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化學反應是指分子破裂成原子，原子重新排列組合生成新分子的過程。在反應中常伴有發光、發熱、變色、生成沉淀物等，判斷一個反應是否為化學反應的依據是反應是否生成新的分子或是分子內部的原子重組。
核反應不屬于化學反應。離子反應是化學反應。

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What are chemical reactions
什么是化學反應?
A chemical reaction is a process in which substances interact to form different substances by breaking forming or rearranging their chemical bonds
化學反應是物質相互作用,通過斷裂、形成或重新排列它們的化學鍵,形成不同的物質的過程·
Substances that take part in a chemical reaction may be ionic or covalent compounds
參與化學反應的物質可以是離子化合物或共價化合物,
as well as the atoms ions or molecules of some elements
也可以是一些元素的原子、離子或分子·
Let's look at an example of a chemical reaction
讓我們來看一個化學反應的例子·
Photosynthesis is a chemical reaction in which plants make food by using the sun's energy to combine carbon dioxide from the air and water from the soil into a sugar called glucose and oxygen
光合作用是一個化學反應,植物利用太陽的能量,將空氣中的二氧化碳和土壤中的水結合,形成一種叫做葡萄糖的糖和氧氣·
In this reaction carbon dioxide and water are called reactants because they're the substances changing by combining or reacting together
在這個反應中,二氧化碳和水被稱為反應物,因為它們通過結合或反應而發生變化·
Glucose and oxygen are called products because they're the new substances produced by the reaction
葡萄糖和氧氣被稱為產物,因為它們是反應生成的新物質·
Scientists write a chemical reaction in the form of a chemical equation
科學家用化學方程式來表示化學反應·
A chemical equation includes the reactants' chemical formulas on the left and the products' chemical formulas on the right
化學方程式包括反應物的化學式在左邊,產物的化學式在右邊·
Notice that the reactants and the products are built from the same elements carbon oxygen and hydrogen
注意,反應物和產物都由相同的元素組成,碳、氧和氫·
Products must contain the same amount and type of elements that were in the reactants
產物必須包含與反應物中相同數量和類型的元素,
and products never contain different elements than the reactants
且產物中不會包含與反應物不同的元素·
In this reaction the chemical formulas for the reactants are CO2 for carbon dioxide and H2O for water
在這個反應中,反應物的化學式是二氧化碳CO2和水H2O·
The chemical formulas for the products are C6H12O6 for glucose and O2 for oxygen
產物的化學式是葡萄糖C6H12O6和氧氣O2·
We are not done yet because the equation is not balanced
我們還沒完成,因為這個方程式沒有平衡·
This is necessary due to the law of conservation of matter
這是必要的,因為質量守恒定律指出,
it states that matter can't be created or destroyed but can only change forms
物質不能被創造或銷毀,只能改變形式·
So to balance the equation
因此,為了平衡方程式,
we have to make sure that the number of atoms of each element in the reactants is equal to those in the products
我們必須確保反應物中每種元素的原子數量等于產物中該元素的數量,
this is because a chemical reaction must abide by the law of conservation of matter no atoms are created or destroyed
因為化學反應必須遵守質量守恒定律,沒有原子被創造或銷毀·
Here's the balanced version of this equation
這是該方程式的平衡版本·
It shows six molecules of carbon dioxide reacting with six molecules of water to produce one molecule of glucose and six molecules of oxygen
它顯示了六個二氧化碳分子與六個水分子反應,生成一個葡萄糖分子和六個氧氣分子·
The numbers in front of the formulas are called coefficients
公式前面的數字被稱為系數·
They indicate the number of molecules of each reactant and product
它們表示每種反應物和產物的分子數量·
If there's no coefficient it's understood to mean one molecule of that substance
如果沒有系數,則意味著該物質的一個分子·
To review
回顧一下,
in a chemical reaction chemical bonds in reacting substances are broken
在化學反應中,反應物中的化學鍵斷裂
then reformed to make different substances
然后后重新形成,生成不同的物質·
The reacting substances are called reactants
反應物稱為反應物·
The substances produced are called products
產生的物質稱為產物·
A chemical equation is a written expression of a chemical reaction
化學方程式是化學反應的書面表達,
it includes the chemical formulas of both the reactants and the products
包含了反應物和產物的化學式·
In a balanced chemical equation
在平衡的化學方程式中,
the number of atoms of each element in the reactants equals those in the products
反應物中每種元素的原子數等于產物中該元素的原子數·