Question

1. choose the best answer.

look at the products of this equation. what is formed that provides evidence of a chemical change?
na₂s(aq) + 2hcl(aq) → 2nacl(aq) + h₂s(g)

gas
water
precipitate

1. choose all that apply.

look at the products of this equation. what two are formed that provide evidence of a chemical change?
2naoh(aq) + h₂co₃(aq) → na₂co₃(s) + 2h₂o(l)

gas
precipitate
water

1. choose the best answer.

which of these is not a double replacement reaction?
3hcl + caco₃ → cacl₂ + h₂o + co₂
2na₃po₄ + 3cacl₂ → 6nacl + ca₃(po₄)₂
h₃po₄ + 3koh → k₃po₄ + 3h₂o
h₂so₄ + 2nano₃ → 2hno₃ + na₂so₄

Explanation:

Step3: Identify chemical change evidence

The reaction $\text{Na}_2\text{S}(aq) + 2\text{HCl}(aq)
ightarrow 2\text{NaCl}(aq) + \text{H}_2\text{S}(g)$ produces $\text{H}_2\text{S}$, a gas. Formation of gas is visible evidence of chemical change.

Step4: Identify chemical change evidence

The reaction $2\text{NaOH}(aq) + \text{H}_2\text{CO}_3(aq)
ightarrow \text{Na}_2\text{CO}_3(s) + 2\text{H}_2\text{O}(l)$ forms solid $\text{Na}_2\text{CO}_3$ (precipitate) and liquid $\text{H}_2\text{O}$. Both are evidence of chemical change.

Step5: Classify reaction type

Double replacement reactions involve swapping ions between two compounds. The reaction $2\text{HCl} + \text{CaCO}_3
ightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$ is a gas-forming reaction that follows the pattern of a single acid reacting with a carbonate, not a double swap of ions between two ionic compounds. The other reactions are double replacement.

Answer:

1. A. gas
2. B. precipitate, C. water
3. A. $2\text{HCl} + \text{CaCO}_3

ightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$