thin, well-supplied absorptive surface.
Why is oxy-ethyne preferred to an air-ethyne mixture for welding?
Pure oxygen supports complete combustion of ethyne and produces a clean blue flame at very high temperature, sufficient to melt metal. Air contains only about 21% oxygen and much nitrogen. Ethyne therefore burns less completely in air, giving a cooler, yellow, sooty flame unsuitable for welding.
Oxygen gives complete combustion and a much hotter, non-sooty flame.
Explain the high melting points of ionic compounds.
Positive and negative ions are arranged in a rigid crystal lattice. Strong electrostatic forces of attraction act in all directions between oppositely charged ions. A large amount of thermal energy is required to overcome these forces and separate the ions, so ionic compounds have high melting and boiling points.
Strong electrostatic attraction in the ionic lattice requires much energy to overcome.
Describe how dilution affects the hydronium-ion concentration of an acidic solution.
Dilution increases the volume of solvent while the amount of acid initially present remains fixed. The hydronium ions become distributed through a larger volume, so the number of \(\ce{H3O+}\) ions per unit volume decreases. Consequently, the pH rises towards 7 and the solution becomes less acidic.
Dilution decreases \([\ce{H3O+}]\) and raises the pH towards 7.