Chemical Reactions Quiz

A chemical reaction is a process where:

• Substances only change shape

• One or more substances change into new substances

• Heat is always released

• Only physical properties change

Answer explanation

A chemical reaction involves the transformation of one or more substances into new substances, which distinguishes it from physical changes. The correct choice highlights this fundamental aspect of chemical reactions.

Which of the following is NOT a chemical reaction?

• Burning of wood

• Melting of ice

• Rusting of iron

• Reaction of acid with a base

Answer explanation

Melting of ice is a physical change, not a chemical reaction, as it only changes the state of water without altering its chemical composition. The other options involve chemical changes.

Burning is a chemical reaction involving:

• Oxygen

• Carbon dioxide

• Hydrogen

• Nitrogen

Answer explanation

Burning is a combustion reaction that requires oxygen to occur. It reacts with a fuel, producing heat and light, while carbon dioxide and water are typically the products. Thus, oxygen is essential for burning.

The ashes formed after burning mainly contain:

• Oxides

• Acids

• Salts

• Bases

Answer explanation

The ashes formed after burning primarily consist of oxides, which are compounds formed when elements react with oxygen. Other options like acids, salts, and bases are not the main components of ashes.

Which equation correctly represents burning of carbon?

• C + O2 → CO

• C + O2 → CO2 + heat

• C + H2 → CH4

• C + O2 → C2O

Answer explanation

The correct equation for burning carbon is C + O2 → CO2 + heat, as it shows carbon reacting with oxygen to produce carbon dioxide and release energy, which is characteristic of combustion.

Sodium reacts with water to form:

• NaCl + H2

• NaOH + H2 + heat

• Na2O + H2

• Na2CO3

Answer explanation

Sodium reacts with water to produce sodium hydroxide (NaOH), hydrogen gas (H2), and releases heat. This is a highly exothermic reaction, making 'NaOH + H2 + heat' the correct answer.

Which metals react violently with water?

• Iron and Copper

• Sodium and Potassium

• Zinc and Magnesium

• Lead and Silver

Answer explanation

Sodium and potassium are alkali metals that react violently with water, producing hydrogen gas and heat. This reaction can be explosive, unlike iron, copper, zinc, magnesium, lead, and silver, which do not react as aggressively.

Which observation is NOT expected when sodium reacts with water?

• Heat is produced

• Bubbles form

• Flames may appear

• Ice forms

Answer explanation

When sodium reacts with water, it produces heat, bubbles, and may even create flames due to the vigorous reaction. However, ice does not form, as the reaction generates heat, preventing freezing.

Metals reacting with acids produce:

• Salt + Hydrogen gas

• Acid + Base

• Water + Salt

• Oxygen + Water

Answer explanation

When metals react with acids, they displace hydrogen ions, resulting in the formation of salt and hydrogen gas. Therefore, the correct answer is Salt + Hydrogen gas.

The test for hydrogen gas is:

• Turns limewater milky

• Pops with a lit splint

• Extinguishes fire

• Burns with a green flame

Answer explanation

The test for hydrogen gas is that it 'pops with a lit splint'. This characteristic sound is due to the rapid combustion of hydrogen when it reacts with oxygen, confirming its presence.