Language of Chemistry | ICSE Class 7 Chemistry Notes

This note covers chemical language, chemical reactions, reactants and products, changes in colour, state and smell, gas evolution, precipitate formation, heat changes, and the reading and writing of word equations.

What is the language of chemistry used for?

Chemistry is the study of substances and the changes they undergo. A chemical reaction is a change in which one or more new substances form. Chemical language gives us a convenient way to describe the substances involved and the changes taking place.

The names of substances can be long. A chemical symbol is a short notation representing an element. An element is a substance made of one kind of atom; an atom is a tiny building block of substances.

A compound contains two or more elements chemically combined in fixed proportions. A chemical formula is a shorthand representation of a substance using element symbols. Symbols and formulae make it easier to represent substances without repeatedly writing their full names.

How do words describe a reaction?

A word equation represents a chemical reaction using the names of the substances. The starting substances are called reactants. The new substances formed are called products. These terms describe the roles that substances have in a particular reaction.

Definition: A word equation shows the change from reactants to products using substance names, with an arrow pointing towards the products.

The arrow → means “forms” or “becomes”. A plus sign + separates different substances on the same side. On the reactant side, it can be read as “reacts with”; on the product side, it can be read as “and”.

For example, magnesium and oxygen are elements; oxygen is a gas present in air. When magnesium burns in oxygen, magnesium oxide forms. Magnesium oxide is the name of the new compound formed in this reaction.

Magnesium + Oxygen → Magnesium oxide

This compact statement preserves the important chemical information: which substances react and what they form. It does not replace the record of what was observed. The appearance of the flame and the powder formed can be described separately alongside the equation.

How does a chemical reaction differ from a physical change?

A physical change changes a physical property without forming a new substance. Physical properties include shape, size, colour and state. Here, state means the solid, liquid or gaseous form of a substance. A physical change is generally reversible.

A chemical reaction forms one or more new substances with different properties. The important question is therefore whether the substance itself has changed. An obvious change in appearance can be useful evidence, but it needs to be interpreted carefully.

What does melting ice show?

When ice melts, solid water becomes liquid water. Its state changes, but it remains water. Freezing the water produces ice again. Melting therefore illustrates why a change of state, by itself, does not establish that a chemical reaction has occurred.

Cutting paper also changes its size and shape without producing a new substance. The pieces remain paper. This example helps explain why “generally reversible” should not be replaced by a claim that every physical change can easily be reversed.

What does burning magnesium show?

Magnesium ribbon burns with a dazzling white flame and forms a white powder, magnesium oxide. The powder is a new substance. The reaction involves magnesium and oxygen, rather than merely a change in the shape of the ribbon.

The drawing shows magnesium ribbon held with tongs near a burner. A watch-glass below collects magnesium oxide. The labels identify the tongs, magnesium ribbon, burner, watch-glass and magnesium oxide.

See Fig. 1.1 in your NCERT textbook

Heat is energy transferred because of a temperature difference. Temperature indicates how hot or cold something is. The burner supplies heat to begin the demonstration. The tongs hold the ribbon, and the watch-glass is a shallow dish that collects the powder.

The tongs and watch-glass are equipment, not reactants. Observing the equipment carefully helps distinguish what is used to carry out the demonstration from the substances that undergo chemical change.

Note: Burning magnesium is a teacher-led demonstration. Do not stare at the burning ribbon. The bright flame and the white powder are observations; the formation of magnesium oxide explains the chemical change.

How can reactants and products be identified?

To identify reactants, look for the substances said to react or undergo chemical change. To identify products, look for the new substances said to form. Read the complete description before writing either list, because a reaction can form more than one product.

An acid turns blue litmus red, while a base turns red litmus blue. Litmus is a natural colour-changing dye used as an indicator, a substance that helps identify an acid or a base.

Consider this description: zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. Zinc is an element. Sulphuric acid is the acid used here; dilute means that the acid is present in a relatively small amount compared with water.

Zinc sulphate is the name of one product, and hydrogen is the gaseous product. Zinc and sulphuric acid belong on the reactant side. Zinc sulphate and hydrogen belong on the product side. All four names are needed to represent this description completely.

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

How does the arrow organise the names?

The left-hand side of a word equation contains the reactants. The right-hand side contains the products. The arrow points from the reactant side to the product side and shows the direction of the reaction being described.

1. Read the statement and identify the substances that undergo the change.
2. Write their names to the left of the arrow, separating them with a plus sign.
3. Identify every substance said to form during the reaction.
4. Write these product names to the right of the arrow, separated by plus signs.

In the zinc example, the plus sign before sulphuric acid joins the reactant names. The plus sign before hydrogen joins the product names. Neither plus sign replaces the arrow. The arrow has the different job of separating the starting substances from the substances formed.

A conical flask, a laboratory container with a broad base and narrow neck, contains zinc granules and dilute sulphuric acid. A cork closes the flask, and a glass tube passes through it. Hydrogen gas is labelled above the liquid inside the flask.

See Fig. 1.2 in your NCERT textbook

The conical flask holds the reacting substances. The flask, cork and glass tube help contain or direct the materials; they do not belong in this word equation.

Which observations may indicate a chemical reaction?

A reaction may take place when two or more reactants come into contact and energy transfer takes place. The word “may” matters: putting substances together does not, by itself, establish that a new substance has formed. The changes must be observed and interpreted.

An observation is something noticed during an activity, such as a colour change or bubbling. An inference is an explanation drawn from the observations and other information. Distinguishing them helps us describe a reaction without claiming more than the evidence shows.

What signs should be recorded?

A mixture contains substances mixed without becoming a new compound. A solution is a uniform mixture of two or more substances. Insoluble means that a substance does not dissolve in the liquid under consideration. A precipitate is an insoluble solid produced during a reaction in solution.

Different reactions can show different signs. Burning magnesium produces a bright flame and white powder. The reaction of potassium iodide with lead acetate produces a yellow precipitate. Potassium iodide and lead acetate are the names of the two starting compounds in that example.

Why should the observations be interpreted carefully?

These signs are clues to a chemical change, rather than a requirement that every reaction must show the complete list. Heat, light, sound, a change in smell, a colour change or gas formation may accompany a chemical change.

For instance, water changes state when ice melts, but no new substance forms. The state change must therefore be considered alongside the nature of the substances. The central feature of a chemical reaction remains the formation of one or more new substances.

A useful record separates the materials used, the observed change and the explanation. “Gas bubbles appeared” records an observation. Naming that gas requires information about the reaction or a suitable test. Bubbles alone do not provide the gas's chemical name.

How do gas-producing reactions help explain word equations?

Gas evolution means that a gas is formed and given off. In a teacher-led demonstration, dilute hydrochloric acid is added to solid sodium carbonate. Hydrochloric acid is the acid used, and sodium carbonate is the starting solid compound. A gas is evolved.

The products are sodium chloride, water and carbon dioxide, the gas given off. Sodium chloride is the chemical name of common salt, a compound that can also be formed by the reaction of an acid with a base. Such compounds are called salts. The complete word equation names all three products, including those that are less obvious than the escaping gas.

Sodium carbonate + Hydrochloric acid → Sodium chloride + Water + Carbon dioxide

This equation illustrates an important distinction: the most visible product is not necessarily the only product. Writing carbon dioxide alone on the right would leave out sodium chloride and water. Observation and a complete account of the reaction need to be used together.

How is carbon dioxide identified?

Lime water is a solution of calcium hydroxide in water. Calcium hydroxide is the dissolved compound. Passing carbon dioxide through lime water produces calcium carbonate, an insoluble white solid, and water. The calcium carbonate makes the lime water appear milky.

Carbon dioxide + Calcium hydroxide → Calcium carbonate + Water

The milky appearance is an observation. Formation of calcium carbonate explains that appearance. This gives an example of how a gas-producing reaction can be followed by a second reaction that helps identify the gas.

A stand supports a test tube containing sodium carbonate and dilute hydrochloric acid. A thistle funnel, a narrow-stemmed funnel for introducing liquid, passes through the cork. A delivery tube, which carries the gas, leads into another test tube containing calcium hydroxide solution.

See Fig. 2.2 in your NCERT textbook

The delivery tube connects the gas-producing apparatus to the receiving test tube. The first tube produces carbon dioxide; the second contains the solution used to test it.

Note: The milky appearance should not be described as permanent under every condition. Passing excess carbon dioxide can make the precipitate dissolve. For the initial test, record that lime water turns milky.

The two equations describe separate reactions. Hydrochloric acid reacts with sodium carbonate in the first. Carbon dioxide then reacts with calcium hydroxide in the second. Keeping the reactions separate prevents reactants and products from the two stages being mixed into an incorrect equation.

How do colour changes and precipitates reveal new substances?

A colour change can help us recognise the formation of a new substance. A precipitate gives another useful sign because a new insoluble solid appears in the liquid. The two observations can occur together, so they need not be treated as separate reactions.

When solutions of potassium iodide and lead acetate react, a yellow precipitate forms. This demonstration therefore illustrates both the appearance of a yellow colour and the formation of an insoluble solid. Record both features in the observation.

The reactants can be identified from the stated starting materials: potassium iodide and lead acetate. The observation establishes the appearance of a yellow precipitate. Its chemical name should come from the reaction information, rather than being guessed solely from its colour.

What happens when iron reacts with copper sulphate?

Copper sulphate is the compound used to make the blue solution in this demonstration. When iron is placed in this solution, iron sulphate forms and copper is deposited. Iron sulphate is the new compound in solution; copper is the deposited element.

The solution changes from blue to green, and a brown deposit of copper appears on the iron. These observations connect the visible changes with the names of the new substances. The reaction can be represented without using formulae.

Copper sulphate + Iron → Iron sulphate + Copper

The full product list contains both iron sulphate and copper. Mentioning only the brown deposit misses the substance formed in solution. Equally, saying only “the solution changes colour” leaves out the name of the new compound responsible for that change.

When comparing this reaction with the yellow-precipitate demonstration, describe the actual observation in each case. The first gives a yellow precipitate; the iron reaction gives a green solution and a brown copper deposit. Specific observations are more informative than a general statement that “something changed”.

What do heat changes and changes in smell tell us?

Chemical changes may involve heat being given off or absorbed. A reaction mixture becoming warmer can provide evidence of heat release.

When hydrochloric acid reacts with sodium hydroxide, heat is evolved. Sodium hydroxide is a base, a substance that reacts with an acid to form a salt and water. In this reaction, the products are sodium chloride and water.

Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

Sodium chloride is the salt produced here. A reaction between an acid and a base that produces salt and water is called neutralisation. The word equation names the substances, while the observation records that heat is released.

Is heat a chemical substance?

Heat is energy, so it should not be described as another chemical substance produced alongside sodium chloride and water. In a written account, distinguish the material products from the energy change. Both are useful information, but they describe different aspects of the reaction.

Similarly, the heat supplied by a burner is not a reactant. Heating can provide energy for a change. An apparatus description may mention the burner, while the chemical description identifies the substance heated and any products formed.

For example, ammonium chloride is the name of a compound used in a heating demonstration. When it is heated, ammonia gas is evolved. Ammonia is the name of the gas identified in this observation. Record the heating and gas evolution as stated.

How should changes in smell be described?

A change in smell may take place, or a new smell may be given off, during a chemical change. This is another possible sign, not something that must accompany every reaction. It belongs alongside colour, state, gas, precipitate and heat observations.

These chemical demonstrations are carried out under teacher supervision. Do not directly inhale laboratory gases or taste reaction mixtures. A report should use the observations provided by the demonstration and distinguish the name of a gas from an unverified guess about its identity.

How can a complete word equation be written and checked?

A good word equation gives a concise account of the stated chemical change. Begin with the names in the description, sort them into reactants and products, and then connect the two groups with the correctly directed arrow.

What is the writing method?

1. Read the complete reaction description, including the names of every substance said to form.
2. Separate the starting substances from the new substances produced during the reaction.
3. Write the reactant names on the left, with plus signs between different reactants.
4. Draw an arrow pointing right, towards the side where the products will be written.
5. Write all product names on the right, with plus signs between different products.
6. Read the equation as a sentence and check that it reproduces the supplied reaction information.

Worked example 1. Magnesium burns in oxygen to form magnesium oxide. Write the word equation and identify the roles of the named substances.

Answer: Magnesium + Oxygen → Magnesium oxide. Magnesium and oxygen are the reactants. Magnesium oxide is the product. The arrow points towards magnesium oxide because it is the new substance formed during this reaction.

Worked example 2. Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen. Write the complete word equation.

Answer: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen. Both zinc sulphate and hydrogen must appear on the product side. Omitting either would give an incomplete account of the supplied description.

Worked example 3. Hydrochloric acid reacts with sodium hydroxide, producing sodium chloride and water while heat is released. Represent the substances in a word equation and state the heat change separately.

Answer: Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water. Heat is released during the reaction. Sodium chloride and water are the material products; the heat release is an accompanying energy change.

What final checks prevent common mistakes?

Check the spelling and completeness of each substance name. Check that the arrow points towards the products and that plus signs separate substances on the same side. Finally, compare the equation with the original sentence to ensure that no reactant or product has been omitted.

Keep observations beside the equation when they are needed. An equation tells us what reacts and what forms. A separate observation can explain that a gas was evolved, the solution changed colour, a precipitate appeared or heat was released during the demonstrated reaction.

Glossary

• Chemical reaction — A change in which one or more new substances are formed.
• Physical change — A change in physical properties that does not form a new substance.
• Chemical symbol — A short notation used to represent an element in chemical language.
• Chemical formula — A shorthand representation of a substance that uses the symbols of elements.
• Word equation — A representation of a chemical reaction using the names of reactants and products.
• Reactants — The starting substances that undergo chemical change during a particular reaction.
• Products — The new substances that are formed during a particular chemical reaction.
• Gas evolution — The formation and giving off of a gas during a reaction.
• Precipitate — An insoluble solid that forms when a reaction takes place in solution.
• Solution — A uniform mixture of two or more substances.
• Lime water — A solution of calcium hydroxide in water used to test for carbon dioxide.
• Neutralisation — A reaction between an acid and a base that forms salt and water.

Common errors and misconceptions

• Misconception: Every change of state is a chemical reaction. Correct: Ice melting into water is a physical change. Formation of a new substance is the defining feature of a chemical reaction.
• Misconception: Every physical change is easily reversible. Correct: Physical changes are generally reversible. Cutting paper changes its size without forming a new substance, although the pieces cannot simply be joined back into the original sheet.
• Misconception: The arrow points towards the reactants. Correct: Reactants are written on the left, and the arrow points towards the products on the right.
• Misconception: A plus sign and an arrow have the same meaning. Correct: Plus signs separate substances on one side. The arrow shows the change from reactants to products.
• Misconception: Bubbles identify a gas as carbon dioxide. Correct: Bubbles indicate gas evolution. The gas's identity needs reaction information or a suitable test, such as carbon dioxide turning lime water milky.
• Misconception: Only the visible gas needs to be written as a product. Correct: The sodium carbonate and hydrochloric acid reaction also forms sodium chloride and water. Its word equation needs all three products.
• Misconception: Every chemical reaction must show every possible sign. Correct: Colour, smell, heat and other changes may accompany a reaction. Different reactions show different observations.
• Misconception: Heat is a chemical substance produced by neutralisation. Correct: Heat is energy. Sodium chloride and water are the material products when hydrochloric acid reacts with sodium hydroxide.

Exam-style questions with model answers

Q1. Magnesium reacts with oxygen to form magnesium oxide. Name the reactants and the product. [2 marks]

1. The reactants are magnesium and oxygen, the two starting substances that undergo the chemical change.
2. The product is magnesium oxide, the new substance formed when magnesium reacts with oxygen.

Q2. Zinc reacts with sulphuric acid to produce zinc sulphate and hydrogen. Write the word equation and explain its plus signs and arrow. [3 marks]

1. The word equation is Zinc + Sulphuric acid → Zinc sulphate + Hydrogen. It includes both named reactants and both named products.
2. The plus signs separate the substances on each side: zinc from sulphuric acid, and zinc sulphate from hydrogen.
3. The arrow points from the reactants on the left towards the products on the right, showing the direction of the stated chemical change.

Q3. Ice melts to form liquid water, which remains water. Magnesium burns in oxygen with a dazzling white flame and produces a white powder called magnesium oxide, a new substance. Classify each change and give a reason for each classification. [4 marks]

1. Melting ice is a physical change because the given description says that the resulting liquid remains water.
2. Its physical state changes from solid to liquid, but the description identifies no new substance being formed.
3. Burning magnesium is a chemical change because magnesium reacts with oxygen and produces magnesium oxide.
4. Magnesium oxide is a new substance. Its formation, rather than the bright appearance of the flame alone, establishes the chemical change.

Q4. Solid sodium carbonate reacts with dilute hydrochloric acid to form sodium chloride, water and carbon dioxide gas. The gas produces a white calcium carbonate precipitate and water when passed through lime water, a solution of calcium hydroxide. State the first reaction's reactants, products, word equation and visible gas observation, then explain the lime-water result. [5 marks]

1. The first reaction has sodium carbonate and hydrochloric acid as its reactants. Sodium carbonate is the solid starting material, and the acid is dilute.
2. Its products are sodium chloride, water and carbon dioxide. All three belong on the product side, even though the escaping gas is especially noticeable.
3. The complete word equation is Sodium carbonate + Hydrochloric acid → Sodium chloride + Water + Carbon dioxide, with the arrow pointing towards the products.
4. Gas evolution is observed as bubbles. The supplied reaction information identifies this gas as carbon dioxide; the bubbles alone do not establish its name.
5. Lime water becomes milky because an insoluble white calcium carbonate precipitate forms. This second reaction also produces water, as stated in the question.

Q5. Iron reacts with blue copper sulphate solution. Green iron sulphate solution and a brown copper deposit form. Write the word equation and describe the two colour observations. [3 marks]

1. The word equation is Copper sulphate + Iron → Iron sulphate + Copper. It places the two starting substances before the arrow and the products after it.
2. The solution changes from blue to green as iron sulphate forms in place of copper sulphate in the described reaction.
3. A brown deposit of copper appears on the iron. This is the second named product and should not be omitted from the equation.

Q6. Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water, releasing heat. Write the word equation and distinguish the material products from the heat change. [3 marks]

1. The word equation is Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water. The two named reactants are placed to the left of the arrow.
2. Sodium chloride and water are the material products formed in the reaction and are written to the right of the arrow.
3. Heat is released as an energy change. It is not another chemical substance alongside sodium chloride and water, so the description should distinguish it from the material products.

Q7. Two solutions, potassium iodide and lead acetate, react to form an insoluble yellow solid. Give the term for such a solid and identify the two visible signs of reaction. [2 marks]

1. An insoluble solid formed during a reaction in solution is called a precipitate; here it is yellow.
2. The two signs are the appearance of a yellow colour and the formation of a solid precipitate.

Q8. A reaction is described as: zinc reacts with sulphuric acid to form zinc sulphate and hydrogen. A pupil writes “Zinc sulphate + Hydrogen → Zinc + Sulphuric acid”. Explain the error and correct the equation. [2 marks]

1. The pupil has reversed the reactant and product sides for the given description, making the arrow point towards the starting substances.
2. The corrected word equation is Zinc + Sulphuric acid → Zinc sulphate + Hydrogen, with the arrow pointing towards the substances formed.

Key takeaways

• A chemical reaction forms one or more new substances; a physical change alters physical properties without forming a new substance.
• Reactants undergo the chemical change, while products are the new substances formed during that particular reaction.
• A word equation uses substance names and an arrow pointing from reactants on the left towards products on the right.
• Plus signs separate different substances on the same side of an equation; they do not replace the reaction arrow.
• Colour, state, smell, gas evolution, precipitate formation and heat release can provide observations associated with chemical reactions.
• The reaction of sodium carbonate with hydrochloric acid forms sodium chloride, water and carbon dioxide, so all three products must be included.
• Carbon dioxide turns lime water milky through calcium carbonate formation; the precipitate can dissolve when excess carbon dioxide is passed.
• Write observations alongside equations when needed, and distinguish material products from accompanying changes such as heat release.

Test yourself

What is the defining feature of a chemical reaction?

One or more new substances form during the change, with properties different from those of the starting substances.

Where are reactants written in a word equation?

Reactants are written on the left-hand side, before the arrow pointing towards the products.

What does the arrow in a word equation mean?

It means “forms” or “becomes” and shows the direction from reactants towards products.

Why does a plus sign appear between product names?

It separates different products formed during the same reaction and can be read as “and”.

Why is melting ice a physical change?

Ice becomes liquid water, but the substance remains water; no new substance is formed.

What is a precipitate?

A precipitate is an insoluble solid that forms during a chemical reaction in solution.

What makes lime water milky when carbon dioxide is passed through it?

Calcium carbonate forms as an insoluble white precipitate, making the lime water appear milky.

Why must a word equation include products that are not gases?

It must represent all the substances formed, including less conspicuous products such as water and substances remaining in solution.