Bicarb, Castile Soap and Hydrogen Peroxide: What Each Is Actually For

There are 4 ingredients doing most of the real work in a fragrance-free house, and any number of recipes online instructing you to combine them in ways that chemically cancel each other out. Knowing what each one is for, and what it is quietly bad at, saves more time than any recipe.

This sits under our wider guide to choosing genuinely low-tox cleaning products in Australia. That article covers labels and marketing. This one is about the 4 things in the cupboard and what they physically do.

Bicarb: a soft abrasive with a side job

Sodium bicarbonate sits at roughly 2.5 on the mineral hardness scale. That number is the entire point of it. It is softer than glass, porcelain, stainless steel and ceramic tile, and harder than most of the soil sitting on top of them, so it grinds off the mess without scoring the surface underneath.

Made into a paste, it lifts baked-on grease from an oven floor, cooked spills off a stovetop, and the grey film off a bath. It is slow. That is fine. Slow abrasion is what you want when the alternative is a caustic chemical eating the finish.

The odour job is real but narrower than advertised. Bicarb is mildly alkaline, so it buffers acidic odour compounds, which covers a lot of what makes a fridge, a bin or a pair of shoes unpleasant. It does considerably less against alkaline odours, which is why it underperforms on ammonia-type smells specifically. Left in an open container it works by contact, so it only reaches what touches it.

There are 2 things bicarb cannot do. It is not a disinfectant, at any concentration you would use at home. And because it is alkaline, it will never dissolve mineral scale or soap scum, both of which need an acid. On a shower screen the abrasion still shifts some film mechanically, but you are working on a mineral deposit with a powder that has no chemical argument with it, and on an acrylic screen you are dulling the surface while you do it.

The bicarb-and-vinegar volcano deserves a plain answer. The fizz is carbon dioxide, and what you are left with in the bowl is a weak sodium acetate solution: less alkaline than the bicarb, less acidic than the vinegar. You have neutralised both. Down a slow drain the physical churn does loosen some build-up, so it is not useless there, but boiling water and a drain brush do more of the work than the chemistry does.

Castile soap: the only real cleaner in the group

Castile is true soap, made by saponifying vegetable oils. Each molecule has a water-loving end and an oil-loving end, which lets it surround grease, lift it off the surface and hold it in suspension so a rinse can carry it away. Nothing else in this article does that. Bicarb scrapes, acids dissolve minerals, peroxide oxidises. Only the soap actually removes oily soil the way you want it removed.

It is concentrated, and most people use far too much. Overdose it and the surplus soap has nothing left to emulsify, so it dries as a film that attracts the next lot of soil. A small squeeze in a bucket. That is genuinely it.

It does not disinfect, and it was never meant to. Soap works by removal, which handles a great deal on a household surface, but that is a different mechanism from killing organisms and worth understanding properly if you have replaced a spray bottle that made antibacterial claims. We covered that distinction in whether natural cleaners actually disinfect.

Why the vinegar-and-castile recipe cannot work

This is the recipe I get asked about most, and it is chemically self-defeating. Soap is the salt of a fatty acid. Add any acid, vinegar or citric or lemon juice, and the acid hands a hydrogen ion back to it, converting the soap into free fatty acid. Free fatty acid is not water soluble, so it falls out of solution as white curds.

The spray you have made has spent its acid, destroyed its surfactant, and now deposits a fine greasy film on whatever you spray it on. The bench feels slightly tacky afterwards, and the usual assumption is that more product is needed. If you want both, use them separately: soap first, rinse properly, acid second.

A last limitation. In hard water, soap reacts with dissolved calcium and magnesium and forms scum, which is precisely the deposit you then need an acid to remove. If your shower goes cloudy faster since switching to castile, that is what is happening, and the answer is less soap and a proper rinse, not more scrubbing.

Hydrogen peroxide: real chemistry, usually stored badly

Household hydrogen peroxide is 3 percent, and unlike most of what gets called natural cleaning, it has genuine oxidising power. It breaks down into water and oxygen, so there is no residue to rinse and nothing left behind on a surface a child will touch. On paper it is close to ideal.

In practice, 3 conditions decide whether it does anything at all.

• It must stay visibly wet on the surface for several minutes. A spray that evaporates in 30 seconds has not done the job.
• The surface has to be clean first. Organic soil consumes peroxide rapidly, so on a dirty surface it is spent before it reaches anything.
• It has to still be active. This is where most bottles fail.

Hydrogen peroxide decomposes on exposure to light, warmth and trace metals. It is sold in an opaque bottle for exactly that reason. Decant it into a clear spray bottle and leave it on the vanity in a Brisbane bathroom and it starts turning into ordinary water, and within weeks you are misting your shower with nothing. Keep it in the original bottle in a dark cupboard, decant only into an opaque sprayer, and replace it rather than nursing a 3 year old bottle.

Be honest about the word disinfectant, too. Peroxide has measurable antimicrobial activity, but disinfectant is a regulated claim tied to specific concentrations, contact times and test methods that a supermarket bottle used casually on a benchtop is not meeting. Useful, yes. A hospital-grade guarantee, no.

And remember it is a mild bleaching agent. It lifts colour. Repeated use on coloured grout, coloured fabric or a timber finish will lighten it, gradually enough that you notice only once the damage is spread across the whole area.

Citric acid: what I actually reach for

Mineral build-up on tapware, that white haze on the shower screen, the ring in the toilet, kettle scale, the crust around the base of a mixer. All of it is mineral, all of it needs an acid, and citric acid is the one in my kit.

It comes as a powder, dissolves in warm water, and you can mix it to whatever strength the job needs: a couple of teaspoons in 500 ml for light haze, considerably stronger and left to dwell for a well-established shower screen. It stores as a dry bag rather than litres of liquid. And it does not put a sharp smell through the house for the next hour, which is exactly what a fragrance-free home is built to avoid.

Vinegar does work on scale. It is a weaker acid at typical strength and it announces itself in every room, so it is a reasonable option rather than the best one. There is more on where it earns its place in our piece on whether vinegar is actually a good cleaner.

What citric acid cannot do is cut grease. It has no surfactant action whatsoever. Acid on an oily surface just sits there. Degrease first, then descale, rinsing between the 2.

What each one will damage

Everything here is capable of ruining something. Match the ingredient to the surface before you match it to the mess.

• Bicarb: acrylic and polycarbonate shower screens, gloss laminate, painted and lacquered surfaces, polished stone (it dulls the polish rather than staining it), and aluminium, which alkalis discolour.
• Castile soap: leaves a film on glass and mirrors, dulls waxed and oiled timber, and builds scum in hard water. Never mix it into an acid solution.
• Hydrogen peroxide: coloured grout, coloured and delicate fabrics, some timber and stone finishes. It keeps oxidising for as long as it sits there.
• Citric acid: marble, limestone, travertine and any carbonate stone. The etch is a physical change to the surface, not a stain, and it does not clean off. Also avoid prolonged contact with aluminium and with coated or brushed tapware finishes.
• Steel wool: excellent on glass shower screens and some stainless, and permanently destructive on acrylic, chrome plating, painted surfaces and anodised finishes. Test somewhere invisible, always.

The part the ingredients do not decide

It would be neat to say the right 4 bottles produce a clean house. They do not. Most of what separates a genuinely clean bathroom from a wiped one is mechanical: a professional HEPA vacuum that holds what it collects instead of redistributing it, the correct brush for the grout profile, fine steel wool where a surface can take it, and enough time on the surface for any of it to matter.

That is the whole reason a fragrance-free clean can work at all. A commodity clean is priced on speed, and harsh chemistry is what compensates for the minutes nobody paid for. Take the harsh chemistry away and you have to pay for the minutes instead, which is how our natural house cleaning is structured: $85 per hour, with technique doing the work the chemicals used to. Grout is the clearest example, and we walked through the method in how to clean grout.

The short version

Soap for anything greasy. Bicarb when you need gentle abrasion or want to absorb an acidic odour. Citric acid for anything mineral. Hydrogen peroxide, stored dark and used on a clean wet surface, when you want oxidising action. Never soap and acid in the same bottle. Never peroxide and vinegar in the same bottle. Never bleach and anything.

You do not need more products than that. Most people already own 3 of the 4 and have simply been combining them into something weaker than either ingredient on its own.