Acids, Bases and Salts | ICSE Class 8 Chemistry Notes

This note covers acids, bases, alkalis and salts; natural and synthetic indicators; litmus, red rose, turmeric and smell tests; neutralisation and its products; reactions with metals and carbonates; and applications in digestion, soil treatment, industrial waste and colour-changing messages.

What are acids, bases and neutral substances?

Substances can be grouped as acidic, basic or neutral. An acidic substance turns blue litmus red; a basic substance turns red litmus blue. Litmus is a natural testing material whose colour helps identify these properties.

A neutral substance is neither acidic nor basic. It does not change either red or blue litmus. An acid-base indicator is a substance that shows different colours in acidic and basic solutions, allowing their nature to be tested.

Which familiar substances contain acids?

Lemon juice, amla juice, tamarind water and vinegar are acidic. These familiar edible substances taste sour. Substances that taste sour tend to contain acids and are acidic in nature. Taste is not a suitable way to investigate an unknown substance.

The names in the table identify particular acids. Different foods can contain the same acid, as lemon and orange illustrate. Conversely, the word “acid” does not identify a single substance: citric acid, acetic acid and lactic acid are different examples.

How do the three groups compare?

Note: Do not taste any unknown substance. Use an indicator to investigate whether a sample is acidic, basic or neutral.

How do bases differ from alkalis?

A base can react with an acid and neutralise, or counteract, its effect. A basic solution turns red litmus blue. Soap solution, baking soda solution, lime water and washing powder solution provide familiar examples of basic substances.

Basic substances are generally slippery to touch, and bases generally taste bitter. However, everything that tastes bitter may not contain a base. Bitter gourd, also called karela, tastes bitter but is not basic in nature.

What makes a base an alkali?

Definition: An alkali is a base that dissolves in water. Not all bases dissolve in water, so “base” and “alkali” do not have identical meanings.

Sodium hydroxide and calcium hydroxide are examples used as alkalis. The distinction concerns whether the base dissolves in water. It does not mean that an alkali belongs to a separate group unrelated to bases.

What is lime water?

Lime water is a solution of calcium hydroxide in water. It is basic. The word “lime” here must not be confused with the citrus fruit similar to lemon. Lime water and lemon juice therefore provide substances of opposite acid-base nature.

Calcium oxide, also called lime or chuna, is used in preparing lime water. After lime is mixed with water and left undisturbed, the liquid is filtered into another container. Filtering separates the liquid from undissolved material.

Although taste and touch describe familiar properties, they are not instructions for handling laboratory bases. Alkalis may cause harm. Never taste or touch them to identify them; use the appropriate indicator test.

How does litmus identify a substance's nature?

Litmus is obtained from lichens. A lichen is an association of a fungus and an alga, two kinds of living organisms. Litmus is available as a solution and as paper strips in red and blue colours.

Litmus solution is purple when it is neither acidic nor basic. The useful test is the response to a sample: acids turn blue litmus red, while bases turn red litmus blue. Neutral substances do not change either paper.

How is the paper test performed?

1. Cut blue litmus paper into small pieces and spread them on a clean, dry white tile.
2. Use a dropper to place one drop of each sample on a separate piece of the blue paper.
3. Record the final colour for every sample, keeping each observation linked to the sample tested.
4. Repeat the procedure with pieces of red litmus paper and record those results separately.
5. Group samples according to whether they turn blue paper red, turn red paper blue, or change neither paper.

Two illustrations show pupils using droppers to test small paper pieces on white tiles. The upper illustration shows blue litmus pieces; the lower illustration shows red litmus pieces. Sample containers stand beside each tile.

Why are both paper colours useful?

A sample that leaves red litmus red might be acidic or neutral. That result alone cannot distinguish the two. Testing blue litmus resolves the uncertainty: an acidic sample makes it red, while a neutral sample leaves it blue.

Similarly, unchanged blue litmus does not by itself prove neutrality. A basic sample also leaves blue litmus blue. Using both colours gives the evidence needed to distinguish the three groups.

Record the observation before the conclusion. “Blue litmus became red” describes what happened; “the sample is acidic” interprets that result. Keeping these separate makes the reasoning clear and helps avoid conclusions based on incomplete tests.

How can red rose petals provide an indicator?

An extract is the coloured liquid obtained here from flower petals. Red rose extract can be used as an acid-base indicator. Fallen petals can be collected for this activity; flowers need not be plucked.

How is the extract prepared?

1. Collect a fistful of fallen red rose petals and wash them with water.
2. Crush the petals using a mortar and pestle, a bowl and tool used for crushing.
3. Put the crushed petals in a glass tumbler. Under adult supervision, pour in enough hot water to immerse them completely.
4. Cover the tumbler and wait for 5 to 10 minutes until the water becomes coloured.
5. Filter the mixture. Keep the filtrate, the liquid that passes through during filtration, as the indicator.

A test tube containing coloured extract stands in a wooden test tube stand. The labelled parts are “Test tube”, “Test tube stand” and “Red rose extract”.

What happens with lemon juice and soap solution?

Place 10 to 20 drops of extract in each of two test tubes. Label the lemon-juice test tube A and the soap-solution test tube B. These letters identify the containers; they are not chemical symbols.

Add 20 to 30 drops of lemon juice to A and 20 to 30 drops of soap solution to B. Observe the colour of each mixture. The extract seems to give red colour in an acidic solution and green colour in a basic solution.

Two test tubes labelled A and B stand side by side. The liquid in A is red; the liquid in B is green. The caption identifies the added samples as lemon juice and soap solution respectively.

The colour changes can be compared with the litmus results for the same samples. Samples that do not change the flower extract can also be compared with the group that changes neither red nor blue litmus.

What can turmeric paper tell us, and what can it not tell us?

Turmeric, also called haldi, is a yellow natural indicator. Its yellow colour turns red in basic solutions but remains unchanged in acidic and neutral solutions. This makes turmeric useful for detecting basic substances.

How is turmeric paper made?

1. Put a spoonful of turmeric in a container and mix it with a little water to make a paste.
2. Dip a piece of filter paper into the paste until the paper becomes yellow.
3. Remove the paper and allow it to dry.
4. Under adult supervision, cut the dried yellow paper into thin strips.
5. Put a drop of each sample on a separate piece of turmeric paper and record the colour.

Part (a) shows yellow turmeric paste in a shallow container. Part (b) shows yellow turmeric paper, including narrow strips beside a wider piece.

Why is unchanged yellow paper inconclusive?

No colour change with turmeric does not mean that a sample is definitely neutral. An acid also leaves turmeric yellow. A second indicator, such as litmus, is needed to distinguish an acidic sample from a neutral one.

A curry stain on white cloth can become reddish-brown when scrubbed with soap because soap is basic and the curry contains turmeric. The stain turns yellow again when the cloth is washed with plenty of water.

The important link is between the indicator and the substance being tested. Turmeric supplies the colour response; soap supplies the basic substance. The colour change is evidence of their interaction, rather than proof that every coloured stain behaves in the same way.

Which other indicators can be used?

Natural indicators come from natural materials. Litmus, red rose extract and turmeric are examples. Beetroot, purple cabbage, Indian blackberry or jamun, and red hibiscus or gudhal can also act as acid-base indicators.

Synthetic indicators are made in laboratories. Methyl orange and phenolphthalein are examples. Their names refer to testing substances, rather than to the acid or base being investigated.

What is an olfactory indicator?

An olfactory indicator is a substance whose odour changes in an acidic or basic medium. “Olfactory” refers to smell, and “medium” here means the surroundings provided by the solution. This test uses an odour response instead of a visible colour change.

1. Place finely chopped onion with clean cotton-cloth or filter-paper strips in a container.
2. Close the container tightly and leave it overnight.
3. Take out two strips and check their odour before adding either sample.
4. Put a few drops of tamarind water on one strip and baking soda solution on the other. Allow the drops to spread.
5. Check the odour again and record any changes separately for the two samples.

This activity compares each strip's odour before and after treatment. It is important to record the observed response rather than assume a result from the sample's appearance. An olfactory indicator is useful when a colour change cannot be observed.

Indicators must also be interpreted according to their individual behaviour. The turmeric colour rule cannot simply be applied to red rose extract or to onion strips: these are different indicators, and the evidence each provides is different.

What happens during neutralisation?

Definition: Neutralisation is the reaction between an acid and a base to form salt and water. Heat is also released in the reaction.

A salt is a compound, a substance made of chemically combined elements, that can be formed when an acid reacts with a base. “Salt” names a group of compounds, not just the common salt added to food. The salt formed depends on the acid and base used.

When an acid solution is mixed with a base solution in sufficient quantity, their acidic and basic effects can neutralise each other. The quantities matter. Adding some base to an acid does not automatically establish that the resulting mixture is neutral.

Acid + Base → Salt + Water + Heat

This is a word equation, a way of describing a reaction using substance names. The plus sign separates substances or products, and the arrow means “produces”. The heat term records the release of heat, also called its evolution.

How do lemon juice and lime water illustrate the change?

1. Take one drop of lemon juice in a test tube and add around twenty drops of water.
2. Add one drop of blue litmus solution. The acidic mixture turns it red.
3. Add lime water slowly, drop by drop, swirling the test tube well.
4. Continue observing until the solution changes from red to blue.
5. Add one drop of lemon juice again and observe the response to adding more acid.

Part (a) shows red liquid in a test tube after blue litmus solution is added. Part (b) shows blue liquid in a test tube after lime water is added.

The red colour initially identifies acidic conditions. The later blue colour shows that the mixture is no longer acidic and that the added lime water has overcome the acidic effect. Blue litmus colour indicates basic conditions; it should not be labelled “neutral”.

What is a named example?

Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

Hydrochloric acid is the acid, sodium hydroxide is the base, and sodium chloride is the salt formed. This example gives names to the substances represented by “acid”, “base”, “salt” and “water” in the general word equation.

Are all salts the same as common salt?

Common salt is sodium chloride, the salt used in food. It is one member of the much larger group called salts. Other examples include sodium carbonate, sodium hydrogencarbonate, copper sulphate and potassium nitrate.

Salt solutions need not all be neutral. They can be acidic, basic or neutral. Sodium chloride is a neutral salt, whereas baking soda and washing soda are basic salts. The name “salt” alone therefore does not predict an indicator result.

Which familiar salts have useful applications?

Non-corrosive describes baking soda's mild nature: it does not have the corrosive, or damaging, action associated with harmful laboratory alkalis. Antacids are substances used to neutralise excess acid in the stomach.

How are baking soda and baking powder different?

Baking powder is a mixture of baking soda and tartaric acid, a weak edible acid. Baking soda is the single named salt sodium hydrogencarbonate. The two names should not be used as though they describe identical materials.

When baking powder is heated or mixed with water, carbon dioxide gas is produced. This gas can cause bread or cake to rise, making it soft and spongy. The example links an acid reacting with a basic substance to a familiar use.

Seawater contains many dissolved salts, from which sodium chloride is separated. Rock salt consists of solid salt deposits. Its large crystals are often brown because of impurities, and it is mined like coal.

How else can acids form salts?

Neutralisation is an important way to form salts, but acids also form salts in other reactions. Two useful patterns involve metals and carbonates. Their gaseous products differ, so the observations must be connected to the reaction actually taking place.

What happens with a metal?

When an acid reacts with a metal, hydrogen gas is usually liberated and a salt forms. Zinc reacting with dilute sulphuric acid illustrates this pattern. “Dilute” means that the acid has been mixed with water to reduce its concentration, the amount of acid in a given volume of solution.

Acid + Metal → Salt + Hydrogen gas

In the zinc activity, gas forms at the metal surface and is passed through soap solution to produce gas-filled bubbles. Bringing a flame near a bubble is a laboratory test performed with teacher assistance, not an unsupervised activity.

What happens with a carbonate?

Carbonates and hydrogencarbonates are groups of compounds represented by sodium carbonate and sodium hydrogencarbonate. Their reactions with acids give the corresponding salt, carbon dioxide gas and water.

Acid + Metal carbonate → Salt + Carbon dioxide + Water

Acid + Metal hydrogencarbonate → Salt + Carbon dioxide + Water

Limestone, chalk and marble are different forms of calcium carbonate. The bubbles seen when acid reacts with a carbonate are evidence that gas is being produced. This bubbling is called effervescence.

Carbon dioxide passed through lime water forms calcium carbonate as a white precipitate, an insoluble solid produced in a solution. With excess carbon dioxide, a soluble product forms instead. Thus the amount of gas passed through also matters.

Keep the patterns separate: acid with a metal usually gives hydrogen; acid with a carbonate gives carbon dioxide and water. Neither pattern should be replaced by the acid-plus-base equation without considering the reacting substances.

How does neutralisation relate to digestion and ant bites?

Neutralisation helps explain everyday situations in which an acidic substance causes an unwanted effect. A suitable basic substance can counteract the acid. The chemical explanation depends on identifying the acidic material and the substance that neutralises it.

Why are antacids used?

The stomach produces hydrochloric acid, which helps digest food. During indigestion, excess acid can cause pain and irritation. Antacids neutralise this excess acid.

Magnesium hydroxide, known as milk of magnesia, is a mild base often used for this purpose. Baking soda is also an ingredient in antacids. These examples explain the role of a base in reducing the effect of excess stomach acid.

The word “antacid” describes the function of the substance. It does not mean that every base is suitable for swallowing. Laboratory alkalis may cause harm, so their use in indicator demonstrations is separate from the use of formulated medicines.

What is the chemistry of the ant-bite example?

Keerthi rests her hand on a tree trunk while watching a butterfly. A red ant bites her, leaving redness and stinging pain. Her brother applies moist baking soda to the affected area, which relieves the pain in this example.

An ant injects an acidic liquid containing formic acid, also called methanoic acid. Moist baking soda is basic and can neutralise the acid's effect. The example illustrates an acid-base interaction at the affected site.

For each explanation, connect the cause, the neutralising substance and the resulting reduction of the acidic effect. Merely naming a base leaves the reason for its use unexplained.

How can soil and factory waste be treated?

Fertilisers are substances added to soil to help plants grow better. Excessive use of chemical fertilisers can make soil acidic. When soil is too acidic, plants do not grow well, and treatment with a basic substance such as lime can help.

Does every unhealthy plant need lime?

Organic matter here includes manure and composted leaves added to soil. Nutrients are substances needed for plant growth. Sometimes plants in neutral soil still show poor health, which can be due to a deficiency of nutrients.

No. Treatment depends on the soil's nature. Acidic and basic soils require different responses. An indicator can help test the nature of the soil before deciding which treatment matches the problem.

In the Farmer's Portal example, a farmer reports that plants are not growing well. Discussion identifies excessive fertiliser use and acidic soil. That information supports lime treatment; the poor growth alone would not establish the same conclusion.

What if acidic waste enters a lake?

Ashwin's friend Gurbir lives near an industrial area and reports a declining fish population in a nearby lake. The decline may be due to factory waste entering the lake. This is a possible cause, not a demonstrated explanation for every decline in fish numbers.

If the factory waste is acidic, basic substances can be added to neutralise it before release into the lake. The condition matters: the proposed acid-base treatment depends on establishing that the waste is acidic.

The order also matters. Treatment is applied to the waste before discharge. The example connects the acidic nature of the waste, treatment with a base and the timing of treatment before release into the lake.

How do colour-changing messages connect chemistry with everyday life?

On 28 February, a school science fair celebrates National Science Day. Siblings Ashwin and Keerthi receive white sheets. When a volunteer sprays a liquid on them, a welcome message appears, raising the question of how colour can reveal hidden writing.

What is one possible explanation?

One possibility is that soap solution was used for the writing and turmeric solution for the spray. Soap is basic, and turmeric turns red in basic conditions. The writing can therefore become visible through the indicator response.

This is a possible explanation of the demonstration, not a claim that the exact liquids used have been proved. The indicator and the writing solution must be considered together when explaining the effect.

Ashwin also makes a greeting card for Guru Purnima. He coats white paper with turmeric paste, dries it and writes a message in Odia, his teacher's mother tongue. A basic solution such as soap solution can produce the colour needed for the writing.

How can soil affect flower colour?

Hydrangea is a plant growing in cooler climates in the Himalayan region and the North-eastern states. Its flowers have different colours depending on the soil: acidic soil produces blue flowers, whereas basic soil produces pink or red flowers.

Who was Prafulla Chandra Ray?

Acharya Prafulla Chandra Ray, also called P.C. Ray, is known as the “Father of Modern Indian Chemistry”. He earned a doctorate in chemistry in the United Kingdom and returned to India, contributing to scientific research.

In 1901, he established India's first pharmaceutical company, a company working in medicines. He wrote about the history of chemistry in India and highlighted ancient Indian scientists' achievements. He also advocated the mother tongue as a medium of instruction.

Glossary

• Acidic substance — A substance that turns blue litmus red when tested with the indicator.
• Basic substance — A substance that turns red litmus blue and can neutralise an acid.
• Neutral substance — A substance that is neither acidic nor basic and changes neither litmus paper.
• Alkali — A base that dissolves in water; not every base is an alkali.
• Acid-base indicator — A substance showing different colours in acidic and basic solutions.
• Litmus — A natural indicator obtained from lichens, available as solution or coloured paper strips.
• Filtrate — The liquid collected after it passes through during the process of filtration.
• Olfactory indicator — A substance whose odour changes in an acidic or basic medium.
• Neutralisation — A reaction between an acid and a base producing salt, water and heat.
• Salt — A compound that can be formed when an acid reacts with a base.
• Antacid — A substance used to neutralise excess stomach acid responsible for pain and irritation.
• Lime water — A solution of calcium hydroxide in water that is basic in nature.
• Effervescence — Bubbling observed when a gas is produced during a chemical reaction.
• Precipitate — An insoluble solid produced in a solution during a chemical reaction.

Common errors and misconceptions

• Misconception: Anything bitter must be a base. Correct: Bases generally taste bitter, but bitter gourd is bitter without being basic. Do not use taste to identify unknown substances.
• Misconception: Unchanged red litmus proves that a sample is neutral. Correct: An acid also leaves red litmus unchanged. The blue-litmus result is needed to distinguish these possibilities.
• Misconception: Yellow turmeric paper proves that a sample is acidic. Correct: Both acidic and neutral samples leave turmeric yellow. Turmeric alone cannot distinguish the two.
• Misconception: Lime water is juice from the lime fruit. Correct: Lime water is calcium hydroxide solution and is basic. It is different from acidic citrus juice.
• Misconception: Adding any amount of base makes an acid neutral. Correct: The quantity matters. A mixture may remain acidic, or may become basic if base is in excess.
• Misconception: All salts are neutral and are the same as table salt. Correct: Common salt is sodium chloride. Other salts exist, and salt solutions can be acidic, basic or neutral.
• Misconception: Poor plant growth always proves that soil is acidic. Correct: Basic soil can also need treatment, and sometimes unhealthy plants in neutral soil may lack nutrients.

Exam-style questions with model answers

Q1. Define an acid-base indicator and state the colour change produced when an acidic solution is tested with blue litmus. [2 marks]

1. An acid-base indicator is a substance that shows different colours in acidic and basic solutions.
2. An acidic solution changes blue litmus to red, providing evidence of the sample's acidic nature.

Q2. A liquid leaves red litmus unchanged, turns blue litmus red and leaves turmeric yellow. Identify its nature and explain what each of the three tests shows. [4 marks]

1. The liquid is acidic. Its change of blue litmus to red supplies the positive evidence needed to identify its acidic nature.
2. Red litmus remains red in the liquid. This result is consistent with an acid, but alone would also allow a neutral sample.
3. Blue litmus turns red because it is responding to the acidic liquid. This distinguishes the liquid from a neutral sample.
4. Turmeric remains yellow in acidic conditions. However, this result alone cannot distinguish an acid from a neutral substance, since both leave turmeric unchanged.

Q3. Prepare red rose extract using a fistful of fallen petals, water for washing, a mortar and pestle, a glass tumbler with a lid, enough hot water to immerse the petals, and filtering materials. The waiting time is 5 to 10 minutes. Explain the five stages, including adult supervision for hot water. [5 marks]

1. Collect a fistful of fallen red rose petals and wash them with water. Use fallen petals rather than plucking flowers for the preparation.
2. Crush the washed petals with a mortar and pestle, then transfer the crushed material to a glass tumbler for extraction.
3. Under adult supervision, add enough hot water to immerse the crushed petals completely. The hot-water step requires care during the activity.
4. Cover the tumbler with a lid and wait for 5 to 10 minutes, until the water becomes coloured by the petals.
5. Filter the mixture and collect the filtrate, meaning the liquid after filtration. This coloured red rose extract is used as the acid-base indicator.

Q4. One drop of lemon juice is mixed with around twenty drops of water. Blue litmus solution makes the mixture red. Lime water is then added slowly until it becomes blue. Explain the initial colour, the role of lime water, the final colour and the general products of neutralisation. [4 marks]

1. Lemon juice is acidic, so it changes the added blue litmus solution to red. The initial colour identifies acidic conditions in the test tube.
2. Lime water is basic calcium hydroxide solution. Adding it gradually counteracts the acidic effect of the lemon juice through neutralisation.
3. The eventual blue colour indicates basic conditions. It shows that the mixture is no longer acidic; blue should not be described as a neutral result.
4. In a neutralisation reaction, an acid reacts with a base to form salt and water, with the release of heat.

Q5. A farmer's soil is confirmed to be too acidic after excessive chemical fertiliser use. Explain the connection with poor growth, name a suitable treatment and explain its action. [3 marks]

1. Excessive use of chemical fertilisers can make soil acidic. When soil is too acidic, plants do not grow well, which explains the problem under the stated conditions.
2. The soil can be treated with lime, which is a base. This choice matches the confirmed acidic nature of the farmer's soil.
3. The basic lime neutralises the acidic nature of the soil. The treatment follows from the indicator or other evidence confirming acidity, rather than poor growth alone.

Q6. Three samples are labelled A, B and C. A turns red litmus blue; B turns yellow turmeric red; C turns red rose extract green. Identify each sample's nature and explain each conclusion. [3 marks]

1. Sample A is basic because bases turn red litmus blue. The observed change is the identifying litmus response for a basic solution.
2. Sample B is basic because turmeric changes from yellow to red in basic conditions. Acidic and neutral samples would leave it yellow.
3. Sample C is basic because green is the response of red rose extract to a basic solution. All three observations therefore indicate basic samples.

Q7. Give two separate points explaining the difference between baking soda and baking powder. [2 marks]

1. Baking soda is sodium hydrogencarbonate, a mild basic salt.
2. Baking powder is a mixture of baking soda and tartaric acid, which is a weak edible acid.

Q8. Acidic factory waste is entering a lake where fish numbers are declining. State the possible connection, the treatment for the waste's acidity and when this treatment should occur. [3 marks]

1. The decline in fish numbers may be due to the factory waste entering the lake. The observation suggests a possible connection, rather than proving every cause of the decline.
2. The waste is stated to be acidic, so basic substances can be added to neutralise its acidic nature through an acid-base reaction.
3. The acidic waste should be neutralised before it is released into the lake. The treatment is directed at the waste before discharge.

Key takeaways

• Acids turn blue litmus red, bases turn red litmus blue, and neutral substances change neither paper.
• Bases generally taste bitter, but bitter taste alone does not establish basic nature; bitter gourd is an exception.
• An alkali is a base that dissolves in water, while not all bases are water-soluble.
• Turmeric detects basic substances through a yellow-to-red change, but cannot distinguish acidic substances from neutral ones.
• Neutralisation forms salt and water with heat release, and the quantities of acid and base affect the resulting mixture.
• Common salt is sodium chloride; other salts include baking soda and washing soda, both of which are basic salts.
• Soil treatment depends on its nature: acidic soil can receive lime, while basic soil can receive organic matter.
• Acidic factory waste can be neutralised with basic substances before discharge; the treatment depends on establishing the waste's acidity.

Test yourself

Why does unchanged blue litmus not prove neutrality?

A basic sample also leaves blue litmus unchanged. Test red litmus as well: a base turns it blue, while a neutral sample does not.

Why can turmeric not separate lemon juice from sugar solution?

Lemon juice is acidic and sugar solution is neutral, but both leave yellow turmeric unchanged.

What does “filtrate” mean when preparing a flower indicator?

It means the liquid obtained after filtration. In the rose activity, this coloured liquid is retained as the indicator.

How does an olfactory indicator differ from a colour indicator?

An olfactory indicator uses a change in odour in acidic or basic conditions, instead of relying on a visible colour change.

What does a blue litmus solution show after sufficient lime water has been added to the lemon-juice mixture?

The blue colour indicates basic conditions and shows that the mixture is no longer acidic. It does not identify a neutral mixture.

Why is manure useful for basic soil?

Organic matter such as manure releases acids that can neutralise the basic nature of the soil.

What possible chemical explanation accounts for soap writing appearing under a turmeric spray?

Soap solution is basic and turns turmeric red, so writing made with soap solution can become visible when turmeric is sprayed over it.

Which gas is produced when an acid reacts with a metal carbonate?

Carbon dioxide is produced, along with the corresponding salt and water. This differs from the hydrogen usually produced in acid-metal reactions.