Question

section b (20marks) answer only two questions from this section 2. (a) (i) explain why aluminium resists corrosion. (ii) what is rust? (b) (i) define dispersion of light. (ii) explain the formation of rainbow in the sky. (c) (i) what is a thermostat? (ii) name two (2) appliances which use thermostats. 3. (a) state any three (3) effects of pests and parasites on crops. (b) (i) define heat. (ii) distinguish between heat and temperature. (c) draw a ray - diagram to show how the image of a point object is seen in a plane - mirror. 4. (a) (i) state any two (2) uses of a plane mirror. (ii) briefly explain why wood is used as a handle for some cooking utensils. (b) classify the following materials as metals and non - metals: al, cu, fe, au, c, o

Explanation:

Step1: Answer 2(a)(i)

Aluminium forms a thin, continuous, impervious layer of aluminium oxide ($Al_2O_3$) on its surface when exposed to air. This oxide layer acts as a barrier and prevents further oxidation of the underlying metal, thus resisting corrosion.

Step2: Answer 2(a)(ii)

Rust is a hydrated iron(III) oxide, mainly $Fe_2O_3\cdot xH_2O$. It forms when iron reacts with oxygen and water in the air.

Step3: Answer 2(b)(i)

Dispersion of light is the phenomenon of splitting of white - light into its constituent colors (spectrum) when it passes through a prism or a similar dispersive medium due to the different refractive indices of different colors of light.

Step4: Answer 2(b)(ii)

A rainbow is formed when sunlight passes through water droplets in the air. The water droplets act as prisms. Sunlight is first refracted as it enters a water - droplet, then internally reflected inside the droplet, and then refracted again as it exits the droplet. Different colors of light are refracted by different amounts (dispersion), creating the rainbow.

Step5: Answer 2(c)(i)

A thermostat is a device that senses temperature and is used to maintain a desired temperature by switching heating or cooling systems on or off.

Step6: Answer 2(c)(ii)

Two household appliances that use thermostats are refrigerators (to maintain a low temperature inside) and electric irons (to maintain a set temperature for ironing).

Step7: Answer 3(a)

1. Pests and parasites reduce crop yield by feeding on the plant parts such as leaves, stems, roots, and fruits.
2. They can transmit diseases to crops, further weakening the plants and reducing productivity.
3. They may cause physical damage to the crops, making them unmarketable (e.g., holes in fruits or leaves).

Step8: Answer 3(b)(i)

Heat is a form of energy that is transferred between two objects or systems due to a temperature difference. It is measured in joules (J).

Step9: Answer 3(b)(ii)

Heat is a form of energy transfer, while temperature is a measure of the average kinetic energy of the particles in a substance. Heat transfer occurs from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached.

Step10: Answer 3(c)

(Here, a ray - diagram description: A point object emits rays of light. These rays strike the plane mirror. According to the law of reflection, the angle of incidence is equal to the angle of reflection. The reflected rays appear to diverge from a point behind the mirror, which is the virtual image of the point object.)

Step11: Answer 4(a)(i)

Two uses of a plane mirror are: 1. In dressing tables for personal grooming. 2. In periscopes (in submarines) to see objects that are not in the direct line of sight.

Step12: Answer 4(a)(ii)

Wood is used as a handle for some cooking utensils because wood is a poor conductor of heat. This prevents the handle from getting too hot during cooking, allowing the user to hold the utensil safely.

Step13: Answer 4(b)

Metals: Al (aluminium), Fe (iron), Cu (copper), Au (gold). Non - metals: C (carbon), O (oxygen).

Answer:

2(a)(i): Aluminium forms a protective oxide layer.
2(a)(ii): Rust is hydrated iron(III) oxide.
2(b)(i): Splitting of white - light into its colors.
2(b)(ii): Sunlight refracts, reflects and disperses through water droplets.
2(c)(i): A device to sense and control temperature.
2(c)(ii): Refrigerators and electric irons.
3(a): Reduce crop yield, transmit diseases, cause physical damage.
3(b)(i): A form of energy transfer due to temperature difference.
3(b)(ii): Heat is energy transfer, temperature is average kinetic energy measure.
3(c): Ray - diagram shows reflection from plane mirror forming virtual image.
4(a)(i): For personal grooming and in periscopes.
4(a)(ii): Wood is a poor heat conductor.
4(b): Metals: Al, Fe, Cu, Au; Non - metals: C, O.