Answer the following questions: 13. What do water droplets formed on a cold surface after breathing out show about the lungs? 14. What is the main reason living organisms respire? 15. Calculate the mo

Molar mass of NH$_3$ = (1 $\times$ Molar mass of N) + (3 $\times$ Molar mass of H)  
Molar mass of NH$_3$ = $(1 \times 14) + (3 \times 1)$  
Molar mass of NH$_3$ = $14 + 3$  
Molar mass of NH$_3$ = $17 \text{ g mol}^{-1}$
*   **B) $17 \text{ g mol}^{-1}$**

16. A buffer solution is important in chemical reactions because it maintains a nearly constant pH. Buffers resist significant changes in pH when small amounts of acid or base are added. * D) maintains a nearly constant pH

17. The observation that best confirms a substance is a base is that it turns red litmus paper to blue. Acids turn blue litmus red, while bases turn red litmus blue.

    • B) turns red litmus paper to blue

18. When salt dissolves completely in water to form a clear solution, the mixture is best described as a homogeneous mixture. In a homogeneous mixture, the components are uniformly distributed throughout.

    • A) a homogeneous mixture

19. The atom is regarded as the basic unit of matter because it retains the chemical properties of an element. It is the smallest particle of an element that still exhibits the characteristic chemical behavior of that element.

    • B) retains the chemical properties of an element

20. In basic science, the states of matter commonly studied are solid, liquid, and gas, which are three states of matter.

    • C) three states of matter

21. To calculate the density of the metal block:

$$
\text{Density} \approx 1.923 \text{ g/cm}^3
$$
Rounding to two decimal places, the density is $1.92 \text{ g/cm}^3$.
*   **D) $1.92 \text{ g cm}^{-3}$**

22. Among the given options, mass is a fundamental quantity. Force, volume, and weight are derived quantities, meaning they can be expressed in terms of fundamental quantities. * A) Mass

23. A chemical bottle labelled with a fire symbol (flame pictogram) indicates that the substance is flammable.

    • C) flammable

24. The activity that best illustrates the harmful use of scientific knowledge is the use of DDT to kill fish in rivers. DDT is a pesticide that was found to be highly toxic to aquatic life and the environment, demonstrating a negative impact of scientific application.

    • D) Use of DDT to kill fish in rivers

25. The correct sequence in the application of the scientific method during an investigation is to first form a hypothesis, then conduct experimentation to test it, and finally, the results contribute to scientific knowledge.

    • C) Hypothesis $\rightarrow$ Experimentation $\rightarrow$ Scientific knowledge