Classify the reaction CaCO_3 → CaO + CO_2. [1 point]

New YorkChemistrySyllabus dot point

How can chemical reactions be classified, and how do you predict the products of each type?

Types of chemical reactions: classify reactions as synthesis, decomposition, single replacement, double replacement or combustion, and use Table J and Table F to predict whether a reaction occurs.

A focused Regents Chemistry answer on classifying reactions as synthesis, decomposition, single replacement, double replacement or combustion, and using the Table J activity series and Table F solubility guidelines to predict products and precipitates.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-13

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Quick answer

The Regents reaction types are: synthesis (combination), where two or more substances form one product ( $\text{A} + \text{B} \rightarrow \text{AB}$ ); decomposition, where one substance breaks into two or more ( $\text{AB} \rightarrow \text{A} + \text{B}$ ); single replacement, where an element replaces an element in a compound ( $\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}$ ); double replacement, where two compounds exchange ions ( $\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$ ); and combustion, where a substance reacts with oxygen, often producing $\text{CO}_2$ and $\text{H}_2\text{O}$ . Use Table J (activity series) to decide if a single replacement occurs, and Table F (solubility) to decide if a double replacement forms a precipitate.

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What this topic is asking
The five reaction types
Predicting single replacement with Table J
Predicting double replacement with Table F
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What this topic is asking

The Core Curriculum asks you to recognize the main types of chemical reaction and to predict products, including using the Table J activity series for single replacement and the Table F solubility guidelines for double replacement. The Regents tests classification in Part A and prediction with the tables in Part B-2 and Part C.

The five reaction types

Key fact

Synthesis (combination): two or more reactants combine into a single product, as in $2\,\text{Mg} + \text{O}_2 \rightarrow 2\,\text{MgO}$ . Decomposition: one compound breaks into simpler substances, as in $2\,\text{H}_2\text{O} \rightarrow 2\,\text{H}_2 + \text{O}_2$ . Single replacement: an element replaces a similar element in a compound, as in $\text{Zn} + 2\,\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$ . Double replacement: two compounds swap partners, as in $\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3$ . Combustion: a fuel reacts with oxygen, releasing energy, as in $\text{CH}_4 + 2\,\text{O}_2 \rightarrow \text{CO}_2 + 2\,\text{H}_2\text{O}$ .

Classifying is a matter of counting reactants and products and seeing how atoms rearrange. Synthesis goes from many to one; decomposition from one to many; single replacement has an element and a compound on each side; double replacement has two compounds exchanging ions.

Predicting single replacement with Table J

For example, zinc is above copper on Table J, so zinc displaces copper from copper sulfate ( $\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}$ ). The reverse, copper in zinc sulfate, does not react, because copper is less active. Table J also includes hydrogen, so it tells you whether a metal will react with an acid to release hydrogen gas.

Predicting double replacement with Table F

In a double replacement, two compounds in solution exchange ions; the reaction "goes" if one product is an insoluble solid (a precipitate), a gas, or water. Table F lists which ionic compounds are soluble. If swapping the ions produces a combination that Table F says is insoluble, that compound precipitates and the reaction occurs. For example, mixing silver nitrate and sodium chloride gives silver chloride, which Table F lists as insoluble, so a white precipitate forms.

Using Table J to predict a reaction

Predict whether each occurs, citing Table J. (a) $\text{Mg} + \text{ZnCl}_2$ ; (b) $\text{Ag} + \text{MgCl}_2$ .

step 1 Identify the free element and the metal in the compound

(a) free magnesium with zinc in $\text{ZnCl}_2$ ; (b) free silver with magnesium in $\text{MgCl}_2$ .

step 2 Compare activities on Table J

(a) magnesium is more active than zinc, so magnesium can replace zinc. (b) silver is much less active than magnesium, so silver cannot replace magnesium.

step 3 State the outcome

(a) the reaction occurs: $\text{Mg} + \text{ZnCl}_2 \rightarrow \text{MgCl}_2 + \text{Zn}$ . (b) no reaction, because the free metal is less active than the one in the compound.

Try this

Q1. Classify the reaction $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ . [1 point]

Cue. Decomposition (one compound breaks into two).

Q2. Using Table J reasoning, state whether magnesium will react with hydrochloric acid. [1 point]

Cue. Yes; magnesium is above hydrogen on Table J, so it replaces hydrogen and releases $\text{H}_2$ gas.

Exam-style practice questions

Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Regents (Part A style)1 marksThe reaction

2\,\text{H}_2 + \text{O}_2 \rightarrow 2\,\text{H}_2\text{O}

is best classified as (1) decomposition (2) synthesis (3) single replacement (4) double replacement

Show worked answer →

A 1-point Part A item on reaction classification. The answer is (2) synthesis.

In a synthesis (combination) reaction, two or more simpler substances combine to form a single, more complex product. Here two elements, hydrogen and oxygen, combine to form one compound, water, which fits the pattern element $+$ element $\rightarrow$ compound. Decomposition is the reverse (one substance breaks into several), and the replacement reactions involve elements swapping into compounds.

Markers reward recognizing the "many reactants to one product" pattern of synthesis.

Regents (Part B-2 style)3 marksConsider the reaction

\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}

. (a) Classify this reaction type. (b) Using Table J, explain why this single-replacement reaction occurs. (c) State what would happen if copper metal were placed in zinc sulfate solution instead.

Show worked answer →

A 3-point constructed-response item using the Table J activity series.

(a) Type (1 point): single replacement (an element replaces an element in a compound).
(b) Explanation (1 point): on Table J zinc is more active than copper, so zinc can replace copper from its compound, and the reaction occurs.
(c) Prediction (1 point): no reaction, because copper is less active than zinc on Table J, so it cannot replace zinc from zinc sulfate.

Markers reward classifying the reaction, citing the relative activity of zinc and copper from Table J, and applying the same rule to predict no reaction in the reverse case.

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Sources & how we know this

Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)