The correct option is 1 and 4 only.
Explanation
The reaction between metals and dilute acids is a fundamental chemical property of metals. In this process, reactive metals displace hydrogen from the acid to form a salt and hydrogen gas. This is classified as a single displacement reaction.
Statement-wise Analysis:
- Statement 1 is Correct: When a reactive metal (such as Zinc, Magnesium, or Iron) reacts with a dilute acid (like Hydrochloric acid or Sulphuric acid), the metal displaces hydrogen from the acid, liberating hydrogen gas ($H_2$).
- Statement 2 is Incorrect: The metal does not remain chemically unchanged. It undergoes a chemical change to form a metal salt. For example, in the reaction $Zn + 2HCl \rightarrow ZnCl_2 + H_2$, Zinc changes to Zinc Chloride.
- Statement 3 is Incorrect: A neutralisation reaction occurs between an acid and a base (or alkali) to produce salt and water. The reaction between a metal and an acid is a displacement reaction (specifically a redox reaction), not neutralisation.
- Statement 4 is Correct: Hydrogen gas is combustible. The standard laboratory test to confirm the presence of hydrogen is to bring a burning candle or matchstick near the evolved gas; it burns with a characteristic "pop" sound.
Key Takeaway:
General Equation: Metal + Dilute Acid $\rightarrow$ Metal Salt + Hydrogen Gas. This reaction is characteristic of metals placed above hydrogen in the reactivity series.