What are the answers to the following questions about states of matter, water treatment, water properties, plant nutrients, and railway tracks?

Here are the answers to the questions:

40. Which statement correctly describes the states of matter commonly studied in basic science?

• C. Four states including plasma
  • The four common states of matter are solid, liquid, gas, and plasma.

41. Which sequence best represents the stages in water treatment?

• A. Sedimentation $\rightarrow$ filtration $\rightarrow$ chlorination
  • A typical sequence for water treatment involves sedimentation (allowing heavy particles to settle), filtration (removing smaller suspended particles), and then chlorination (disinfection to kill microorganisms).

42. Which property of water involves a chemical change?

• D. Reaction with sodium
  • Freezing, boiling, and evaporation are physical changes where water changes state but remains $\text{H}_2\text{O}$. Reacting with sodium involves a chemical reaction where new substances (sodium hydroxide and hydrogen gas) are formed.

43. Which set of elements represents the major nutrients required by plants for healthy growth?

• B. Nitrogen, phosphorus and potassium
  • Nitrogen (N), Phosphorus (P), and Potassium (K) are the three primary macronutrients (NPK) essential for plant growth.

44. Gaps are left between railway tracks mainly to

• D. allow for expansion
  • Railway tracks are made of metal, which expands when heated (e.g., by the sun) and contracts when cooled. Gaps are left to prevent buckling or cracking due to thermal expansion.

45. Which device converts electrical energy to sound energy using an electromagnet?

• C. Loudspeaker
  • A loudspeaker uses an electromagnet to convert electrical signals into mechanical vibrations, which then produce sound waves.

46. The time required for one complete vibration of a wave is called

• D. period
  • The period of a wave is the time it takes for one complete cycle or vibration to occur.

47. Which property of sound enables us to distinguish between high and low notes?

• C. Pitch
  • Pitch is determined by the frequency of a sound wave. Higher frequency sounds have a higher pitch (high notes), and lower frequency sounds have a lower pitch (low notes).

48. When a torchlight is switched on, the energy transformation involved is

• D. chemical $\rightarrow$ electrical $\rightarrow$ light
  • The batteries in a torchlight store chemical energy, which is converted into electrical energy to power the bulb, producing light (and some heat).

49. Which sequence correctly shows the movement of food through the mammalian alimentary canal?

• B. Mouth $\rightarrow$ oesophagus $\rightarrow$ stomach $\rightarrow$ small intestine
  • Food enters through the mouth, passes down the oesophagus, into the stomach, and then into the small intestine for further digestion and absorption.

50. Animals depend on green plants mainly because photosynthesis

• B. produces food and oxygen
  • Green plants perform photosynthesis, converting light energy into chemical energy (food) and releasing oxygen as a byproduct. Animals consume plants or other animals that eat plants for food and require oxygen for respiration.