Choose a course and topic, attempt each question and open its complete worked solution when you are ready.
What is an electric circuit?
An electric circuit is a closed, continuous conducting path containing a source and components through which electric charge can flow. If the path is open, no steady current flows.
A closed conducting path for electric current.
What is the power of accommodation of the eye?
Accommodation is the eye's ability to alter the curvature and therefore focal length/power of its lens so that objects at different distances form sharp images on the fixed retina. Ciliary muscles contract for a near object, making the lens thicker and more powerful; they relax for a distant object.
The ability of the eye lens to change focal length and focus at different distances.
Define the principal focus of a concave mirror.
When rays parallel to the principal axis strike a concave mirror, the reflected rays meet at a point on the axis. This point is the principal focus F. It is real and lies in front of the mirror midway between pole P and centre of curvature C for a paraxial spherical mirror.
The point on the principal axis where paraxial reflected rays converge.
In an asexually reproducing population, trait A occurs in 10% and trait B in 60% of individuals. Which trait probably appeared earlier?
A new heritable variation begins in one lineage and is passed to its descendants. Other factors being comparable, a variation found in a larger fraction has had more generations in which to spread. Trait B, present in 60%, is therefore more likely to be older. This is a probability, because selection can also change frequencies.
Trait B is more likely to have arisen earlier.
Why is accurate DNA copying central to reproduction?
DNA carries the instructions for body design and cellular function. Copying passes these instructions from parent to offspring, preserving the basic features of the species. Copying is highly accurate but not perfect; small changes produce heritable variation. Thus DNA copying provides both continuity and the variation needed for long-term survival.
It transfers hereditary information while occasional errors create variation.
Distinguish a reflex action from walking.
A reflex is a rapid, automatic response to a stimulus, usually coordinated through a reflex arc in the spinal cord; withdrawal from a hot object is an example. Walking is a voluntary, learned and coordinated movement initiated by the brain. It uses sensory feedback, motor areas, cerebellum and repeated muscle activity, although its rhythm can become largely automatic.
Reflex: rapid involuntary spinal response. Walking: voluntary, learned brain-coordinated movement.
Why can diffusion alone not supply enough oxygen to a large multicellular organism such as a human?
In a large body most cells are far from the external surface, the surface-area-to-volume ratio is small, and diffusion is effective only over short distances. Human cells also use oxygen rapidly. Diffusion through many cell layers would therefore be far too slow. A respiratory surface and circulatory system provide rapid bulk transport.
Large distances, a low surface-area-to-volume ratio and high demand make diffusion alone too slow.
Construct the electron-dot structure of carbon dioxide, \(\ce{CO2}\), and identify the bonds.
Carbon has four valence electrons and each oxygen has six. Carbon shares two pairs with each oxygen, giving every atom an octet:
Each C=O is a double covalent bond; every oxygen also retains two lone pairs.
\(\ce{O=C=O}\), with two lone pairs on each O and two C=O double bonds.
Give one suitable metal for each description: (i) liquid at room temperature(ii) soft enough to cut with a knife(iii) best conductor of heat(iv) poor conductor of heat.
(i) Mercury is liquid at room temperature. (ii) Sodium is soft and can be cut with a knife. (iii) Silver has the highest thermal conductivity among metals. (iv) Lead and mercury are comparatively poor conductors of heat; lead is an acceptable example.
(i) Mercury(ii) sodium(iii) silver(iv) lead.
Three unlabelled test tubes contain distilled water, an acid and a base. Devise a sequence using only red litmus paper to identify all three liquids.
Dip separate strips of red litmus into the three liquids. The liquid that turns red litmus blue is the base.
Use the blue strip produced in this test on the two remaining liquids. The liquid that changes it back to red is the acid. The liquid that produces no colour change is distilled water.
Fresh strips and clean droppers should be used to avoid mixing the samples.