What are the definitions of mass number and atomic number, and why should passengers wear seatbelts?

Molar mass of HCl ($M$) = Atomic mass of H + Atomic mass of Cl

Volume ($V$) = $500 \text{ cm}^3 = 0.500 \text{ dm}^3$

2 (c) State five characteristics of images formed in a plane mirror.

• Virtual (cannot be projected onto a screen)
• Erect (upright)
• Laterally inverted (left and right are swapped)
• Same size as the object
• Same distance behind the mirror as the object is in front

2 (d) State five benefits a nation derives from agriculture.

• Food security for its population.
• Employment opportunities for a significant portion of the workforce.
• Generation of foreign exchange through export of agricultural products.
• Provision of raw materials for industries (e.g., textiles, food processing).
• Rural development and poverty reduction in agricultural areas.

3 (a) (i) What is relative density?
Relative density (or specific gravity) is the ratio of the density of a substance to the density of a reference substance, usually water at 4°C. It is a dimensionless quantity.

3 (a) (ii) Calculate the relative density of brine, if its density is 1.6 g/cm$^3$. (Density of water = 1 g/cm$^3$)

Density of brine = $1.6 \text{ g/cm}^3$
Density of water = $1 \text{ g/cm}^3$

3 (b) Describe, briefly, how urine is formed in humans.
Urine formation occurs in the kidneys through three main processes: glomerular filtration, where blood plasma is filtered in the glomerulus to form a filtrate; tubular reabsorption, where essential substances like water, glucose, and salts are reabsorbed from the filtrate back into the blood in the renal tubules; and tubular secretion, where waste products and excess ions are actively secreted from the blood into the filtrate. The remaining fluid, now concentrated with wastes, becomes urine.

3 (c) State five ways by which the fertility of soil can be enhanced.

• Adding organic matter (e.g., compost, manure) to improve soil structure and nutrient retention.
• Practicing crop rotation to prevent nutrient depletion and break pest cycles.
• Using cover crops to protect the soil from erosion and add nitrogen.
• Applying appropriate fertilizers (organic or inorganic) to replenish specific nutrients.
• Controlling soil pH through liming or acidification to optimize nutrient availability.

3 (d) (i) What is hard water?
Hard water is water that contains a high concentration of dissolved mineral ions, primarily calcium (Ca$^{2+}$) and magnesium (Mg$^{2+}$) ions.

3 (d) (ii) Give three disadvantages of hard water.

• Forms scum with soap, reducing its lathering ability and effectiveness.
• Causes limescale buildup in pipes, kettles, and water heaters, reducing efficiency and potentially causing damage.
• Can lead to increased consumption of soap and detergents to achieve cleaning.

4 (a) Describe how igneous rocks are formed.
Igneous rocks are formed from the cooling and solidification of molten rock material. This molten rock, called magma when it is beneath the Earth's surface, or lava when it erupts onto the surface, cools and crystallizes. The rate of cooling determines the crystal size: slow cooling underground forms large crystals (intrusive igneous rocks), while rapid cooling on the surface forms small crystals or glassy textures (extrusive igneous rocks).

4 (b) (i) What is meant by the term classification as used in science?
Classification in science refers to the systematic arrangement of organisms or objects into groups or categories based on shared characteristics, similarities, and evolutionary relationships. This helps in organizing knowledge, identifying relationships, and making predictions.